Internet Engineering Task Force (IETF) M. Jones Request for Comments: 8392 Microsoft Category: Standards Track E. Wahlstroem ISSN: 2070-1721 S. Erdtman Spotify AB H. Tschofenig ARM Ltd. May 2018
CBOR Web Token (CWT)
Abstract
CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON.
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 https://www.rfc-editor.org/info/rfc8392.
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Copyright Notice
Copyright (c) 2018 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 (https://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.
The JSON Web Token (JWT) [RFC7519] is a standardized security token format that has found use in OAuth 2.0 and OpenID Connect deployments, among other applications. JWT uses JSON Web Signature (JWS) [RFC7515] and JSON Web Encryption (JWE) [RFC7516] to secure the contents of the JWT, which is a set of claims represented in JSON. The use of JSON for encoding information is popular for Web and native applications, but it is considered inefficient for some Internet of Things (IoT) systems that use low-power radio technologies.
An alternative encoding of claims is defined in this document. Instead of using JSON, as provided by JWTs, this specification uses CBOR [RFC7049] and calls this new structure "CBOR Web Token (CWT)", which is a compact means of representing secured claims to be transferred between two parties. CWT is closely related to JWT. It references the JWT claims and both its name and pronunciation are derived from JWT (the suggested pronunciation of CWT is the same as the English word "cot"). To protect the claims contained in CWTs, the CBOR Object Signing and Encryption (COSE) [RFC8152] specification is used.
In JSON, maps are called objects and only have one kind of map key: a string. CBOR uses 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.
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.
This document reuses terminology from JWT [RFC7519] and COSE [RFC8152].
StringOrURI The "StringOrURI" term in this specification has the same meaning and processing rules as the JWT "StringOrURI" term defined in Section 2 of [RFC7519], except that it is represented as a CBOR text string instead of a JSON text string.
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NumericDate The "NumericDate" term in this specification has the same meaning and processing rules as the JWT "NumericDate" term defined in Section 2 of [RFC7519], except that it is represented as a CBOR numeric date (from Section 2.4.1 of [RFC7049]) instead of a JSON number. The encoding is modified so that the leading tag 1 (epoch-based date/time) MUST be omitted.
Claim Name The human-readable name used to identify a claim.
Claim Key The CBOR map key used to identify a claim.
Claim Value The CBOR map value representing the value of the claim.
CWT Claims Set The CBOR map that contains the claims conveyed by the CWT.
The set of claims that a CWT must contain to be considered valid is context dependent and is outside the scope of this specification. Specific applications of CWTs will require implementations to understand and process some claims in particular ways. However, in the absence of such requirements, all claims that are not understood by implementations MUST be ignored.
To keep CWTs as small as possible, the Claim Keys are represented using integers or text strings. Section 4 summarizes all keys used to identify the claims defined in this document.
None of the claims defined below are intended to be mandatory to use or implement. Rather, they provide a starting point for a set of useful, interoperable claims. Applications using CWTs should define which specific claims they use and when they are required or optional.
The "iss" (issuer) claim has the same meaning and processing rules as the "iss" claim defined in Section 4.1.1 of [RFC7519], except that the value is a StringOrURI, as defined in Section 2 of this specification. The Claim Key 1 is used to identify this claim.
The "sub" (subject) claim has the same meaning and processing rules as the "sub" claim defined in Section 4.1.2 of [RFC7519], except that the value is a StringOrURI, as defined in Section 2 of this specification. The Claim Key 2 is used to identify this claim.
The "aud" (audience) claim has the same meaning and processing rules as the "aud" claim defined in Section 4.1.3 of [RFC7519], except that the value of the audience claim is a StringOrURI when it is not an array or each of the audience array element values is a StringOrURI when the audience claim value is an array. (StringOrURI is defined in Section 2 of this specification.) The Claim Key 3 is used to identify this claim.
The "exp" (expiration time) claim has the same meaning and processing rules as the "exp" claim defined in Section 4.1.4 of [RFC7519], except that the value is a NumericDate, as defined in Section 2 of this specification. The Claim Key 4 is used to identify this claim.
The "nbf" (not before) claim has the same meaning and processing rules as the "nbf" claim defined in Section 4.1.5 of [RFC7519], except that the value is a NumericDate, as defined in Section 2 of this specification. The Claim Key 5 is used to identify this claim.
The "iat" (issued at) claim has the same meaning and processing rules as the "iat" claim defined in Section 4.1.6 of [RFC7519], except that the value is a NumericDate, as defined in Section 2 of this specification. The Claim Key 6 is used to identify this claim.
