This document is obsolete. Please
refer to RFC 9052 and RFC 9053.
Internet Engineering Task Force (IETF) J. Schaad Request for Comments: 8152 August Cellars Category: Standards Track July 2017 ISSN: 2070-1721
CBOR Object Signing and Encryption (COSE)
Abstract
Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need for the ability to have basic security services defined for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8152.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Schaad Standards Track [Page 1]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Table of Contents
1. Introduction ....................................................4 1.1. Design Changes from JOSE ...................................5 1.2. Requirements Terminology ...................................6 1.3. CBOR Grammar ...............................................6 1.4. CBOR-Related Terminology ...................................7 1.5. Document Terminology .......................................8 2. Basic COSE Structure ............................................8 3. Header Parameters ..............................................10 3.1. Common COSE Headers Parameters ............................12 4. Signing Objects ................................................16 4.1. Signing with One or More Signers ..........................16 4.2. Signing with One Signer ...................................18 4.3. Externally Supplied Data ..................................19 4.4. Signing and Verification Process ..........................20 4.5. Computing Counter Signatures ..............................22 5. Encryption Objects .............................................22 5.1. Enveloped COSE Structure ..................................23 5.1.1. Content Key Distribution Methods ...................24 5.2. Single Recipient Encrypted ................................25 5.3. How to Encrypt and Decrypt for AEAD Algorithms ............26 5.4. How to Encrypt and Decrypt for AE Algorithms ..............28 6. MAC Objects ....................................................29 6.1. MACed Message with Recipients .............................30 6.2. MACed Messages with Implicit Key ..........................31 6.3. How to Compute and Verify a MAC ...........................32 7. Key Objects ....................................................33 7.1. COSE Key Common Parameters ................................34 8. Signature Algorithms ...........................................37 8.1. ECDSA .....................................................38 8.1.1. Security Considerations ............................40 8.2. Edwards-Curve Digital Signature Algorithms (EdDSAs) .......40 8.2.1. Security Considerations ............................41 9. Message Authentication Code (MAC) Algorithms ...................42 9.1. Hash-Based Message Authentication Codes (HMACs) ...........42 9.1.1. Security Considerations ............................44 9.2. AES Message Authentication Code (AES-CBC-MAC) .............44 9.2.1. Security Considerations ............................45 10. Content Encryption Algorithms .................................45 10.1. AES GCM ..................................................46 10.1.1. Security Considerations ...........................47 10.2. AES CCM ..................................................47 10.2.1. Security Considerations ...........................50 10.3. ChaCha20 and Poly1305 ....................................50 10.3.1. Security Considerations ...........................51 11. Key Derivation Functions (KDFs) ...............................51
Schaad Standards Track [Page 2]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
11.1. HMAC-Based Extract-and-Expand Key Derivation Function (HKDF) ..........................................52 11.2. Context Information Structure ............................54 12. Content Key Distribution Methods ..............................60 12.1. Direct Encryption ........................................60 12.1.1. Direct Key ........................................61 12.1.2. Direct Key with KDF ...............................61 12.2. Key Wrap ................................................63 12.2.1. AES Key Wrap ......................................64 12.3. Key Transport ...........................................65 12.4. Direct Key Agreement ....................................65 12.4.1. ECDH ..............................................66 12.4.2. Security Considerations ...........................69 12.5. Key Agreement with Key Wrap ..............................69 12.5.1. ECDH ..............................................70 13. Key Object Parameters .........................................72 13.1. Elliptic Curve Keys ......................................73 13.1.1. Double Coordinate Curves ..........................73 13.2. Octet Key Pair ...........................................74 13.3. Symmetric Keys ...........................................75 14. CBOR Encoder Restrictions .....................................76 15. Application Profiling Considerations ..........................76 16. IANA Considerations ...........................................78 16.1. CBOR Tag Assignment ......................................78 16.2. COSE Header Parameters Registry ..........................78 16.3. COSE Header Algorithm Parameters Registry ................79 16.4. COSE Algorithms Registry .................................79 16.5. COSE Key Common Parameters Registry ......................81 16.6. COSE Key Type Parameters Registry ........................81 16.7. COSE Key Types Registry ..................................82 16.8. COSE Elliptic Curves Registry ............................83 16.9. Media Type Registrations .................................84 16.9.1. COSE Security Message .............................84 16.9.2. COSE Key Media Type ...............................85 16.10. CoAP Content-Formats Registry ...........................87 16.11. Expert Review Instructions ..............................87 17. Security Considerations .......................................88 18. References ....................................................90 18.1. Normative References .....................................90 18.2. Informative References ...................................92 Appendix A. Guidelines for External Data Authentication of Algorithms ............................................96 A.1. Algorithm Identification ..................................96 A.2. Counter Signature without Headers .........................99 Appendix B. Two Layers of Recipient Information ..................100 Appendix C. Examples .............................................102 C.1. Examples of Signed Messages ..............................103 C.1.1. Single Signature ..................................103
Schaad Standards Track [Page 3]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
C.1.2. Multiple Signers ..................................103 C.1.3. Counter Signature .................................104 C.1.4. Signature with Criticality ........................105 C.2. Single Signer Examples ...................................106 C.2.1. Single ECDSA Signature ...........................106 C.3. Examples of Enveloped Messages ...........................107 C.3.1. Direct ECDH .......................................107 C.3.2. Direct Plus Key Derivation ........................108 C.3.3. Counter Signature on Encrypted Content ............109 C.3.4. Encrypted Content with External Data ..............111 C.4. Examples of Encrypted Messages ...........................111 C.4.1. Simple Encrypted Message ..........................111 C.4.2. Encrypted Message with a Partial IV ...............112 C.5. Examples of MACed Messages ...............................112 C.5.1. Shared Secret Direct MAC ..........................112 C.5.2. ECDH Direct MAC ...................................113 C.5.3. Wrapped MAC .......................................114 C.5.4. Multi-Recipient MACed Message .....................115 C.6. Examples of MAC0 Messages ................................117 C.6.1. Shared Secret Direct MAC ..........................117 C.7. COSE Keys ................................................117 C.7.1. Public Keys .......................................117 C.7.2. Private Keys ......................................119 Acknowledgments ..................................................121 Author's Address .................................................121
There has been an increased focus on small, constrained devices that make up the Internet of Things (IoT). One of the standards that has come out of this process is "Concise Binary Object Representation (CBOR)" [RFC7049]. CBOR extended the data model of the JavaScript Object Notation (JSON) [RFC7159] by allowing for binary data, among other changes. CBOR is being adopted by several of the IETF working groups dealing with the IoT world as their encoding of data structures. CBOR was designed specifically to be both small in terms of messages transport and implementation size and be a schema-free decoder. A need exists to provide message security services for IoT, and using CBOR as the message-encoding format makes sense.
The JOSE working group produced a set of documents [RFC7515] [RFC7516] [RFC7517] [RFC7518] using JSON that specified how to process encryption, signatures, and Message Authentication Code (MAC) operations and how to encode keys using JSON. This document defines the CBOR Object Signing and Encryption (COSE) standard, which does the same thing for the CBOR encoding format. While there is a strong
Schaad Standards Track [Page 4]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
attempt to keep the flavor of the original JSON Object Signing and Encryption (JOSE) documents, two considerations are taken into account:
o CBOR has capabilities that are not present in JSON and are appropriate to use. One example of this is the fact that CBOR has a method of encoding binary directly without first converting it into a base64-encoded string.
o COSE is not a direct copy of the JOSE specification. In the process of creating COSE, decisions that were made for JOSE were re-examined. In many cases, different results were decided on as the criteria were not always the same.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
When the words appear in lowercase, this interpretation does not apply.
There is currently no standard CBOR grammar available for use by specifications. The CBOR structures are therefore described in prose.
The document was developed by first working on the grammar and then developing the prose to go with it. An artifact of this is that the prose was written using the primitive type strings defined by CBOR Data Definition Language (CDDL) [CDDL]. In this specification, the following primitive types are used:
any -- non-specific value that permits all CBOR values to be placed here.
bool -- a boolean value (true: major type 7, value 21; false: major type 7, value 20).
bstr -- byte string (major type 2).
int -- an unsigned integer or a negative integer.
nil -- a null value (major type 7, value 22).
nint -- a negative integer (major type 1).
tstr -- a UTF-8 text string (major type 3).
uint -- an unsigned integer (major type 0).
Two syntaxes from CDDL appear in this document as shorthand. These are:
FOO / BAR -- indicates that either FOO or BAR can appear here.
[+ FOO] -- indicates that the type FOO appears one or more times in an array.
Schaad Standards Track [Page 6]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
As well as the prose description, a version of a CBOR grammar is presented in CDDL. Since CDDL has not been published in an RFC, this grammar may not work with the final version of CDDL. The CDDL grammar is informational; the prose description is normative.
The collected CDDL can be extracted from the XML version of this document via the following XPath expression below. (Depending on the XPath evaluator one is using, it may be necessary to deal with > as an entity.)
//artwork[@type='CDDL']/text()
CDDL expects the initial non-terminal symbol to be the first symbol in the file. For this reason, the first fragment of CDDL is presented here.
; This is defined to make the tool quieter: Internal_Types = Sig_structure / Enc_structure / MAC_structure / COSE_KDF_Context
The non-terminal Internal_Types is defined for dealing with the automated validation tools used during the writing of this document. It references those non-terminals that are used for security computations but are not emitted for transport.
In JSON, maps are called objects and only have one kind of map key: a string. In COSE, we use strings, negative integers, and unsigned integers as map keys. The integers are used for compactness of encoding and easy comparison. The inclusion of strings allows for an additional range of short encoded values to be used as well. Since the word "key" is mainly used in its other meaning, as a cryptographic key, we use the term "label" for this usage as a map key.
The presence of a label in a COSE map that is not a string or an integer is an error. Applications can either fail processing or process messages with incorrect labels; however, they MUST NOT create messages with incorrect labels.
A CDDL grammar fragment defines the non-terminal 'label', as in the previous paragraph, and 'values', which permits any value to be used.
label = int / tstr values = any
Schaad Standards Track [Page 7]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
In this document, we use the following terminology:
Byte is a synonym for octet.
Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use in constrained systems. It is defined in [RFC7252].
Authenticated Encryption (AE) [RFC5116] algorithms are those encryption algorithms that provide an authentication check of the contents algorithm with the encryption service.
Authenticated Encryption with Authenticated Data (AEAD) [RFC5116] algorithms provide the same content authentication service as AE algorithms, but they additionally provide for authentication of non- encrypted data as well.
The COSE object structure is designed so that there can be a large amount of common code when parsing and processing the different types of security messages. All of the message structures are built on the CBOR array type. The first three elements of the array always contain the same information:
1. The set of protected header parameters wrapped in a bstr.
2. The set of unprotected header parameters as a map.
3. The content of the message. The content is either the plaintext or the ciphertext as appropriate. The content may be detached, but the location is still used. The content is wrapped in a bstr when present and is a nil value when detached.
Elements after this point are dependent on the specific message type.
COSE messages are also built using the concept of layers to separate different types of cryptographic concepts. As an example of how this works, consider the COSE_Encrypt message (Section 5.1). This message type is broken into two layers: the content layer and the recipient layer. In the content layer, the plaintext is encrypted and information about the encrypted message is placed. In the recipient layer, the content encryption key (CEK) is encrypted and information about how it is encrypted for each recipient is placed. A single layer version of the encryption message COSE_Encrypt0 (Section 5.2) is provided for cases where the CEK is pre-shared.
Schaad Standards Track [Page 8]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Identification of which type of message has been presented is done by the following methods:
1. The specific message type is known from the context. This may be defined by a marker in the containing structure or by restrictions specified by the application protocol.
2. The message type is identified by a CBOR tag. Messages with a CBOR tag are known in this specification as tagged messages, while those without the CBOR tag are known as untagged messages. This document defines a CBOR tag for each of the message structures. These tags can be found in Table 1.
3. When a COSE object is carried in a media type of 'application/ cose', the optional parameter 'cose-type' can be used to identify the embedded object. The parameter is OPTIONAL if the tagged version of the structure is used. The parameter is REQUIRED if the untagged version of the structure is used. The value to use with the parameter for each of the structures can be found in Table 1.
4. When a COSE object is carried as a CoAP payload, the CoAP Content-Format Option can be used to identify the message content. The CoAP Content-Format values can be found in Table 26. The CBOR tag for the message structure is not required as each security message is uniquely identified.
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The following CDDL fragment identifies all of the top messages defined in this document. Separate non-terminals are defined for the tagged and the untagged versions of the messages.
The structure of COSE has been designed to have two buckets of information that are not considered to be part of the payload itself, but are used for holding information about content, algorithms, keys, or evaluation hints for the processing of the layer. These two buckets are available for use in all of the structures except for keys. While these buckets are present, they may not all be usable in all instances. For example, while the protected bucket is defined as part of the recipient structure, some of the algorithms used for recipient structures do not provide for authenticated data. If this is the case, the protected bucket is left empty.
Both buckets are implemented as CBOR maps. The map key is a 'label' (Section 1.4). The value portion is dependent on the definition for the label. Both maps use the same set of label/value pairs. The integer and string values for labels have been divided into several sections including a standard range, a private range, and a range that is dependent on the algorithm selected. The defined labels can be found in the "COSE Header Parameters" IANA registry (Section 16.2).
Two buckets are provided for each layer:
protected: Contains parameters about the current layer that are to be cryptographically protected. This bucket MUST be empty if it is not going to be included in a cryptographic computation. This bucket is encoded in the message as a binary object. This value is obtained by CBOR encoding the protected map and wrapping it in a bstr object. Senders SHOULD encode a zero-length map as a zero- length string rather than as a zero-length map (encoded as h'a0'). The zero-length binary encoding is preferred because it is both shorter and the version used in the serialization structures for cryptographic computation. After encoding the map, the value is
Schaad Standards Track [Page 10]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
wrapped in the binary object. Recipients MUST accept both a zero- length binary value and a zero-length map encoded in the binary value. The wrapping allows for the encoding of the protected map to be transported with a greater chance that it will not be altered in transit. (Badly behaved intermediates could decode and re-encode, but this will result in a failure to verify unless the re-encoded byte string is identical to the decoded byte string.) This avoids the problem of all parties needing to be able to do a common canonical encoding.
unprotected: Contains parameters about the current layer that are not cryptographically protected.