The "cti" (CWT ID) claim has the same meaning and processing rules as the "jti" claim defined in Section 4.1.7 of [RFC7519], except that the value is a byte string. The Claim Key 7 is used to identify this claim.
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4. Summary of the Claim Names, Keys, and Value Types
+------+-----+----------------------------------+ | Name | Key | Value Type | +------+-----+----------------------------------+ | iss | 1 | text string | | sub | 2 | text string | | aud | 3 | text string | | exp | 4 | integer or floating-point number | | nbf | 5 | integer or floating-point number | | iat | 6 | integer or floating-point number | | cti | 7 | byte string | +------+-----+----------------------------------+
Table 1: Summary of the Claim Names, Keys, and Value Types
The claim values defined in this specification MUST NOT be prefixed with any CBOR tag. For instance, while CBOR tag 1 (epoch-based date/ time) could logically be prefixed to values of the "exp", "nbf", and "iat" claims, this is unnecessary since the representation of the claim values is already specified by the claim definitions. Tagging claim values would only take up extra space without adding information. However, this does not prohibit future claim definitions from requiring the use of CBOR tags for those specific claims.
How to determine that a CBOR data structure is a CWT is application dependent. In some cases, this information is known from the application context, such as from the position of the CWT in a data structure at which the value must be a CWT. One method of indicating that a CBOR object is a CWT is the use of the "application/cwt" content type by a transport protocol.
This section defines the CWT CBOR tag as another means for applications to declare that a CBOR data structure is a CWT. Its use is optional and is intended for use in cases in which this information would not otherwise be known.
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If present, the CWT tag MUST prefix a tagged object using one of the COSE CBOR tags. In this example, the COSE_Mac0 tag is used. The actual COSE_Mac0 object has been excluded from this example.
/ CWT CBOR tag / 61( / COSE_Mac0 CBOR tag / 17( / COSE_Mac0 object / ) )
To create a CWT, the following steps are performed. The order of the steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps.
1. Create a CWT Claims Set containing the desired claims.
2. Let the Message be the binary representation of the CWT Claims Set.
3. Create a COSE Header containing the desired set of Header Parameters. The COSE Header MUST be valid per the [RFC8152] specification.
4. Depending upon whether the CWT is signed, MACed, or encrypted, there are three cases:
* If the CWT is signed, create a COSE_Sign/COSE_Sign1 object using the Message as the COSE_Sign/COSE_Sign1 Payload; all steps specified in [RFC8152] for creating a COSE_Sign/ COSE_Sign1 object MUST be followed.
* Else, if the CWT is MACed, create a COSE_Mac/COSE_Mac0 object using the Message as the COSE_Mac/COSE_Mac0 Payload; all steps specified in [RFC8152] for creating a COSE_Mac/COSE_Mac0 object MUST be followed.
* Else, if the CWT is a COSE_Encrypt/COSE_Encrypt0 object, create a COSE_Encrypt/COSE_Encrypt0 using the Message as the plaintext for the COSE_Encrypt/COSE_Encrypt0 object; all steps specified in [RFC8152] for creating a COSE_Encrypt/ COSE_Encrypt0 object MUST be followed.
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5. If a nested signing, MACing, or encryption operation will be performed, let the Message be the tagged COSE_Sign/COSE_Sign1, COSE_Mac/COSE_Mac0, or COSE_Encrypt/COSE_Encrypt0, and return to Step 3.
6. If needed by the application, prepend the COSE object with the appropriate COSE CBOR tag to indicate the type of the COSE object. If needed by the application, prepend the COSE object with the CWT CBOR tag to indicate that the COSE object is a CWT.
When validating a CWT, the following steps are performed. The order of the steps is not significant in cases where there are no dependencies between the inputs and outputs of the steps. If any of the listed steps fail, then the CWT MUST be rejected -- that is, treated by the application as invalid input.
1. Verify that the CWT is a valid CBOR object.
2. If the object begins with the CWT CBOR tag, remove it and verify that one of the COSE CBOR tags follows it.
3. If the object is tagged with one of the COSE CBOR tags, remove it and use it to determine the type of the CWT, COSE_Sign/ COSE_Sign1, COSE_Mac/COSE_Mac0, or COSE_Encrypt/COSE_Encrypt0. If the object does not have a COSE CBOR tag, the COSE message type is determined from the application context.