Only parameters that deal with the current layer are to be placed at that layer. As an example of this, the parameter 'content type' describes the content of the message being carried in the message. As such, this parameter is placed only in the content layer and is not placed in the recipient or signature layers. In principle, one should be able to process any given layer without reference to any other layer. With the exception of the COSE_Sign structure, the only data that needs to cross layers is the cryptographic key.
The buckets are present in all of the security objects defined in this document. The fields in order are the 'protected' bucket (as a CBOR 'bstr' type) and then the 'unprotected' bucket (as a CBOR 'map' type). The presence of both buckets is required. The parameters that go into the buckets come from the IANA "COSE Header Parameters" registry (Section 16.2). Some common parameters are defined in the next section, but a number of parameters are defined throughout this document.
Labels in each of the maps MUST be unique. When processing messages, if a label appears multiple times, the message MUST be rejected as malformed. Applications SHOULD verify that the same label does not occur in both the protected and unprotected headers. If the message is not rejected as malformed, attributes MUST be obtained from the protected bucket before they are obtained from the unprotected bucket.
Schaad Standards Track [Page 11]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The following CDDL fragment represents the two header buckets. A group "Headers" is defined in CDDL that represents the two buckets in which attributes are placed. This group is used to provide these two fields consistently in all locations. A type is also defined that represents the map of common headers.
This section defines a set of common header parameters. A summary of these parameters can be found in Table 2. This table should be consulted to determine the value of label and the type of the value.
The set of header parameters defined in this section are:
alg: This parameter is used to indicate the algorithm used for the security processing. This parameter MUST be authenticated where the ability to do so exists. This support is provided by AEAD algorithms or construction (COSE_Sign, COSE_Sign0, COSE_Mac, and COSE_Mac0). This authentication can be done either by placing the header in the protected header bucket or as part of the externally supplied data. The value is taken from the "COSE Algorithms" registry (see Section 16.4).
crit: The parameter is used to indicate which protected header labels an application that is processing a message is required to understand. Parameters defined in this document do not need to be included as they should be understood by all implementations. When present, this parameter MUST be placed in the protected header bucket. The array MUST have at least one value in it. Not all labels need to be included in the 'crit' parameter. The rules for deciding which header labels are placed in the array are:
* Integer labels in the range of 0 to 8 SHOULD be omitted.
Schaad Standards Track [Page 12]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
* Integer labels in the range -1 to -128 can be omitted as they are algorithm dependent. If an application can correctly process an algorithm, it can be assumed that it will correctly process all of the common parameters associated with that algorithm. Integer labels in the range -129 to -65536 SHOULD be included as these would be less common parameters that might not be generally supported.
* Labels for parameters required for an application MAY be omitted. Applications should have a statement if the label can be omitted.
The header parameter values indicated by 'crit' can be processed by either the security library code or an application using a security library; the only requirement is that the parameter is processed. If the 'crit' value list includes a value for which the parameter is not in the protected bucket, this is a fatal error in processing the message.
content type: This parameter is used to indicate the content type of the data in the payload or ciphertext fields. Integers are from the "CoAP Content-Formats" IANA registry table [COAP.Formats]. Text values following the syntax of "<type-name>/<subtype-name>" where <type-name> and <subtype-name> are defined in Section 4.2 of [RFC6838]. Leading and trailing whitespace is also omitted. Textual content values along with parameters and subparameters can be located using the IANA "Media Types" registry. Applications SHOULD provide this parameter if the content structure is potentially ambiguous.
kid: This parameter identifies one piece of data that can be used as input to find the needed cryptographic key. The value of this parameter can be matched against the 'kid' member in a COSE_Key structure. Other methods of key distribution can define an equivalent field to be matched. Applications MUST NOT assume that 'kid' values are unique. There may be more than one key with the same 'kid' value, so all of the keys associated with this 'kid' may need to be checked. The internal structure of 'kid' values is not defined and cannot be relied on by applications. Key identifier values are hints about which key to use. This is not a security-critical field. For this reason, it can be placed in the unprotected headers bucket.
IV: This parameter holds the Initialization Vector (IV) value. For some symmetric encryption algorithms, this may be referred to as a nonce. The IV can be placed in the unprotected header as modifying the IV will cause the decryption to yield plaintext that is readily detectable as garbled.
Schaad Standards Track [Page 13]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Partial IV: This parameter holds a part of the IV value. When using the COSE_Encrypt0 structure, a portion of the IV can be part of the context associated with the key. This field is used to carry a value that causes the IV to be changed for each message. The IV can be placed in the unprotected header as modifying the IV will cause the decryption to yield plaintext that is readily detectable as garbled. The 'Initialization Vector' and 'Partial Initialization Vector' parameters MUST NOT both be present in the same security layer.
The message IV is generated by the following steps:
1. Left-pad the Partial IV with zeros to the length of IV.
2. XOR the padded Partial IV with the context IV.
counter signature: This parameter holds one or more counter signature values. Counter signatures provide a method of having a second party sign some data. The counter signature parameter can occur as an unprotected attribute in any of the following structures: COSE_Sign1, COSE_Signature, COSE_Encrypt, COSE_recipient, COSE_Encrypt0, COSE_Mac, and COSE_Mac0. These structures all have the same beginning elements, so that a consistent calculation of the counter signature can be computed. Details on computing counter signatures are found in Section 4.5.
Schaad Standards Track [Page 14]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The CDDL fragment that represents the set of headers defined in this section is given below. Each of the headers is tagged as optional because they do not need to be in every map; headers required in specific maps are discussed above.
COSE supports two different signature structures. COSE_Sign allows for one or more signatures to be applied to the same content. COSE_Sign1 is restricted to a single signer. The structures cannot be converted between each other; as the signature computation includes a parameter identifying which structure is being used, the converted structure will fail signature validation.
The COSE_Sign structure allows for one or more signatures to be applied to a message payload. Parameters relating to the content and parameters relating to the signature are carried along with the signature itself. These parameters may be authenticated by the signature, or just present. An example of a parameter about the content is the content type. Examples of parameters about the signature would be the algorithm and key used to create the signature and counter signatures.
When more than one signature is present, the successful validation of one signature associated with a given signer is usually treated as a successful signature by that signer. However, there are some application environments where other rules are needed. An application that employs a rule other than one valid signature for each signer must specify those rules. Also, where simple matching of the signer identifier is not sufficient to determine whether the signatures were generated by the same signer, the application specification must describe how to determine which signatures were generated by the same signer. Support for different communities of recipients is the primary reason that signers choose to include more than one signature.
For example, the COSE_Sign structure might include signatures generated with the Edwards-curve Digital Signature Algorithm (EdDSA) [RFC8032] and with the Elliptic Curve Digital Signature Algorithm (ECDSA) [DSS]. This allows recipients to verify the signature associated with one algorithm or the other. More-detailed information on multiple signature evaluations can be found in [RFC5752].
Schaad Standards Track [Page 16]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The signature structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Sign structure is identified by the CBOR tag 98. The CDDL fragment that represents this is:
COSE_Sign_Tagged = #6.98(COSE_Sign)
A COSE Signed Message is defined in two parts. The CBOR object that carries the body and information about the body is called the COSE_Sign structure. The CBOR object that carries the signature and information about the signature is called the COSE_Signature structure. Examples of COSE Signed Messages can be found in Appendix C.1.
The COSE_Sign structure is a CBOR array. The fields of the array in order are:
payload: This field contains the serialized content to be signed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately ("detached content"), then a nil CBOR object is placed in this location, and it is the responsibility of the application to ensure that it will be transported without changes.
Note: When a signature with a message recovery algorithm is used (Section 8), the maximum number of bytes that can be recovered is the length of the payload. The size of the payload is reduced by the number of bytes that will be recovered. If all of the bytes of the payload are consumed, then the payload is encoded as a zero-length binary string rather than as being absent.
signatures: This field is an array of signatures. Each signature is represented as a COSE_Signature structure.
Schaad Standards Track [Page 17]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The CDDL fragment that represents the above text for COSE_Sign follows.
signature: This field contains the computed signature value. The type of the field is a bstr. Algorithms MUST specify padding if the signature value is not a multiple of 8 bits.
The CDDL fragment that represents the above text for COSE_Signature follows.
The COSE_Sign1 signature structure is used when only one signature is going to be placed on a message. The parameters dealing with the content and the signature are placed in the same pair of buckets rather than having the separation of COSE_Sign.
The structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Sign1 structure is identified by the CBOR tag 18. The CDDL fragment that represents this is:
COSE_Sign1_Tagged = #6.18(COSE_Sign1)
The CBOR object that carries the body, the signature, and the information about the body and signature is called the COSE_Sign1 structure. Examples of COSE_Sign1 messages can be found in Appendix C.2.
Schaad Standards Track [Page 18]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The COSE_Sign1 structure is a CBOR array. The fields of the array in order are:
One of the features offered in the COSE document is the ability for applications to provide additional data to be authenticated, but that is not carried as part of the COSE object. The primary reason for supporting this can be seen by looking at the CoAP message structure [RFC7252], where the facility exists for options to be carried before the payload. Examples of data that can be placed in this location would be the CoAP code or CoAP options. If the data is in the header section, then it is available for proxies to help in performing its operations. For example, the Accept Option can be used by a proxy to determine if an appropriate value is in the proxy's cache. But the sender can prevent a proxy from changing the set of values that it will accept by including that value in the resulting authentication tag. However, it may also be desired to protect these values so that if they are modified in transit, it can be detected.
This document describes the process for using a byte array of externally supplied authenticated data; however, the method of constructing the byte array is a function of the application. Applications that use this feature need to define how the externally supplied authenticated data is to be constructed. Such a construction needs to take into account the following issues:
o If multiple items are included, applications need to ensure that the same byte string is not produced if there are different inputs. This could occur by appending the strings 'AB' and 'CDE'
Schaad Standards Track [Page 19]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
or by appending the strings 'ABC' and 'DE'. This is usually addressed by making fields a fixed width and/or encoding the length of the field as part of the output. Using options from CoAP [RFC7252] as an example, these fields use a TLV structure so they can be concatenated without any problems.
o If multiple items are included, an order for the items needs to be defined. Using options from CoAP as an example, an application could state that the fields are to be ordered by the option number.
o Applications need to ensure that the byte stream is going to be the same on both sides. Using options from CoAP might give a problem if the same relative numbering is kept. An intermediate node could insert or remove an option, changing how the relative number is done. An application would need to specify that the relative number must be re-encoded to be relative only to the options that are in the external data.
In order to create a signature, a well-defined byte stream is needed. The Sig_structure is used to create the canonical form. This signing and verification process takes in the body information (COSE_Sign or COSE_Sign1), the signer information (COSE_Signature), and the application data (external source). A Sig_structure is a CBOR array. The fields of the Sig_structure in order are:
1. A text string identifying the context of the signature. The context string is:
"Signature" for signatures using the COSE_Signature structure.
"Signature1" for signatures using the COSE_Sign1 structure.
"CounterSignature" for signatures used as counter signature attributes.
2. The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used.
3. The protected attributes from the signer structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used. This field is omitted for the COSE_Sign1 signature structure.
Schaad Standards Track [Page 20]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
4. The protected attributes from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero- length binary string. (See Section 4.3 for application guidance on constructing this field.)
5. The payload to be signed encoded in a bstr type. The payload is placed here independent of how it is transported.
The CDDL fragment that describes the above text is:
1. Create a Sig_structure and populate it with the appropriate fields.
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string, using the encoding described in Section 14.
3. Call the signature creation algorithm passing in K (the key to sign with), alg (the algorithm to sign with), and ToBeSigned (the value to sign).
4. Place the resulting signature value in the 'signature' field of the array.
The steps for verifying a signature are:
1. Create a Sig_structure object and populate it with the appropriate fields.
2. Create the value ToBeSigned by encoding the Sig_structure to a byte string, using the encoding described in Section 14.
3. Call the signature verification algorithm passing in K (the key to verify with), alg (the algorithm used sign with), ToBeSigned (the value to sign), and sig (the signature to be verified).
Schaad Standards Track [Page 21]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
In addition to performing the signature verification, the application may also perform the appropriate checks to ensure that the key is correctly paired with the signing identity and that the signing identity is authorized before performing actions.
Counter signatures provide a method of associating a different signature generated by different signers with some piece of content. This is normally used to provide a signature on a signature allowing for a proof that a signature existed at a given time (i.e., a Timestamp). In this document, we allow for counter signatures to exist in a greater number of environments. As an example, it is possible to place a counter signature in the unprotected attributes of a COSE_Encrypt object. This would allow for an intermediary to either verify that the encrypted byte stream has not been modified, without being able to decrypt it, or assert that an encrypted byte stream either existed at a given time or passed through it in terms of routing (i.e., a proxy signature).
An example of a counter signature on a signature can be found in Appendix C.1.3. An example of a counter signature in an encryption object can be found in Appendix C.3.3.
The creation and validation of counter signatures over the different items relies on the fact that the objects have the same structure. The elements are a set of protected attributes, a set of unprotected attributes, and a body, in that order. This means that the Sig_structure can be used in a uniform manner to get the byte stream for processing a signature. If the counter signature is going to be computed over a COSE_Encrypt structure, the body_protected and payload items can be mapped into the Sig_structure in the same manner as from the COSE_Sign structure.
It should be noted that only a signature algorithm with appendix (see Section 8) can be used for counter signatures. This is because the body should be able to be processed without having to evaluate the counter signature, and this is not possible for signature schemes with message recovery.
5. Encryption Objects
COSE supports two different encryption structures. COSE_Encrypt0 is used when a recipient structure is not needed because the key to be used is known implicitly. COSE_Encrypt is used the rest of the time. This includes cases where there are multiple recipients or a recipient algorithm other than direct is used.
Schaad Standards Track [Page 22]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The enveloped structure allows for one or more recipients of a message. There are provisions for parameters about the content and parameters about the recipient information to be carried in the message. The protected parameters associated with the content are authenticated by the content encryption algorithm. The protected parameters associated with the recipient are authenticated by the recipient algorithm (when the algorithm supports it). Examples of parameters about the content are the type of the content and the content encryption algorithm. Examples of parameters about the recipient are the recipient's key identifier and the recipient's encryption algorithm.
The same techniques and structures are used for encrypting both the plaintext and the keys. This is different from the approach used by both "Cryptographic Message Syntax (CMS)" [RFC5652] and "JSON Web Encryption (JWE)" [RFC7516] where different structures are used for the content layer and for the recipient layer. Two structures are defined: COSE_Encrypt to hold the encrypted content and COSE_recipient to hold the encrypted keys for recipients. Examples of encrypted messages can be found in Appendix C.3.
The COSE_Encrypt structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt structure is identified by the CBOR tag 96. The CDDL fragment that represents this is:
COSE_Encrypt_Tagged = #6.96(COSE_Encrypt)
The COSE_Encrypt structure is a CBOR array. The fields of the array in order are:
ciphertext: This field contains the ciphertext encoded as a bstr. If the ciphertext is to be transported independently of the control information about the encryption process (i.e., detached content), then the field is encoded as a nil value.
recipients: This field contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient.
Schaad Standards Track [Page 23]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The CDDL fragment that corresponds to the above text is:
ciphertext: This field contains the encrypted key encoded as a bstr. All encoded keys are symmetric keys; the binary value of the key is the content. If there is not an encrypted key, then this field is encoded as a nil value.
recipients: This field contains an array of recipient information structures. The type for the recipient information structure is a COSE_recipient (an example of this can be found in Appendix B). If there are no recipient information structures, this element is absent.
The CDDL fragment that corresponds to the above text for COSE_recipient is:
An encrypted message consists of an encrypted content and an encrypted CEK for one or more recipients. The CEK is encrypted for each recipient, using a key specific to that recipient. The details of this encryption depend on which class the recipient algorithm falls into. Specific details on each of the classes can be found in Section 12. A short summary of the five content key distribution methods is:
direct: The CEK is the same as the identified previously distributed symmetric key or is derived from a previously distributed secret. No CEK is transported in the message.
Schaad Standards Track [Page 24]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
symmetric key-encryption keys (KEK): The CEK is encrypted using a previously distributed symmetric KEK. Also known as key wrap.
key agreement: The recipient's public key and a sender's private key are used to generate a pairwise secret, a Key Derivation Function (KDF) is applied to derive a key, and then the CEK is either the derived key or encrypted by the derived key.
key transport: The CEK is encrypted with the recipient's public key. No key transport algorithms are defined in this document.
passwords: The CEK is encrypted in a KEK that is derived from a password. No password algorithms are defined in this document.
The COSE_Encrypt0 encrypted structure does not have the ability to specify recipients of the message. The structure assumes that the recipient of the object will already know the identity of the key to be used in order to decrypt the message. If a key needs to be identified to the recipient, the enveloped structure ought to be used.
Examples of encrypted messages can be found in Appendix C.3.
The COSE_Encrypt0 structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Encrypt0 structure is identified by the CBOR tag 16. The CDDL fragment that represents this is:
COSE_Encrypt0_Tagged = #6.16(COSE_Encrypt0)
The COSE_Encrypt0 structure is a CBOR array. The fields of the array in order are:
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
5.3. How to Encrypt and Decrypt for AEAD Algorithms
The encryption algorithm for AEAD algorithms is fairly simple. The first step is to create a consistent byte stream for the authenticated data structure. For this purpose, we use an Enc_structure. The Enc_structure is a CBOR array. The fields of the Enc_structure in order are:
1. A text string identifying the context of the authenticated data structure. The context string is:
"Encrypt0" for the content encryption of a COSE_Encrypt0 data structure.
"Encrypt" for the first layer of a COSE_Encrypt data structure (i.e., for content encryption).
"Enc_Recipient" for a recipient encoding to be placed in an COSE_Encrypt data structure.
"Mac_Recipient" for a recipient encoding to be placed in a MACed message structure.
"Rec_Recipient" for a recipient encoding to be placed in a recipient structure.
2. The protected attributes from the body structure encoded in a bstr type. If there are no protected attributes, a bstr of length zero is used.
3. The protected attributes from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero- length bstr. (See Section 4.3 for application guidance on constructing this field.)
The CDDL fragment that describes the above text is:
1. Create an Enc_structure and populate it with the appropriate fields.
Schaad Standards Track [Page 26]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
2. Encode the Enc_structure to a byte stream (Additional Authenticated Data (AAD)), using the encoding described in Section 14.
3. Determine the encryption key (K). This step is dependent on the class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 12.3), key wrap keys (Section 12.2.1), or pre-shared secrets.
Direct Encryption and Direct Key Agreement: The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 12.1.2 and 12.4.1.
Other: The key is randomly or pseudorandomly generated.
4. Call the encryption algorithm with K (the encryption key), P (the plaintext), and AAD. Place the returned ciphertext into the 'ciphertext' field of the structure.
5. For recipients of the message, recursively perform the encryption algorithm for that recipient, using K (the encryption key) as the plaintext.
How to decrypt a message:
1. Create an Enc_structure and populate it with the appropriate fields.
2. Encode the Enc_structure to a byte stream (AAD), using the encoding described in Section 14.
3. Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 12.3), key wrap keys (Section 12.2.1), or pre-shared secrets.
Schaad Standards Track [Page 27]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Direct Encryption and Direct Key Agreement: The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 12.1.2 and 12.4.1.
Other: The key is determined by decoding and decrypting one of the recipient structures.
4. Call the decryption algorithm with K (the decryption key to use), C (the ciphertext), and AAD.
2. Verify that there was no external additional authenticated data supplied for this operation.
3. Determine the encryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 12.3), key wrap keys (Section 12.2.1), or pre-shared secrets.
Direct Encryption and Direct Key Agreement: The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 12.1.2 and 12.4.1.
Other: The key is randomly generated.
4. Call the encryption algorithm with K (the encryption key to use) and P (the plaintext). Place the returned ciphertext into the 'ciphertext' field of the structure.
5. For recipients of the message, recursively perform the encryption algorithm for that recipient, using K (the encryption key) as the plaintext.
Schaad Standards Track [Page 28]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
How to decrypt a message:
1. Verify that the 'protected' field is empty.
2. Verify that there was no external additional authenticated data supplied for this operation.
3. Determine the decryption key. This step is dependent on the class of recipient algorithm being used. For:
No Recipients: The key to be used is determined by the algorithm and key at the current layer. Examples are key transport keys (Section 12.3), key wrap keys (Section 12.2.1), or pre-shared secrets.
Direct Encryption and Direct Key Agreement: The key is determined by the key and algorithm in the recipient structure. The encryption algorithm and size of the key to be used are inputs into the KDF used for the recipient. (For direct, the KDF can be thought of as the identity operation.) Examples of these algorithms are found in Sections 12.1.2 and 12.4.1.
Other: The key is determined by decoding and decrypting one of the recipient structures.
4. Call the decryption algorithm with K (the decryption key to use) and C (the ciphertext).
COSE supports two different MAC structures. COSE_MAC0 is used when a recipient structure is not needed because the key to be used is implicitly known. COSE_MAC is used for all other cases. These include a requirement for multiple recipients, the key being unknown, and a recipient algorithm of other than direct.
In this section, we describe the structure and methods to be used when doing MAC authentication in COSE. This document allows for the use of all of the same classes of recipient algorithms as are allowed for encryption.
When using MAC operations, there are two modes in which they can be used. The first is just a check that the content has not been changed since the MAC was computed. Any class of recipient algorithm can be used for this purpose. The second mode is to both check that the content has not been changed since the MAC was computed and to use the recipient algorithm to verify who sent it. The classes of
Schaad Standards Track [Page 29]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
recipient algorithms that support this are those that use a pre- shared secret or do static-static (SS) key agreement (without the key wrap step). In both of these cases, the entity that created and sent the message MAC can be validated. (This knowledge of the sender assumes that there are only two parties involved and that you did not send the message to yourself.) The origination property can be obtained with both of the MAC message structures.
The multiple recipient MACed message uses two structures: the COSE_Mac structure defined in this section for carrying the body and the COSE_recipient structure (Section 5.1) to hold the key used for the MAC computation. Examples of MACed messages can be found in Appendix C.5.
The MAC structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Mac structure is identified by the CBOR tag 97. The CDDL fragment that represents this is:
COSE_Mac_Tagged = #6.97(COSE_Mac)
The COSE_Mac structure is a CBOR array. The fields of the array in order are:
payload: This field contains the serialized content to be MACed. If the payload is not present in the message, the application is required to supply the payload separately. The payload is wrapped in a bstr to ensure that it is transported without changes. If the payload is transported separately (i.e., detached content), then a nil CBOR value is placed in this location, and it is the responsibility of the application to ensure that it will be transported without changes.
In this section, we describe the structure and methods to be used when doing MAC authentication for those cases where the recipient is implicitly known.
The MACed message uses the COSE_Mac0 structure defined in this section for carrying the body. Examples of MACed messages with an implicit key can be found in Appendix C.6.
The MAC structure can be encoded as either tagged or untagged depending on the context it will be used in. A tagged COSE_Mac0 structure is identified by the CBOR tag 17. The CDDL fragment that represents this is:
COSE_Mac0_Tagged = #6.17(COSE_Mac0)
The COSE_Mac0 structure is a CBOR array. The fields of the array in order are:
In order to get a consistent encoding of the data to be authenticated, the MAC_structure is used to have a canonical form. The MAC_structure is a CBOR array. The fields of the MAC_structure in order are:
1. A text string that identifies the structure that is being encoded. This string is "MAC" for the COSE_Mac structure. This string is "MAC0" for the COSE_Mac0 structure.
2. The protected attributes from the COSE_MAC structure. If there are no protected attributes, a zero-length bstr is used.
3. The protected attributes from the application encoded as a bstr type. If this field is not supplied, it defaults to a zero- length binary string. (See Section 4.3 for application guidance on constructing this field.)
4. The payload to be MACed encoded in a bstr type. The payload is placed here independent of how it is transported.
The CDDL fragment that corresponds to the above text is:
A COSE Key structure is built on a CBOR map object. The set of common parameters that can appear in a COSE Key can be found in the IANA "COSE Key Common Parameters" registry (Section 16.5). Additional parameters defined for specific key types can be found in the IANA "COSE Key Type Parameters" registry (Section 16.6).
A COSE Key Set uses a CBOR array object as its underlying type. The values of the array elements are COSE Keys. A COSE Key Set MUST have at least one element in the array. Examples of COSE Key Sets can be found in Appendix C.7.
Each element in a COSE Key Set MUST be processed independently. If one element in a COSE Key Set is either malformed or uses a key that is not understood by an application, that key is ignored and the other keys are processed normally.
The element "kty" is a required element in a COSE_Key map.
Schaad Standards Track [Page 33]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The CDDL grammar describing COSE_Key and COSE_KeySet is:
This document defines a set of common parameters for a COSE Key object. Table 3 provides a summary of the parameters defined in this section. There are also parameters that are defined for specific key types. Key-type-specific parameters can be found in Section 13.
Schaad Standards Track [Page 34]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
+---------+-------+----------------+------------+-------------------+ | Name | Label | CBOR Type | Value | Description | | | | | Registry | | +---------+-------+----------------+------------+-------------------+ | kty | 1 | tstr / int | COSE Key | Identification of | | | | | Common | the key type | | | | | Parameters | | | | | | | | | kid | 2 | bstr | | Key | | | | | | identification | | | | | | value -- match to | | | | | | kid in message | | | | | | | | alg | 3 | tstr / int | COSE | Key usage | | | | | Algorithms | restriction to | | | | | | this algorithm | | | | | | | | key_ops | 4 | [+ (tstr/int)] | | Restrict set of | | | | | | permissible | | | | | | operations | | | | | | | | Base IV | 5 | bstr | | Base IV to be | | | | | | xor-ed with | | | | | | Partial IVs | +---------+-------+----------------+------------+-------------------+
Table 3: Key Map Labels
kty: This parameter is used to identify the family of keys for this structure and, thus, the set of key-type-specific parameters to be found. The set of values defined in this document can be found in Table 21. This parameter MUST be present in a key object. Implementations MUST verify that the key type is appropriate for the algorithm being processed. The key type MUST be included as part of the trust decision process.
alg: This parameter is used to restrict the algorithm that is used with the key. If this parameter is present in the key structure, the application MUST verify that this algorithm matches the algorithm for which the key is being used. If the algorithms do not match, then this key object MUST NOT be used to perform the cryptographic operation. Note that the same key can be in a different key structure with a different or no algorithm specified; however, this is considered to be a poor security practice.
Schaad Standards Track [Page 35]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
kid: This parameter is used to give an identifier for a key. The identifier is not structured and can be anything from a user- provided string to a value computed on the public portion of the key. This field is intended for matching against a 'kid' parameter in a message in order to filter down the set of keys that need to be checked.
key_ops: This parameter is defined to restrict the set of operations that a key is to be used for. The value of the field is an array of values from Table 4. Algorithms define the values of key ops that are permitted to appear and are required for specific operations. The set of values matches that in [RFC7517] and [W3C.WebCrypto].
Base IV: This parameter is defined to carry the base portion of an IV. It is designed to be used with the Partial IV header parameter defined in Section 3.1. This field provides the ability to associate a Partial IV with a key that is then modified on a per message basis with the Partial IV.
Extreme care needs to be taken when using a Base IV in an application. Many encryption algorithms lose security if the same IV is used twice.
If different keys are derived for each sender, using the same Base IV with Partial IVs starting at zero is likely to ensure that the IV would not be used twice for a single key. If different keys are derived for each sender, starting at the same Base IV is likely to satisfy this condition. If the same key is used for multiple senders, then the application needs to provide for a method of dividing the IV space up between the senders. This could be done by providing a different base point to start from or a different Partial IV to start with and restricting the number of messages to be sent before rekeying.
Schaad Standards Track [Page 36]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
+---------+-------+-------------------------------------------------+ | Name | Value | Description | +---------+-------+-------------------------------------------------+ | sign | 1 | The key is used to create signatures. Requires | | | | private key fields. | | verify | 2 | The key is used for verification of signatures. | | encrypt | 3 | The key is used for key transport encryption. | | decrypt | 4 | The key is used for key transport decryption. | | | | Requires private key fields. | | wrap | 5 | The key is used for key wrap encryption. | | key | | | | unwrap | 6 | The key is used for key wrap decryption. | | key | | Requires private key fields. | | derive | 7 | The key is used for deriving keys. Requires | | key | | private key fields. | | derive | 8 | The key is used for deriving bits not to be | | bits | | used as a key. Requires private key fields. | | MAC | 9 | The key is used for creating MACs. | | create | | | | MAC | 10 | The key is used for validating MACs. | | verify | | | +---------+-------+-------------------------------------------------+
There are two signature algorithm schemes. The first is signature with appendix. In this scheme, the message content is processed and a signature is produced; the signature is called the appendix. This is the scheme used by algorithms such as ECDSA and the RSA Probabilistic Signature Scheme (RSASSA-PSS). (In fact, the SSA in RSASSA-PSS stands for Signature Scheme with Appendix.)
The second scheme is signature with message recovery (an example of such an algorithm is [PVSig]). In this scheme, the message content is processed, but part of it is included in the signature. Moving bytes of the message content into the signature allows for smaller signatures; the signature size is still potentially large, but the message content has shrunk. This has implications for systems implementing these algorithms and for applications that use them. The first is that the message content is not fully available until
Schaad Standards Track [Page 37]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
after a signature has been validated. Until that point, the part of the message contained inside of the signature is unrecoverable. The second is that the security analysis of the strength of the signature is very much based on the structure of the message content. Messages that are highly predictable require additional randomness to be supplied as part of the signature process. In the worst case, it becomes the same as doing a signature with appendix. Finally, in the event that multiple signatures are applied to a message, all of the signature algorithms are going to be required to consume the same number of bytes of message content. This means that the mixing of the different schemes in a single message is not supported, and if a recovery signature scheme is used, then the same amount of content needs to be consumed by all of the signatures.
The signature functions for this scheme are:
signature, message sent = Sign(message content, key)
Signature algorithms are used with the COSE_Signature and COSE_Sign1 structures. At this time, only signatures with appendixes are defined for use with COSE; however, considerable interest has been expressed in using a signature with message recovery algorithm due to the effective size reduction that is possible. Implementations will need to keep this in mind for later possible integration.
ECDSA [DSS] defines a signature algorithm using ECC. Implementations SHOULD use a deterministic version of ECDSA such as the one defined in [RFC6979]. The use of a deterministic signature algorithm allows for systems to avoid relying on random number generators in order to avoid generating the same value of 'k' (the per-message random value). Biased generation of the value 'k' can be attacked, and collisions of this value leads to leaked keys. It additionally allows for doing deterministic tests for the signature algorithm. The use of deterministic ECDSA does not lessen the need to have good random number generation when creating the private key.
The ECDSA signature algorithm is parameterized with a hash function (h). In the event that the length of the hash function output is greater than the group of the key, the leftmost bytes of the hash output are used.
Schaad Standards Track [Page 38]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The algorithms defined in this document can be found in Table 5.
This document defines ECDSA to work only with the curves P-256, P-384, and P-521. This document requires that the curves be encoded using the 'EC2' (2 coordinate elliptic curve) key type. Implementations need to check that the key type and curve are correct when creating and verifying a signature. Other documents can define it to work with other curves and points in the future.
In order to promote interoperability, it is suggested that SHA-256 be used only with curve P-256, SHA-384 be used only with curve P-384, and SHA-512 be used with curve P-521. This is aligned with the recommendation in Section 4 of [RFC5480].
The signature algorithm results in a pair of integers (R, S). These integers will be the same length as the length of the key used for the signature process. The signature is encoded by converting the integers into byte strings of the same length as the key size. The length is rounded up to the nearest byte and is left padded with zero bits to get to the correct length. The two integers are then concatenated together to form a byte string that is the resulting signature.
Using the function defined in [RFC8017], the signature is: Signature = I2OSP(R, n) | I2OSP(S, n) where n = ceiling(key_length / 8)
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'EC2'.
o If the 'alg' field is present, it MUST match the ECDSA signature algorithm being used.
o If the 'key_ops' field is present, it MUST include 'sign' when creating an ECDSA signature.
Schaad Standards Track [Page 39]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o If the 'key_ops' field is present, it MUST include 'verify' when verifying an ECDSA signature.
The security strength of the signature is no greater than the minimum of the security strength associated with the bit length of the key and the security strength of the hash function.
Note: Use of this technique is a good idea even when good random number generation exists. Doing so both reduces the possibility of having the same value of 'k' in two signature operations and allows for reproducible signature values, which helps testing.
There are two substitution attacks that can theoretically be mounted against the ECDSA signature algorithm.
o Changing the curve used to validate the signature: If one changes the curve used to validate the signature, then potentially one could have two messages with the same signature, each computed under a different curve. The only requirement on the new curve is that its order be the same as the old one and it be acceptable to the client. An example would be to change from using the curve secp256r1 (aka P-256) to using secp256k1. (Both are 256-bit curves.) We currently do not have any way to deal with this version of the attack except to restrict the overall set of curves that can be used.
o Change the hash function used to validate the signature: If one either has two different hash functions of the same length or can truncate a hash function down, then one could potentially find collisions between the hash functions rather than within a single hash function (for example, truncating SHA-512 to 256 bits might collide with a SHA-256 bit hash value). As the hash algorithm is part of the signature algorithm identifier, this attack is mitigated by including a signature algorithm identifier in the protected header.
8.2. Edwards-Curve Digital Signature Algorithms (EdDSAs)
[RFC8032] describes the elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). In that document, the signature algorithm is instantiated using parameters for edwards25519 and edwards448 curves. The document additionally describes two variants of the EdDSA algorithm: Pure EdDSA, where no hash function is applied to the content before signing, and HashEdDSA, where a hash function is applied to the content before signing and the result of that hash function is signed. For EdDSA, the content to be signed (either the
Schaad Standards Track [Page 40]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
message or the pre-hash value) is processed twice inside of the signature algorithm. For use with COSE, only the pure EdDSA version is used. This is because it is not expected that extremely large contents are going to be needed and, based on the arrangement of the message structure, the entire message is going to need to be held in memory in order to create or verify a signature. This means that there does not appear to be a need to be able to do block updates of the hash, followed by eliminating the message from memory. Applications can provide the same features by defining the content of the message as a hash value and transporting the COSE object (with the hash value) and the content as separate items.
The algorithms defined in this document can be found in Table 6. A single signature algorithm is defined, which can be used for multiple curves.
+-------+-------+-------------+ | Name | Value | Description | +-------+-------+-------------+ | EdDSA | -8 | EdDSA | +-------+-------+-------------+
Table 6: EdDSA Algorithm Values
[RFC8032] describes the method of encoding the signature value.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'OKP' (Octet Key Pair).
o The 'crv' field MUST be present, and it MUST be a curve defined for this signature algorithm.
o If the 'alg' field is present, it MUST match 'EdDSA'.
o If the 'key_ops' field is present, it MUST include 'sign' when creating an EdDSA signature.
o If the 'key_ops' field is present, it MUST include 'verify' when verifying an EdDSA signature.
How public values are computed is not the same when looking at EdDSA and Elliptic Curve Diffie-Hellman (ECDH); for this reason, they should not be used with the other algorithm.
Schaad Standards Track [Page 41]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
If batch signature verification is performed, a well-seeded cryptographic random number generator is REQUIRED. Signing and non- batch signature verification are deterministic operations and do not need random numbers of any kind.
Message Authentication Codes (MACs) provide data authentication and integrity protection. They provide either no or very limited data origination. A MAC, for example, can be used to prove the identity of the sender to a third party.
MACs use the same scheme as signature with appendix algorithms. The message content is processed and an authentication code is produced. The authentication code is frequently called a tag.
The MAC functions are:
tag = MAC_Create(message content, key)
valid = MAC_Verify(message content, key, tag)
MAC algorithms can be based on either a block cipher algorithm (i.e., AES-MAC) or a hash algorithm (i.e., a Hash-based Message Authentication Code (HMAC)). This document defines a MAC algorithm using each of these constructions.
MAC algorithms are used in the COSE_Mac and COSE_Mac0 structures.
HMAC [RFC2104] [RFC4231] was designed to deal with length extension attacks. The algorithm was also designed to allow for new hash algorithms to be directly plugged in without changes to the hash function. The HMAC design process has been shown as solid since, while the security of hash algorithms such as MD5 has decreased over time; the security of HMAC combined with MD5 has not yet been shown to be compromised [RFC6151].
The HMAC algorithm is parameterized by an inner and outer padding, a hash function (h), and an authentication tag value length. For this specification, the inner and outer padding are fixed to the values set in [RFC2104]. The length of the authentication tag corresponds to the difficulty of producing a forgery. For use in constrained environments, we define a set of HMAC algorithms that are truncated.
Schaad Standards Track [Page 42]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
There are currently no known issues with truncation; however, the security strength of the message tag is correspondingly reduced in strength. When truncating, the leftmost tag length bits are kept and transmitted.
The algorithms defined in this document can be found in Table 7.
Some recipient algorithms carry the key while others derive a key from secret data. For those algorithms that carry the key (such as AES Key Wrap), the size of the HMAC key SHOULD be the same size as the underlying hash function. For those algorithms that derive the key (such as ECDH), the derived key MUST be the same size as the underlying hash function.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the HMAC algorithm being used.
o If the 'key_ops' field is present, it MUST include 'MAC create' when creating an HMAC authentication tag.
o If the 'key_ops' field is present, it MUST include 'MAC verify' when verifying an HMAC authentication tag.
Implementations creating and validating MAC values MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
Schaad Standards Track [Page 43]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
HMAC has proved to be resistant to attack even when used with weakened hash algorithms. The current best known attack is to brute force the key. This means that key size is going to be directly related to the security of an HMAC operation.
AES-CBC-MAC is defined in [MAC]. (Note that this is not the same algorithm as AES Cipher-Based Message Authentication Code (AES-CMAC) [RFC4493].)
AES-CBC-MAC is parameterized by the key length, the authentication tag length, and the IV used. For all of these algorithms, the IV is fixed to all zeros. We provide an array of algorithms for various key lengths and tag lengths. The algorithms defined in this document are found in Table 8.
Keys may be obtained either from a key structure or from a recipient structure. Implementations creating and validating MAC values MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-MAC algorithm being used.
Schaad Standards Track [Page 44]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o If the 'key_ops' field is present, it MUST include 'MAC create' when creating an AES-MAC authentication tag.
o If the 'key_ops' field is present, it MUST include 'MAC verify' when verifying an AES-MAC authentication tag.
A number of attacks exist against Cipher Block Chaining Message Authentication Code (CBC-MAC) that need to be considered.
o A single key must only be used for messages of a fixed and known length. If this is not the case, an attacker will be able to generate a message with a valid tag given two message and tag pairs. This can be addressed by using different keys for messages of different lengths. The current structure mitigates this problem, as a specific encoding structure that includes lengths is built and signed. (CMAC also addresses this issue.)
o Cipher Block Chaining (CBC) mode, if the same key is used for both encryption and authentication operations, an attacker can produce messages with a valid authentication code.
o If the IV can be modified, then messages can be forged. This is addressed by fixing the IV to all zeros.
Content encryption algorithms provide data confidentiality for potentially large blocks of data using a symmetric key. They provide integrity on the data that was encrypted; however, they provide either no or very limited data origination. (One cannot, for example, be used to prove the identity of the sender to a third party.) The ability to provide data origination is linked to how the CEK is obtained.
COSE restricts the set of legal content encryption algorithms to those that support authentication both of the content and additional data. The encryption process will generate some type of authentication value, but that value may be either explicit or implicit in terms of the algorithm definition. For simplicity's sake, the authentication code will normally be defined as being appended to the ciphertext stream. The encryption functions are:
The Galois/Counter Mode (GCM) mode is a generic authenticated encryption block cipher mode defined in [AES-GCM]. The GCM mode is combined with the AES block encryption algorithm to define an AEAD cipher.
The GCM mode is parameterized by the size of the authentication tag and the size of the nonce. This document fixes the size of the nonce at 96 bits. The size of the authentication tag is limited to a small set of values. For this document however, the size of the authentication tag is fixed at 128 bits.
The set of algorithms defined in this document are in Table 9.
+---------+-------+------------------------------------------+ | Name | Value | Description | +---------+-------+------------------------------------------+ | A128GCM | 1 | AES-GCM mode w/ 128-bit key, 128-bit tag | | A192GCM | 2 | AES-GCM mode w/ 192-bit key, 128-bit tag | | A256GCM | 3 | AES-GCM mode w/ 256-bit key, 128-bit tag | +---------+-------+------------------------------------------+
Table 9: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-GCM algorithm being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting.
Schaad Standards Track [Page 46]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting.
When using AES-GCM, the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted.
o The total amount of data encrypted for a single key MUST NOT exceed 2^39 - 256 bits. An explicit check is required only in environments where it is expected that it might be exceeded.
Consideration was given to supporting smaller tag values; the constrained community would desire tag sizes in the 64-bit range. Doing so drastically changes both the maximum messages size (generally not an issue) and the number of times that a key can be used. Given that Counter with CBC-MAC (CCM) is the usual mode for constrained environments, restricted modes are not supported.
CCM is a generic authentication encryption block cipher mode defined in [RFC3610]. The CCM mode is combined with the AES block encryption algorithm to define a commonly used content encryption algorithm used in constrained devices.
The CCM mode has two parameter choices. The first choice is M, the size of the authentication field. The choice of the value for M involves a trade-off between message growth (from the tag) and the probability that an attacker can undetectably modify a message. The second choice is L, the size of the length field. This value requires a trade-off between the maximum message size and the size of the Nonce.
It is unfortunate that the specification for CCM specified L and M as a count of bytes rather than a count of bits. This leads to possible misunderstandings where AES-CCM-8 is frequently used to refer to a version of CCM mode where the size of the authentication is 64 bits and not 8 bits. These values have traditionally been specified as bit counts rather than byte counts. This document will follow the convention of using bit counts so that it is easier to compare the different algorithms presented in this document.
We define a matrix of algorithms in this document over the values of L and M. Constrained devices are usually operating in situations where they use short messages and want to avoid doing recipient- specific cryptographic operations. This favors smaller values of
Schaad Standards Track [Page 47]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
both L and M. Less-constrained devices will want to be able to use larger messages and are more willing to generate new keys for every operation. This favors larger values of L and M.
The following values are used for L:
16 bits (2): This limits messages to 2^16 bytes (64 KiB) in length. This is sufficiently long for messages in the constrained world. The nonce length is 13 bytes allowing for 2^(13*8) possible values of the nonce without repeating.
64 bits (8): This limits messages to 2^64 bytes in length. The nonce length is 7 bytes allowing for 2^56 possible values of the nonce without repeating.
The following values are used for M:
64 bits (8): This produces a 64-bit authentication tag. This implies that there is a 1 in 2^64 chance that a modified message will authenticate.
128 bits (16): This produces a 128-bit authentication tag. This implies that there is a 1 in 2^128 chance that a modified message will authenticate.
Schaad Standards Track [Page 48]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES-CCM algorithm being used.
Schaad Standards Track [Page 49]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting.
When using AES-CCM, the following restrictions MUST be enforced:
o The key and nonce pair MUST be unique for every message encrypted. Note that the value of L influences the number of unique nonces.
o The total number of times the AES block cipher is used MUST NOT exceed 2^61 operations. This limitation is the sum of times the block cipher is used in computing the MAC value and in performing stream encryption operations. An explicit check is required only in environments where it is expected that it might be exceeded.
[RFC3610] additionally calls out one other consideration of note. It is possible to do a pre-computation attack against the algorithm in cases where portions of the plaintext are highly predictable. This reduces the security of the key size by half. Ways to deal with this attack include adding a random portion to the nonce value and/or increasing the key size used. Using a portion of the nonce for a random value will decrease the number of messages that a single key can be used for. Increasing the key size may require more resources in the constrained device. See Sections 5 and 10 of [RFC3610] for more information.
ChaCha20 and Poly1305 combined together is an AEAD mode that is defined in [RFC7539]. This is an algorithm defined to be a cipher that is not AES and thus would not suffer from any future weaknesses found in AES. These cryptographic functions are designed to be fast in software-only implementations.
The ChaCha20/Poly1305 AEAD construction defined in [RFC7539] has no parameterization. It takes a 256-bit key and a 96-bit nonce, as well as the plaintext and additional data as inputs and produces the ciphertext as an option. We define one algorithm identifier for this algorithm in Table 11.
Schaad Standards Track [Page 50]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
+-------------------+-------+---------------------------------------+ | Name | Value | Description | +-------------------+-------+---------------------------------------+ | ChaCha20/Poly1305 | 24 | ChaCha20/Poly1305 w/ 256-bit key, | | | | 128-bit tag | +-------------------+-------+---------------------------------------+
Table 11: Algorithm Value for AES-GCM
Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the ChaCha20/Poly1305 algorithm being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting.
The key and nonce values MUST be a unique pair for every invocation of the algorithm. Nonce counters are considered to be an acceptable way of ensuring that they are unique.
KDFs are used to take some secret value and generate a different one. The secret value comes in three flavors:
o Secrets that are uniformly random: This is the type of secret that is created by a good random number generator.
o Secrets that are not uniformly random: This is type of secret that is created by operations like key agreement.
o Secrets that are not random: This is the type of secret that people generate for things like passwords.
Schaad Standards Track [Page 51]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
General KDFs work well with the first type of secret, can do reasonably well with the second type of secret, and generally do poorly with the last type of secret. None of the KDFs in this section are designed to deal with the type of secrets that are used for passwords. Functions like PBES2 [RFC8018] need to be used for that type of secret.
The same KDF can be set up to deal with the first two types of secrets in a different way. The KDF defined in Section 11.1 is such a function. This is reflected in the set of algorithms defined for the HMAC-based Extract-and-Expand Key Derivation Function (HKDF).
When using KDFs, one component that is included is context information. Context information is used to allow for different keying information to be derived from the same secret. The use of context-based keying material is considered to be a good security practice.
This document defines a single context structure and a single KDF. These elements are used for all of the recipient algorithms defined in this document that require a KDF process. These algorithms are defined in Sections 12.1.2, 12.4.1, and 12.5.1.
11.1. HMAC-Based Extract-and-Expand Key Derivation Function (HKDF)
The HKDF key derivation algorithm is defined in [RFC5869].
The HKDF algorithm takes these inputs:
secret -- a shared value that is secret. Secrets may be either previously shared or derived from operations like a Diffie-Hellman (DH) key agreement.
salt -- an optional value that is used to change the generation process. The salt value can be either public or private. If the salt is public and carried in the message, then the 'salt' algorithm header parameter defined in Table 13 is used. While [RFC5869] suggests that the length of the salt be the same as the length of the underlying hash value, any amount of salt will improve the security as different key values will be generated. This parameter is protected by being included in the key computation and does not need to be separately authenticated. The salt value does not need to be unique for every message sent.
length -- the number of bytes of output that need to be generated.
Schaad Standards Track [Page 52]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
context information -- Information that describes the context in which the resulting value will be used. Making this information specific to the context in which the material is going to be used ensures that the resulting material will always be tied to that usage. The context structure defined in Section 11.2 is used by the KDFs in this document.
PRF -- The underlying pseudorandom function to be used in the HKDF algorithm. The PRF is encoded into the HKDF algorithm selection.
HKDF is defined to use HMAC as the underlying PRF. However, it is possible to use other functions in the same construct to provide a different KDF that is more appropriate in the constrained world. Specifically, one can use AES-CBC-MAC as the PRF for the expand step, but not for the extract step. When using a good random shared secret of the correct length, the extract step can be skipped. For the AES algorithm versions, the extract step is always skipped.
The extract step cannot be skipped if the secret is not uniformly random, for example, if it is the result of an ECDH key agreement step. This implies that the AES HKDF version cannot be used with ECDH. If the extract step is skipped, the 'salt' value is not used as part of the HKDF functionality.
The algorithms defined in this document are found in Table 12.
+---------------+-----------------+---------------------------------+ | Name | PRF | Description | +---------------+-----------------+---------------------------------+ | HKDF SHA-256 | HMAC with | HKDF using HMAC SHA-256 as the | | | SHA-256 | PRF | | HKDF SHA-512 | HMAC with | HKDF using HMAC SHA-512 as the | | | SHA-512 | PRF | | HKDF AES- | AES-CBC-MAC-128 | HKDF using AES-MAC as the PRF | | MAC-128 | | w/ 128-bit key | | HKDF AES- | AES-CBC-MAC-256 | HKDF using AES-MAC as the PRF | | MAC-256 | | w/ 256-bit key | +---------------+-----------------+---------------------------------+
Table 12: HKDF Algorithms
Schaad Standards Track [Page 53]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The context information structure is used to ensure that the derived keying material is "bound" to the context of the transaction. The context information structure used here is based on that defined in [SP800-56A]. By using CBOR for the encoding of the context information structure, we automatically get the same type and length separation of fields that is obtained by the use of ASN.1. This means that there is no need to encode the lengths for the base elements, as it is done by the encoding used in JOSE (Section 4.6.2 of [RFC7518]).
The context information structure refers to PartyU and PartyV as the two parties that are doing the key derivation. Unless the application protocol defines differently, we assign PartyU to the entity that is creating the message and PartyV to the entity that is receiving the message. By doing this association, different keys will be derived for each direction as the context information is different in each direction.
The context structure is built from information that is known to both entities. This information can be obtained from a variety of sources:
o Fields can be defined by the application. This is commonly used to assign fixed names to parties, but it can be used for other items such as nonces.
o Fields can be defined by usage of the output. Examples of this are the algorithm and key size that are being generated.
Schaad Standards Track [Page 54]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Fields can be defined by parameters from the message. We define a set of parameters in Table 14 that can be used to carry the values associated with the context structure. Examples of this are identities and nonce values. These parameters are designed to be placed in the unprotected bucket of the recipient structure; they do not need to be in the protected bucket since they already are included in the cryptographic computation by virtue of being included in the context structure.
Schaad Standards Track [Page 55]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
We define a CBOR object to hold the context information. This object is referred to as COSE_KDF_Context. The object is based on a CBOR array type. The fields in the array are:
AlgorithmID: This field indicates the algorithm for which the key material will be used. This normally is either a key wrap algorithm identifier or a content encryption algorithm identifier. The values are from the "COSE Algorithms" registry. This field is required to be present. The field exists in the context information so that if the same environment is used for different algorithms, then completely different keys will be generated for each of those algorithms. This practice means if algorithm A is broken and thus is easier to find, the key derived for algorithm B will not be the same as the key derived for algorithm A.
PartyUInfo: This field holds information about party U. The PartyUInfo is encoded as a CBOR array. The elements of PartyUInfo are encoded in the order presented. The elements of the PartyUInfo array are:
identity: This contains the identity information for party U. The identities can be assigned in one of two manners. First, a protocol can assign identities based on roles. For example, the roles of "client" and "server" may be assigned to different entities in the protocol. Each entity would then use the correct label for the data they send or receive. The second way for a protocol to assign identities is to use a name based on a naming system (i.e., DNS, X.509 names).
We define an algorithm parameter 'PartyU identity' that can be used to carry identity information in the message. However, identity information is often known as part of the protocol and can thus be inferred rather than made explicit. If identity information is carried in the message, applications SHOULD have a way of validating the supplied identity information. The identity information does not need to be specified and is set to nil in that case.
nonce: This contains a nonce value. The nonce can either be implicit from the protocol or be carried as a value in the unprotected headers.
We define an algorithm parameter 'PartyU nonce' that can be used to carry this value in the message; however, the nonce value could be determined by the application and the value determined from elsewhere.
Schaad Standards Track [Page 58]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
This option does not need to be specified and is set to nil in that case.
other: This contains other information that is defined by the protocol. This option does not need to be specified and is set to nil in that case.
PartyVInfo: This field holds information about party V. The content of the structure is the same as for the PartyUInfo but for party V.
SuppPubInfo: This field contains public information that is mutually known to both parties.
keyDataLength: This is set to the number of bits of the desired output value. This practice means if algorithm A can use two different key lengths, the key derived for longer key size will not contain the key for shorter key size as a prefix.
protected: This field contains the protected parameter field. If there are no elements in the protected field, then use a zero- length bstr.
other: This field is for free form data defined by the application. An example is that an application could define two different strings to be placed here to generate different keys for a data stream versus a control stream. This field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly included.
SuppPrivInfo: This field contains private information that is mutually known private information. An example of this information would be a preexisting shared secret. (This could, for example, be used in combination with an ECDH key agreement to provide a secondary proof of identity.) The field is optional and will only be present if the application defines a structure for this information. Applications that define this SHOULD use CBOR to encode the data so that types and lengths are correctly included.
Schaad Standards Track [Page 59]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The following CDDL fragment corresponds to the text above.
Content key distribution methods (recipient algorithms) can be defined into a number of different classes. COSE has the ability to support many classes of recipient algorithms. In this section, a number of classes are listed, and then a set of algorithms are specified for each of the classes. The names of the recipient algorithm classes used here are the same as those defined in [RFC7516]. Other specifications use different terms for the recipient algorithm classes or do not support some of the recipient algorithm classes.
The direct encryption class algorithms share a secret between the sender and the recipient that is used either directly or after manipulation as the CEK. When direct encryption mode is used, it MUST be the only mode used on the message.
The COSE_Recipient structure for the recipient is organized as follows:
o The 'protected' field MUST be a zero-length item unless it is used in the computation of the content key.
o The 'alg' parameter MUST be present.
o A parameter identifying the shared secret SHOULD be present.
Schaad Standards Track [Page 60]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o The 'ciphertext' field MUST be a zero-length item.
This recipient algorithm is the simplest; the identified key is directly used as the key for the next layer down in the message. There are no algorithm parameters defined for this algorithm. The algorithm identifier value is assigned in Table 15.
When this algorithm is used, the protected field MUST be zero length. The key type MUST be 'Symmetric'.
+--------+-------+-------------------+ | Name | Value | Description | +--------+-------+-------------------+ | direct | -6 | Direct use of CEK | +--------+-------+-------------------+
This recipient algorithm has several potential problems that need to be considered:
o These keys need to have some method to be regularly updated over time. All of the content encryption algorithms specified in this document have limits on how many times a key can be used without significant loss of security.
o These keys need to be dedicated to a single algorithm. There have been a number of attacks developed over time when a single key is used for multiple different algorithms. One example of this is the use of a single key for both the CBC encryption mode and the CBC-MAC authentication mode.
o Breaking one message means all messages are broken. If an adversary succeeds in determining the key for a single message, then the key for all messages is also determined.
These recipient algorithms take a common shared secret between the two parties and applies the HKDF function (Section 11.1), using the context structure defined in Section 11.2 to transform the shared
Schaad Standards Track [Page 61]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
secret into the CEK. The 'protected' field can be of non-zero length. Either the 'salt' parameter of HKDF or the 'PartyU nonce' parameter of the context structure MUST be present. The salt/nonce parameter can be generated either randomly or deterministically. The requirement is that it be a unique value for the shared secret in question.
If the salt/nonce value is generated randomly, then it is suggested that the length of the random value be the same length as the hash function underlying HKDF. While there is no way to guarantee that it will be unique, there is a high probability that it will be unique. If the salt/nonce value is generated deterministically, it can be guaranteed to be unique, and thus there is no length requirement.
A new IV must be used for each message if the same key is used. The IV can be modified in a predictable manner, a random manner, or an unpredictable manner (i.e., encrypting a counter).
The IV used for a key can also be generated from the same HKDF functionality as the key is generated. If HKDF is used for generating the IV, the algorithm identifier is set to "IV- GENERATION".
When these algorithms are used, the key type MUST be 'symmetric'.
The set of algorithms defined in this document can be found in Table 16.
The shared secret needs to have some method to be regularly updated over time. The shared secret forms the basis of trust. Although not used directly, it should still be subject to scheduled rotation.
While these methods do not provide for perfect forward secrecy, as the same shared secret is used for all of the keys generated, if the key for any single message is discovered, only the message (or series of messages) using that derived key are compromised. A new key derivation step will generate a new key that requires the same amount of work to get the key.
In key wrap mode, the CEK is randomly generated and that key is then encrypted by a shared secret between the sender and the recipient. All of the currently defined key wrap algorithms for COSE are AE algorithms. Key wrap mode is considered to be superior to direct encryption if the system has any capability for doing random key generation. This is because the shared key is used to wrap random data rather than data that has some degree of organization and may in fact be repeating the same content. The use of key wrap loses the weak data origination that is provided by the direct encryption algorithms.
The COSE_Encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be absent if the key wrap algorithm is an AE algorithm.
o The 'recipients' field is normally absent, but can be used. Applications MUST deal with a recipient field being present, not being able to decrypt that recipient is an acceptable way of dealing with it. Failing to process the message is not an acceptable way of dealing with it.
Schaad Standards Track [Page 63]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o The plaintext to be encrypted is the key from next layer down (usually the content layer).
o At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the shared secret.
The AES Key Wrap algorithm is defined in [RFC3394]. This algorithm uses an AES key to wrap a value that is a multiple of 64 bits. As such, it can be used to wrap a key for any of the content encryption algorithms defined in this document. The algorithm requires a single fixed parameter, the initial value. This is fixed to the value specified in Section 2.2.3.1 of [RFC3394]. There are no public parameters that vary on a per-invocation basis. The protected header field MUST be empty.
Keys may be obtained either from a key structure or from a recipient structure. Implementations encrypting and decrypting MUST validate that the key type, key length, and algorithm are correct and appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'Symmetric'.
o If the 'alg' field is present, it MUST match the AES Key Wrap algorithm being used.
o If the 'key_ops' field is present, it MUST include 'encrypt' or 'wrap key' when encrypting.
o If the 'key_ops' field is present, it MUST include 'decrypt' or 'unwrap key' when decrypting.
Key transport mode is also called key encryption mode in some standards. Key transport mode differs from key wrap mode in that it uses an asymmetric encryption algorithm rather than a symmetric encryption algorithm to protect the key. This document does not define any key transport mode algorithms.
When using a key transport algorithm, the COSE_Encrypt structure for the recipient is organized as follows:
o The 'protected' field MUST be absent.
o The plaintext to be encrypted is the key from the next layer down (usually the content layer).
o At a minimum, the 'unprotected' field MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the asymmetric key.
The 'direct key agreement' class of recipient algorithms uses a key agreement method to create a shared secret. A KDF is then applied to the shared secret to derive a key to be used in protecting the data. This key is normally used as a CEK or MAC key, but could be used for other purposes if more than two layers are in use (see Appendix B).
The most commonly used key agreement algorithm is Diffie-Hellman, but other variants exist. Since COSE is designed for a store and forward environment rather than an online environment, many of the DH variants cannot be used as the receiver of the message cannot provide any dynamic key material. One side effect of this is that perfect forward secrecy (see [RFC4949]) is not achievable. A static key will always be used for the receiver of the COSE object.
Two variants of DH that are supported are:
Ephemeral-Static (ES) DH: where the sender of the message creates a one-time DH key and uses a static key for the recipient. The use of the ephemeral sender key means that no additional random input is needed as this is randomly generated for each message.
Schaad Standards Track [Page 65]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Static-Static DH: where a static key is used for both the sender and the recipient. The use of static keys allows for the recipient to get a weak version of data origination for the message. When static-static key agreement is used, then some piece of unique data for the KDF is required to ensure that a different key is created for each message.
When direct key agreement mode is used, there MUST be only one recipient in the message. This method creates the key directly, and that makes it difficult to mix with additional recipients. If multiple recipients are needed, then the version with key wrap needs to be used.
The COSE_Encrypt structure for the recipient is organized as follows:
o At a minimum, headers MUST contain the 'alg' parameter and SHOULD contain a parameter identifying the recipient's asymmetric key.
o The headers SHOULD identify the sender's key for the static-static versions and MUST contain the sender's ephemeral key for the ephemeral-static versions.
The mathematics for ECDH can be found in [RFC6090]. In this document, the algorithm is extended to be used with the two curves defined in [RFC7748].
ECDH is parameterized by the following:
o Curve Type/Curve: The curve selected controls not only the size of the shared secret, but the mathematics for computing the shared secret. The curve selected also controls how a point in the curve is represented and what happens for the identity points on the curve. In this specification, we allow for a number of different curves to be used. A set of curves are defined in Table 22. The math used to obtain the computed secret is based on the curve selected and not on the ECDH algorithm. For this reason, a new algorithm does not need to be defined for each of the curves.
o Computed Secret to Shared Secret: Once the computed secret is known, the resulting value needs to be converted to a byte string to run the KDF. The x-coordinate is used for all of the curves defined in this document. For curves X25519 and X448, the resulting value is used directly as it is a byte string of a known length. For the P-256, P-384, and P-521 curves, the x-coordinate is run through the I2OSP function defined in [RFC8017], using the same computation for n as is defined in Section 8.1.
Schaad Standards Track [Page 66]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Ephemeral-Static or Static-Static: The key agreement process may be done using either a static or an ephemeral key for the sender's side. When using ephemeral keys, the sender MUST generate a new ephemeral key for every key agreement operation. The ephemeral key is placed in the 'ephemeral key' parameter and MUST be present for all algorithm identifiers that use ephemeral keys. When using static keys, the sender MUST either generate a new random value or create a unique value. For the KDFs used, this means either the 'salt' parameter for HKDF (Table 13) or the 'PartyU nonce' parameter for the context structure (Table 14) MUST be present (both can be present if desired). The value in the parameter MUST be unique for the pair of keys being used. It is acceptable to use a global counter that is incremented for every static-static operation and use the resulting value. When using static keys, the static key should be identified to the recipient. The static key can be identified either by providing the key ('static key') or by providing a key identifier for the static key ('static key id'). Both of these parameters are defined in Table 19.
o Key Derivation Algorithm: The result of an ECDH key agreement process does not provide a uniformly random secret. As such, it needs to be run through a KDF in order to produce a usable key. Processing the secret through a KDF also allows for the introduction of context material: how the key is going to be used and one-time material for static-static key agreement. All of the algorithms defined in this document use one of the HKDF algorithms defined in Section 11.1 with the context structure defined in Section 11.2.
o Key Wrap Algorithm: No key wrap algorithm is used. This is represented in Table 18 as 'none'. The key size for the context structure is the content layer encryption algorithm size.
The set of direct ECDH algorithms defined in this document are found in Table 18.
Schaad Standards Track [Page 67]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
This document defines these algorithms to be used with the curves P-256, P-384, P-521, X25519, and X448. Implementations MUST verify that the key type and curve are correct. Different curves are restricted to different key types. Implementations MUST verify that the curve and algorithm are appropriate for the entities involved.
When using a COSE key for this algorithm, the following checks are made:
o The 'kty' field MUST be present, and it MUST be 'EC2' or 'OKP'.
o If the 'alg' field is present, it MUST match the key agreement algorithm being used.
o If the 'key_ops' field is present, it MUST include 'derive key' or 'derive bits' for the private key.
o If the 'key_ops' field is present, it MUST be empty for the public key.
There is a method of checking that points provided from external entities are valid. For the 'EC2' key format, this can be done by checking that the x and y values form a point on the curve. For the 'OKP' format, there is no simple way to do point validation.
Consideration was given to requiring that the public keys of both entities be provided as part of the key derivation process (as recommended in Section 6.1 of [RFC7748]). This was not done as COSE is used in a store and forward format rather than in online key exchange. In order for this to be a problem, either the receiver public key has to be chosen maliciously or the sender has to be malicious. In either case, all security evaporates anyway.
A proof of possession of the private key associated with the public key is recommended when a key is moved from untrusted to trusted (either by the end user or by the entity that is responsible for making trust statements on keys).
Key Agreement with Key Wrap uses a randomly generated CEK. The CEK is then encrypted using a key wrap algorithm and a key derived from the shared secret computed by the key agreement algorithm. The function for this would be:
ECDH with Key Agreement is parameterized by the same parameters as for ECDH; see Section 12.4.1, with the following modifications:
o Key Wrap Algorithm: Any of the key wrap algorithms defined in Section 12.2.1 are supported. The size of the key used for the key wrap algorithm is fed into the KDF. The set of identifiers are found in Table 20.
Schaad Standards Track [Page 70]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The COSE_Key object defines a way to hold a single key object. It is still required that the members of individual key types be defined. This section of the document is where we define an initial set of members for specific key types.
For each of the key types, we define both public and private members. The public members are what is transmitted to others for their usage. Private members allow for the archival of keys by individuals. However, there are some circumstances in which private keys may be distributed to entities in a protocol. Examples include: entities that have poor random number generation, centralized key creation for multi-cast type operations, and protocols in which a shared secret is used as a bearer token for authorization purposes.
Key types are identified by the 'kty' member of the COSE_Key object. In this document, we define four values for the member:
+-----------+-------+-----------------------------------------------+ | Name | Value | Description | +-----------+-------+-----------------------------------------------+ | OKP | 1 | Octet Key Pair | | EC2 | 2 | Elliptic Curve Keys w/ x- and y-coordinate | | | | pair | | Symmetric | 4 | Symmetric Keys | | Reserved | 0 | This value is reserved | +-----------+-------+-----------------------------------------------+
Table 21: Key Type Values
Schaad Standards Track [Page 72]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Two different key structures are defined for elliptic curve keys. One version uses both an x-coordinate and a y-coordinate, potentially with point compression ('EC2'). This is the traditional EC point representation that is used in [RFC5480]. The other version uses only the x-coordinate as the y-coordinate is either to be recomputed or not needed for the key agreement operation ('OKP').
Applications MUST check that the curve and the key type are consistent and reject a key if they are not.
+---------+-------+----------+------------------------------------+ | Name | Value | Key Type | Description | +---------+-------+----------+------------------------------------+ | P-256 | 1 | EC2 | NIST P-256 also known as secp256r1 | | P-384 | 2 | EC2 | NIST P-384 also known as secp384r1 | | P-521 | 3 | EC2 | NIST P-521 also known as secp521r1 | | X25519 | 4 | OKP | X25519 for use w/ ECDH only | | X448 | 5 | OKP | X448 for use w/ ECDH only | | Ed25519 | 6 | OKP | Ed25519 for use w/ EdDSA only | | Ed448 | 7 | OKP | Ed448 for use w/ EdDSA only | +---------+-------+----------+------------------------------------+
The traditional way of sending ECs has been to send either both the x-coordinate and y-coordinate or the x-coordinate and a sign bit for the y-coordinate. The latter encoding has not been recommended in the IETF due to potential IPR issues. However, for operations in constrained environments, the ability to shrink a message by not sending the y-coordinate is potentially useful.
For EC keys with both coordinates, the 'kty' member is set to 2 (EC2). The key parameters defined in this section are summarized in Table 23. The members that are defined for this key type are:
crv: This contains an identifier of the curve to be used with the key. The curves defined in this document for this key type can be found in Table 22. Other curves may be registered in the future, and private curves can be used as well.
x: This contains the x-coordinate for the EC point. The integer is converted to an octet string as defined in [SEC1]. Leading zero octets MUST be preserved.
Schaad Standards Track [Page 73]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
y: This contains either the sign bit or the value of the y-coordinate for the EC point. When encoding the value y, the integer is converted to an octet string (as defined in [SEC1]) and encoded as a CBOR bstr. Leading zero octets MUST be preserved. The compressed point encoding is also supported. Compute the sign bit as laid out in the Elliptic-Curve-Point-to- Octet-String Conversion function of [SEC1]. If the sign bit is zero, then encode y as a CBOR false value; otherwise, encode y as a CBOR true value. The encoding of the infinity point is not supported.
d: This contains the private key.
For public keys, it is REQUIRED that 'crv', 'x', and 'y' be present in the structure. For private keys, it is REQUIRED that 'crv' and 'd' be present in the structure. For private keys, it is RECOMMENDED that 'x' and 'y' also be present, but they can be recomputed from the required elements and omitting them saves on space.
+-------+------+-------+--------+-----------------------------------+ | Key | Name | Label | CBOR | Description | | Type | | | Type | | +-------+------+-------+--------+-----------------------------------+ | 2 | crv | -1 | int / | EC identifier - Taken from the | | | | | tstr | "COSE Elliptic Curves" registry | | 2 | x | -2 | bstr | x-coordinate | | 2 | y | -3 | bstr / | y-coordinate | | | | | bool | | | 2 | d | -4 | bstr | Private key | +-------+------+-------+--------+-----------------------------------+
A new key type is defined for Octet Key Pairs (OKP). Do not assume that keys using this type are elliptic curves. This key type could be used for other curve types (for example, mathematics based on hyper-elliptic surfaces).
The key parameters defined in this section are summarized in Table 24. The members that are defined for this key type are:
crv: This contains an identifier of the curve to be used with the key. The curves defined in this document for this key type can be found in Table 22. Other curves may be registered in the future and private curves can be used as well.
Schaad Standards Track [Page 74]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
x: This contains the x-coordinate for the EC point. The octet string represents a little-endian encoding of x.
d: This contains the private key.
For public keys, it is REQUIRED that 'crv' and 'x' be present in the structure. For private keys, it is REQUIRED that 'crv' and 'd' be present in the structure. For private keys, it is RECOMMENDED that 'x' also be present, but it can be recomputed from the required elements and omitting it saves on space.
+------+-------+-------+--------+-----------------------------------+ | Name | Key | Label | Type | Description | | | Type | | | | +------+-------+-------+--------+-----------------------------------+ | crv | 1 | -1 | int / | EC identifier - Taken from the | | | | | tstr | "COSE Key Common Parameters" | | | | | | registry | | x | 1 | -2 | bstr | x-coordinate | | d | 1 | -4 | bstr | Private key | +------+-------+-------+--------+-----------------------------------+
Occasionally it is required that a symmetric key be transported between entities. This key structure allows for that to happen.
For symmetric keys, the 'kty' member is set to 4 ('Symmetric'). The member that is defined for this key type is:
k: This contains the value of the key.
This key structure does not have a form that contains only public members. As it is expected that this key structure is going to be transmitted, care must be taken that it is never transmitted accidentally or insecurely. For symmetric keys, it is REQUIRED that 'k' be present in the structure.
+------+----------+-------+------+-------------+ | Name | Key Type | Label | Type | Description | +------+----------+-------+------+-------------+ | k | 4 | -1 | bstr | Key Value | +------+----------+-------+------+-------------+
Table 25: Symmetric Key Parameters
Schaad Standards Track [Page 75]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
There has been an attempt to limit the number of places where the document needs to impose restrictions on how the CBOR Encoder needs to work. We have managed to narrow it down to the following restrictions:
o The restriction applies to the encoding of the Sig_structure, the Enc_structure, and the MAC_structure.
o The rules for "Canonical CBOR" (Section 3.9 of RFC 7049) MUST be used in these locations. The main rule that needs to be enforced is that all lengths in these structures MUST be encoded such that they are using definite lengths, and the minimum length encoding is used.
o Applications MUST NOT generate messages with the same label used twice as a key in a single map. Applications MUST NOT parse and process messages with the same label used twice as a key in a single map. Applications can enforce the parse and process requirement by using parsers that will fail the parse step or by using parsers that will pass all keys to the application, and the application can perform the check for duplicate keys.
This document is designed to provide a set of security services, but not implementation requirements for specific usage. The interoperability requirements are provided for how each of the individual services are used and how the algorithms are to be used for interoperability. The requirements about which algorithms and which services are needed are deferred to each application.
An example of a profile can be found in [OSCOAP] where two profiles are being developed. One is for carrying content by itself, and the other is for carrying content in combination with CoAP headers.
It is intended that a profile of this document be created that defines the interoperability requirements for that specific application. This section provides a set of guidelines and topics that need to be considered when profiling this document.
o Applications need to determine the set of messages defined in this document that they will be using. The set of messages corresponds fairly directly to the set of security services that are needed and to the security levels needed.
Schaad Standards Track [Page 76]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Applications may define new header parameters for a specific purpose. Applications will often times select specific header parameters to use or not to use. For example, an application would normally state a preference for using either the IV or the Partial IV parameter. If the Partial IV parameter is specified, then the application would also need to define how the fixed portion of the IV would be determined.
o When applications use externally defined authenticated data, they need to define how that data is encoded. This document assumes that the data will be provided as a byte stream. More information can be found in Section 4.3.
o Applications need to determine the set of security algorithms that are to be used. When selecting the algorithms to be used as the mandatory-to-implement set, consideration should be given to choosing different types of algorithms when two are chosen for a specific purpose. An example of this would be choosing HMAC- SHA512 and AES-CMAC as different MAC algorithms; the construction is vastly different between these two algorithms. This means that a weakening of one algorithm would be unlikely to lead to a weakening of the other algorithms. Of course, these algorithms do not provide the same level of security and thus may not be comparable for the desired security functionality.
o Applications may need to provide some type of negotiation or discovery method if multiple algorithms or message structures are permitted. The method can be as simple as requiring preconfiguration of the set of algorithms to providing a discovery method built into the protocol. S/MIME provided a number of different ways to approach the problem that applications could follow:
* Advertising in the message (S/MIME capabilities) [RFC5751].
* Advertising in the certificate (capabilities extension) [RFC4262].
* Minimum requirements for the S/MIME, which have been updated over time [RFC2633] [RFC5751] (note that [RFC2633] has been obsoleted by [RFC5751]).
Schaad Standards Track [Page 77]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
IANA has assigned the following tags from the "CBOR Tags" registry. The tags for COSE_Sign1, COSE_Encrypt0, and COSE_Mac0 were assigned in the 1 to 23 value range (one byte long when encoded). The tags for COSE_Sign, COSE_Encrypt, and COSE_Mac were assigned in the 24 to 255 value range (two bytes long when encoded).
IANA has created a new registry titled "COSE Header Parameters". The registry has been created to use the "Expert Review Required" registration procedure [RFC8126]. Guidelines for the experts are provided in Section 16.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the registry are:
Name: The name is present to make it easier to refer to and discuss the registration entry. The value is not used in the protocol. Names are to be unique in the table.
Label: This is the value used for the label. The label can be either an integer or a string. Registration in the table is based on the value of the label requested. Integer values between 1 and 255 and strings of length 1 are designated as "Standards Action". Integer values from 256 to 65535 and strings of length 2 are designated as "Specification Required". Integer values of greater than 65535 and strings of length greater than 2 are designated as "Expert Review". Integer values in the range -1 to -65536 are "delegated to the COSE Header Algorithm Parameters registry". Integer values less than -65536 are marked as private use.
Value Type: This contains the CBOR type for the value portion of the label.
Value Registry: This contains a pointer to the registry used to contain values where the set is limited.
Description: This contains a brief description of the header field.
Schaad Standards Track [Page 78]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Reference: This contains a pointer to the specification defining the header field (where public).
The initial contents of the registry can be found in Tables 2 and 27. All of the entries in the "References" column of this registry point to this document.
Additionally, the label of 0 is to be marked as 'Reserved'.
IANA has created a new registry titled "COSE Header Algorithm Parameters". The registry uses the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 16.11.
The columns of the registry are:
Name: The name is present to make it easier to refer to and discuss the registration entry. The value is not used in the protocol.
Algorithm: The algorithm(s) that this registry entry is used for. This value is taken from the "COSE Algorithms" registry. Multiple algorithms can be specified in this entry. For the table, the algorithm/label pair MUST be unique.
Label: This is the value used for the label. The label is an integer in the range of -1 to -65536.
Type: This contains the CBOR type for the value portion of the label.
Description: This contains a brief description of the header field.
Reference: This contains a pointer to the specification defining the header field (where public).
The initial contents of the registry can be found in Tables 13, 14, and 19. All of the entries in the "References" column of this registry point to this document.
IANA has created a new registry titled "COSE Algorithms". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in
Schaad Standards Track [Page 79]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Section 16.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the registry are:
Name: A value that can be used to identify an algorithm in documents for easier comprehension. The name SHOULD be unique. However, the 'Value' field is what is used to identify the algorithm, not the 'name' field.
Value: The value to be used to identify this algorithm. Algorithm values MUST be unique. The value can be a positive integer, a negative integer, or a string. Integer values between -256 and 255 and strings of length 1 are designated as "Standards Action". Integer values from -65536 to 65535 and strings of length 2 are designated as "Specification Required". Integer values greater than 65535 and strings of length greater than 2 are designated as "Expert Review". Integer values less than -65536 are marked as private use.
Description: A short description of the algorithm.
Reference: A document where the algorithm is defined (if publicly available).
Recommended: Does the IETF have a consensus recommendation to use the algorithm? The legal values are 'Yes', 'No', and 'Deprecated'.
The initial contents of the registry can be found in Tables 5, 6, 7, 8, 9, 10, 11, 15, 16, 17, 18, and 20. All of the entries in the "References" column of this registry point to this document. All of the entries in the "Recommended" column are set to "Yes".
Additionally, the label of 0 is to be marked as 'Reserved'.
NOTE: The assignment of algorithm identifiers in this document was done so that positive numbers were used for the first layer objects (COSE_Sign, COSE_Sign1, COSE_Encrypt, COSE_Encrypt0, COSE_Mac, and COSE_Mac0). Negative numbers were used for second layer objects (COSE_Signature and COSE_recipient). Expert reviewers should consider this practice, but are not expected to be restricted by this precedent.
Schaad Standards Track [Page 80]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
IANA has created a new registry titled "COSE Key Common Parameters". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 16.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the registry are:
Name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
Label: The value to be used to identify this algorithm. Key map labels MUST be unique. The label can be a positive integer, a negative integer, or a string. Integer values between 0 and 255 and strings of length 1 are designated as "Standards Action". Integer values from 256 to 65535 and strings of length 2 are designated as "Specification Required". Integer values of greater than 65535 and strings of length greater than 2 are designated as "Expert Review". Integer values in the range -65536 to -1 are "used for key parameters specific to a single algorithm delegated to the COSE Key Type Parameters registry". Integer values less than -65536 are marked as private use.
CBOR Type: This field contains the CBOR type for the field.
Value Registry: This field denotes the registry that values come from, if one exists.
Description: This field contains a brief description for the field.
Reference: This contains a pointer to the public specification for the field if one exists.
This registry has been initially populated by the values in Table 3. All of the entries in the "References" column of this registry point to this document.
IANA has created a new registry titled "COSE Key Type Parameters". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 16.11.
Schaad Standards Track [Page 81]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
The columns of the table are:
Key Type: This field contains a descriptive string of a key type. This should be a value that is in the "COSE Key Common Parameters" registry and is placed in the 'kty' field of a COSE Key structure.
Name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
Label: The label is to be unique for every value of key type. The range of values is from -65536 to -1. Labels are expected to be reused for different keys.
CBOR Type: This field contains the CBOR type for the field.
Description: This field contains a brief description for the field.
Reference: This contains a pointer to the public specification for the field if one exists.
This registry has been initially populated by the values in Tables 23, 24, and 25. All of the entries in the "References" column of this registry point to this document.
IANA has created a new registry titled "COSE Key Types". The registry has been created to use the "Expert Review Required" registration procedure. Expert review guidelines are provided in Section 16.11.
The columns of this table are:
Name: This is a descriptive name that enables easier reference to the item. The name MUST be unique. It is not used in the encoding.
Value: This is the value used to identify the curve. These values MUST be unique. The value can be a positive integer, a negative integer, or a string.
Description: This field contains a brief description of the curve.
References: This contains a pointer to the public specification for the curve if one exists.
Schaad Standards Track [Page 82]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
This registry has been initially populated by the values in Table 21. The specification column for all of these entries will be this document.
IANA has created a new registry titled "COSE Elliptic Curves". The registry has been created to use the "Expert Review Required" registration procedure. Guidelines for the experts are provided in Section 16.11. It should be noted that, in addition to the expert review, some portions of the registry require a specification, potentially a Standards Track RFC, be supplied as well.
The columns of the table are:
Name: This is a descriptive name that enables easier reference to the item. It is not used in the encoding.
Value: This is the value used to identify the curve. These values MUST be unique. The integer values from -256 to 255 are designated as "Standards Action". The integer values from 256 to 65535 and -65536 to -257 are designated as "Specification Required". Integer values over 65535 are designated as "Expert Review". Integer values less than -65536 are marked as private use.
Key Type: This designates the key type(s) that can be used with this curve.
Description: This field contains a brief description of the curve.
Reference: This contains a pointer to the public specification for the curve if one exists.
Recommended: Does the IETF have a consensus recommendation to use the algorithm? The legal values are 'Yes', 'No', and 'Deprecated'.
This registry has been initially populated by the values in Table 22. All of the entries in the "References" column of this registry point to this document. All of the entries in the "Recommended" column are set to "Yes".
Schaad Standards Track [Page 83]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
This section registers the 'application/cose' media type in the "Media Types" registry. These media types are used to indicate that the content is a COSE message.
Type name: application
Subtype name: cose
Required parameters: N/A
Optional parameters: cose-type
Encoding considerations: binary
Security considerations: See the Security Considerations section of RFC 8152.
This section registers the 'application/cose-key' and 'application/ cose-key-set' media types in the "Media Types" registry. These media types are used to indicate, respectively, that content is a COSE_Key or COSE_KeySet object.
The template for registering 'application/cose-key' is:
Type name: application
Subtype name: cose-key
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See the Security Considerations section of RFC 8152.
All of the IANA registries established in this document are defined as expert review. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason, so they should be given substantial latitude.
Expert reviewers should take into consideration the following points:
o Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered, and that the point is likely to be used in
Schaad Standards Track [Page 87]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
deployments. The zones tagged as private use are intended for testing purposes and closed environments; code points in other ranges should not be assigned for testing.
o Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for.
o Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size.
o When algorithms are registered, vanity registrations should be discouraged. One way to do this is to require registrations to provide additional documentation on security analysis of the algorithm. Another thing that should be considered is requesting an opinion on the algorithm from the Crypto Forum Research Group (CFRG). Algorithms that do not meet the security requirements of the community and the messages structures should not be registered.
There are a number of security considerations that need to be taken into account by implementers of this specification. The security considerations that are specific to an individual algorithm are placed next to the description of the algorithm. While some considerations have been highlighted here, additional considerations may be found in the documents listed in the references.
Implementations need to protect the private key material for any individuals. There are some cases in this document that need to be highlighted on this issue.
o Using the same key for two different algorithms can leak information about the key. It is therefore recommended that keys be restricted to a single algorithm.
Schaad Standards Track [Page 88]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Use of 'direct' as a recipient algorithm combined with a second recipient algorithm exposes the direct key to the second recipient.
o Several of the algorithms in this document have limits on the number of times that a key can be used without leaking information about the key.
The use of ECDH and direct plus KDF (with no key wrap) will not directly lead to the private key being leaked; the one way function of the KDF will prevent that. There is, however, a different issue that needs to be addressed. Having two recipients requires that the CEK be shared between two recipients. The second recipient therefore has a CEK that was derived from material that can be used for the weak proof of origin. The second recipient could create a message using the same CEK and send it to the first recipient; the first recipient would, for either static-static ECDH or direct plus KDF, make an assumption that the CEK could be used for proof of origin even though it is from the wrong entity. If the key wrap step is added, then no proof of origin is implied and this is not an issue.
Although it has been mentioned before, the use of a single key for multiple algorithms has been demonstrated in some cases to leak information about a key, provide the opportunity for attackers to forge integrity tags, or gain information about encrypted content. Binding a key to a single algorithm prevents these problems. Key creators and key consumers are strongly encouraged not only to create new keys for each different algorithm, but to include that selection of algorithm in any distribution of key material and strictly enforce the matching of algorithms in the key structure to algorithms in the message structure. In addition to checking that algorithms are correct, the key form needs to be checked as well. Do not use an 'EC2' key where an 'OKP' key is expected.
Before using a key for transmission, or before acting on information received, a trust decision on a key needs to be made. Is the data or action something that the entity associated with the key has a right to see or a right to request? A number of factors are associated with this trust decision. Some of the ones that are highlighted here are:
o What are the permissions associated with the key owner?
o Is the cryptographic algorithm acceptable in the current context?
o Have the restrictions associated with the key, such as algorithm or freshness, been checked and are they correct?
Schaad Standards Track [Page 89]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Is the request something that is reasonable, given the current state of the application?
o Have any security considerations that are part of the message been enforced (as specified by the application or 'crit' parameter)?
There are a large number of algorithms presented in this document that use nonce values. For all of the nonces defined in this document, there is some type of restriction on the nonce being a unique value either for a key or for some other conditions. In all of these cases, there is no known requirement on the nonce being both unique and unpredictable; under these circumstances, it's reasonable to use a counter for creation of the nonce. In cases where one wants the pattern of the nonce to be unpredictable as well as unique, one can use a key created for that purpose and encrypt the counter to produce the nonce value.
One area that has been starting to get exposure is doing traffic analysis of encrypted messages based on the length of the message. This specification does not provide for a uniform method of providing padding as part of the message structure. An observer can distinguish between two different strings (for example, 'YES' and 'NO') based on the length for all of the content encryption algorithms that are defined in this document. This means that it is up to the applications to document how content padding is to be done in order to prevent or discourage such analysis. (For example, the strings could be defined as 'YES' and 'NO '.)
[AES-GCM] National Institute of Standards and Technology, "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", NIST Special Publication 800-38D, DOI 10.6028/NIST.SP.800-38D, November 2007, <https://csrc.nist.gov/publications/nistpubs/800- 38D/SP-800-38D.pdf>.
[DSS] National Institute of Standards and Technology, "Digital Signature Standard (DSS)", FIPS PUB 186-4, DOI 10.6028/NIST.FIPS.186-4, July 2013, <http://nvlpubs.nist.gov/nistpubs/FIPS/
Schaad Standards Track [Page 90]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
[CDDL] Vigano, C. and H. Birkholz, "CBOR data definition language (CDDL): a notational convention to express CBOR data structures", Work in Progress, draft-greevenbosch-appsawg- cbor-cddl-09, March 2017.
[OSCOAP] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security of CoAP (OSCOAP)", Work in Progress, draft-ietf-core-object-security-03, May 2017.
[PVSig] Brown, D. and D. Johnson, "Formal Security Proofs for a Signature Scheme with Partial Message Recovery", DOI 10.1007/3-540-45353-9_11, LNCS Volume 2020, June 2000.
[RFC5990] Randall, J., Kaliski, B., Brainard, J., and S. Turner, "Use of the RSA-KEM Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 5990, DOI 10.17487/RFC5990, September 2010, <http://www.rfc-editor.org/info/rfc5990>.
[SP800-56A] Barker, E., Chen, L., Roginsky, A., and M. Smid, "Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm Cryptography", NIST Special Publication 800-56A, Revision 2, DOI 10.6028/NIST.SP.800-56Ar2, May 2013, <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/ NIST.SP.800-56Ar2.pdf>.
Schaad Standards Track [Page 94]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Appendix A. Guidelines for External Data Authentication of Algorithms
A portion of the working group has expressed a strong desire to relax the rule that the algorithm identifier be required to appear in each level of a COSE object. There are two basic reasons that have been advanced to support this position. First, the resulting message will be smaller if the algorithm identifier is omitted from the most common messages in a CoAP environment. Second, there is a potential bug that will arise if full checking is not done correctly between the different places that an algorithm identifier could be placed (the message itself, an application statement, the key structure that the sender possesses, and the key structure the recipient possesses).
This appendix lays out how such a change can be made and the details that an application needs to specify in order to use this option. Two different sets of details are specified: those needed to omit an algorithm identifier and those needed to use a variant on the counter signature attribute that contains no attributes about itself.
In this section, three sets of recommendations are laid out. The first set of recommendations apply to having an implicit algorithm identified for a single layer of a COSE object. The second set of recommendations apply to having multiple implicit algorithms identified for multiple layers of a COSE object. The third set of recommendations apply to having implicit algorithms for multiple COSE object constructs.
The key words from RFC 2119 are deliberately not used here. This specification can provide recommendations, but it cannot enforce them.
This set of recommendations applies to the case where an application is distributing a fixed algorithm along with the key information for use in a single COSE object. This normally applies to the smallest of the COSE objects, specifically COSE_Sign1, COSE_Mac0, and COSE_Encrypt0, but could apply to the other structures as well.
The following items should be taken into account:
o Applications need to list the set of COSE structures that implicit algorithms are to be used in. Applications need to require that the receipt of an explicit algorithm identifier in one of these structures will lead to the message being rejected. This requirement is stated so that there will never be a case where there is any ambiguity about the question of which algorithm should be used, the implicit or the explicit one. This applies
Schaad Standards Track [Page 96]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
even if the transported algorithm identifier is a protected attribute. This applies even if the transported algorithm is the same as the implicit algorithm.
o Applications need to define the set of information that is to be considered to be part of a context when omitting algorithm identifiers. At a minimum, this would be the key identifier (if needed), the key, the algorithm, and the COSE structure it is used with. Applications should restrict the use of a single key to a single algorithm. As noted for some of the algorithms in this document, the use of the same key in different related algorithms can lead to leakage of information about the key, leakage about the data or the ability to perform forgeries.
o In many cases, applications that make the algorithm identifier implicit will also want to make the context identifier implicit for the same reason. That is, omitting the context identifier will decrease the message size (potentially significantly depending on the length of the identifier). Applications that do this will need to describe the circumstances where the context identifier is to be omitted and how the context identifier is to be inferred in these cases. (An exhaustive search over all of the keys would normally not be considered to be acceptable.) An example of how this can be done is to tie the context to a transaction identifier. Both would be sent on the original message, but only the transaction identifier would need to be sent after that point as the context is tied into the transaction identifier. Another way would be to associate a context with a network address. All messages coming from a single network address can be assumed to be associated with a specific context. (In this case, the address would normally be distributed as part of the context.)
o Applications cannot rely on key identifiers being unique unless they take significant efforts to ensure that they are computed in such a way as to create this guarantee. Even when an application does this, the uniqueness might be violated if the application is run in different contexts (i.e., with a different context provider) or if the system combines the security contexts from different applications together into a single store.
o Applications should continue the practice of protecting the algorithm identifier. Since this is not done by placing it in the protected attributes field, applications should define an application-specific external data structure that includes this value. This external data field can be used as such for content encryption, MAC, and signature algorithms. It can be used in the SuppPrivInfo field for those algorithms that use a KDF to derive a
Schaad Standards Track [Page 97]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
key value. Applications may also want to protect other information that is part of the context structure as well. It should be noted that those fields, such as the key or a Base IV, are protected by virtue of being used in the cryptographic computation and do not need to be included in the external data field.
The second case is having multiple implicit algorithm identifiers specified for a multiple layer COSE object. An example of how this would work is the encryption context that an application specifies, which contains a content encryption algorithm, a key wrap algorithm, a key identifier, and a shared secret. The sender omits sending the algorithm identifier for both the content layer and the recipient layer leaving only the key identifier. The receiver then uses the key identifier to get the implicit algorithm identifiers.
The following additional items need to be taken into consideration:
o Applications that want to support this will need to define a structure that allows for, and clearly identifies, both the COSE structure to be used with a given key and the structure and algorithm to be used for the secondary layer. The key for the secondary layer is computed as normal from the recipient layer.
The third case is having multiple implicit algorithm identifiers, but targeted at potentially unrelated layers or different COSE objects. There are a number of different scenarios where this might be applicable. Some of these scenarios are:
o Two contexts are distributed as a pair. Each of the contexts is for use with a COSE_Encrypt message. Each context will consist of distinct secret keys and IVs and potentially even different algorithms. One context is for sending messages from party A to party B, and the second context is for sending messages from party B to party A. This means that there is no chance for a reflection attack to occur as each party uses different secret keys to send its messages; a message that is reflected back to it would fail to decrypt.
o Two contexts are distributed as a pair. The first context is used for encryption of the message, and the second context is used to place a counter signature on the message. The intention is that the second context can be distributed to other entities independently of the first context. This allows these entities to validate that the message came from an individual without being able to decrypt the message and see the content.
Schaad Standards Track [Page 98]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
o Two contexts are distributed as a pair. The first context contains a key for dealing with MACed messages, and the second context contains a key for dealing with encrypted messages. This allows for a unified distribution of keys to participants for different types of messages that have different keys, but where the keys may be used in a coordinated manner.
For these cases, the following additional items need to be considered:
o Applications need to ensure that the multiple contexts stay associated. If one of the contexts is invalidated for any reason, all of the contexts associated with it should also be invalidated.
There is a group of people who want to have a counter signature parameter that is directly tied to the value being signed, and thus the authenticated and unauthenticated buckets can be removed from the message being sent. The focus on this is an even smaller size, as all of the information on the process of creating the counter signature is implicit rather than being explicitly carried in the message. This includes not only the algorithm identifier as presented above, but also items such as the key identification, which is always external to the signature structure. This means that the entities that are doing the validation of the counter signature are required to infer which key is to be used from context rather than being explicit. One way of doing this would be to presume that all data coming from a specific port (or to a specific URL) is to be validated by a specific key. (Note that this does not require that the key identifier be part of the value signed as it does not serve a cryptographic purpose. If the key validates the counter signature, then it should be presumed that the entity associated with that key produced the signature.)
When computing the signature for the bare counter signature header, the same Sig_structure defined in Section 4.4 is used. The sign_protected field is omitted, as there is no protected header field in this counter signature header. The value of "CounterSignature0" is placed in the context field of the Sig_stucture.
Schaad Standards Track [Page 99]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
+-------------------+-------+-------+-------+-----------------------+ | Name | Label | Value | Value | Description | | | | Type | | | +-------------------+-------+-------+-------+-----------------------+ | CounterSignature0 | 9 | bstr | | Counter signature | | | | | | with implied signer | | | | | | and headers | +-------------------+-------+-------+-------+-----------------------+
All of the currently defined recipient algorithm classes only use two layers of the COSE_Encrypt structure. The first layer is the message content, and the second layer is the content key encryption. However, if one uses a recipient algorithm such as the RSA Key Encapsulation Mechanism (RSA-KEM) (see Appendix A of RSA-KEM [RFC5990]), then it makes sense to have three layers of the COSE_Encrypt structure.
These layers would be:
o Layer 0: The content encryption layer. This layer contains the payload of the message.
o Layer 1: The encryption of the CEK by a KEK.
o Layer 2: The encryption of a long random secret using an RSA key and a key derivation function to convert that secret into the KEK.
This is an example of what a triple layer message would look like. The message has the following layers:
o Layer 0: Has a content encrypted with AES-GCM using a 128-bit key.
o Layer 1: Uses the AES Key Wrap algorithm with a 128-bit key.
o Layer 2: Uses ECDH Ephemeral-Static direct to generate the layer 1 key.
Schaad Standards Track [Page 100]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
In effect, this example is a decomposed version of using the ECDH-ES+A128KW algorithm.
This appendix includes a set of examples that show the different features and message types that have been defined in this document. To make the examples easier to read, they are presented using the extended CBOR diagnostic notation (defined in [CDDL]) rather than as a binary dump.
A GitHub project has been created at <https://github.com/cose-wg/ Examples> that contains not only the examples presented in this document, but a more complete set of testing examples as well. Each example is found in a JSON file that contains the inputs used to create the example, some of the intermediate values that can be used in debugging the example and the output of the example presented in both a hex and a CBOR diagnostic notation format. Some of the examples at the site are designed failure testing cases; these are clearly marked as such in the JSON file. If errors in the examples in this document are found, the examples on GitHub will be updated, and a note to that effect will be placed in the JSON file.
As noted, the examples are presented using the CBOR's diagnostic notation. A Ruby-based tool exists that can convert between the diagnostic notation and binary. This tool can be installed with the command line:
gem install cbor-diag
The diagnostic notation can be converted into binary files using the following command line:
diag2cbor.rb < inputfile > outputfile
The examples can be extracted from the XML version of this document via an XPath expression as all of the artwork is tagged with the attribute type='CBORdiag'. (Depending on the XPath evaluator one is using, it may be necessary to deal with > as an entity.)
//artwork[@type='CDDL']/text()
Schaad Standards Track [Page 102]
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
RFC 8152 CBOR Object Signing and Encryption (COSE) July 2017
Acknowledgments
This document is a product of the COSE working group of the IETF.
The following individuals are to blame for getting me started on this project in the first place: Richard Barnes, Matt Miller, and Martin Thomson.
The initial version of the specification was based to some degree on the outputs of the JOSE and S/MIME working groups.
The following individuals provided input into the final form of the document: Carsten Bormann, John Bradley, Brain Campbell, Michael B. Jones, Ilari Liusvaara, Francesca Palombini, Ludwig Seitz, and Goran Selander.