4. Verify that the resulting COSE Header includes only parameters and values whose syntax and semantics are both understood and supported or that are specified as being ignored when not understood.
5. Depending upon whether the CWT is a signed, MACed, or encrypted, there are three cases:
* If the CWT is a COSE_Sign/COSE_Sign1, follow the steps specified in Section 4 of [RFC8152] ("Signing Objects") for validating a COSE_Sign/COSE_Sign1 object. Let the Message be the COSE_Sign/COSE_Sign1 payload.
* Else, if the CWT is a COSE_Mac/COSE_Mac0, follow the steps specified in Section 6 of [RFC8152] ("MAC Objects") for validating a COSE_Mac/COSE_Mac0 object. Let the Message be the COSE_Mac/COSE_Mac0 payload.
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* Else, if the CWT is a COSE_Encrypt/COSE_Encrypt0 object, follow the steps specified in Section 5 of [RFC8152] ("Encryption Objects") for validating a COSE_Encrypt/ COSE_Encrypt0 object. Let the Message be the resulting plaintext.
6. If the Message begins with a COSE CBOR tag, then the Message is a CWT that was the subject of nested signing, MACing, or encryption operations. In this case, return to Step 1, using the Message as the CWT.
7. Verify that the Message is a valid CBOR map; let the CWT Claims Set be this CBOR map.
The security of the CWT relies upon on the protections offered by COSE. Unless the claims in a CWT are protected, an adversary can modify, add, or remove claims.
Since the claims conveyed in a CWT may be used to make authorization decisions, it is not only important to protect the CWT in transit but also to ensure that the recipient can authenticate the party that assembled the claims and created the CWT. Without trust of the recipient in the party that created the CWT, no sensible authorization decision can be made. Furthermore, the creator of the CWT needs to carefully evaluate each claim value prior to including it in the CWT so that the recipient can be assured of the validity of the information provided.
Syntactically, the signing and encryption operations for Nested CWTs may be applied in any order; however, if both signing and encryption are necessary, producers normally should sign the message and then encrypt the result (thus encrypting the signature). This prevents attacks in which the signature is stripped, leaving just an encrypted message, as well as providing privacy for the signer. Furthermore, signatures over encrypted text are not considered valid in many jurisdictions.
IANA has created the "CBOR Web Token (CWT) Claims" registry [IANA.CWT.Claims].
Registration requests are evaluated using the criteria described in the Claim Key instructions in the registration template below after a three-week review period on the cwt-reg-review@ietf.org mailing list, on the advice of one or more Designated Experts [RFC8126]. However, to allow for the allocation of values prior to publication, the Designated Experts may approve registration once they are satisfied that such a specification will be published.
Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register claim: example"). Registration requests that are undetermined for a period longer than 21 days can be brought to the IESG's attention (using the iesg@ietf.org mailing list) for resolution.
Criteria that should be applied by the Designated Experts includes determining whether the proposed registration duplicates existing functionality, whether it is likely to be of general applicability or whether it is useful only for a single application, and whether the registration description is clear. Registrations for the limited set of values between -256 and 255 and strings of length 1 are to be restricted to claims with general applicability.
IANA must only accept registry updates from the Designated Experts and should direct all requests for registration to the review mailing list.
It is suggested that multiple Designated Experts be appointed who are able to represent the perspectives of different applications using this specification in order to enable broadly informed review of registration decisions. In cases where a registration decision could be perceived as creating a conflict of interest for a particular Expert, that Expert should defer to the judgment of the other Experts.
Since a high degree of overlap is expected between the contents of the "CBOR Web Token (CWT) Claims" registry and the "JSON Web Token Claims" registry, overlap in the corresponding pools of Designated Experts would be useful to help ensure that an appropriate level of coordination between the registries is maintained.
Claim Name: The human-readable name requested (e.g., "iss").
Claim Description: Brief description of the claim (e.g., "Issuer").
JWT Claim Name: Claim Name of the equivalent JWT claim, as registered in [IANA.JWT.Claims]. CWT claims should normally have a corresponding JWT claim. If a corresponding JWT claim would not make sense, the Designated Experts can choose to accept registrations for which the JWT Claim Name is listed as "N/A".
Claim Key: CBOR map key for the claim. Different ranges of values use different registration policies [RFC8126]. Integer values from -256 to 255 and strings of length 1 are designated as Standards Action. Integer values from -65536 to -257 and from 256 to 65535 along with 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.
Claim Value Type(s): CBOR types that can be used for the claim value.
Change Controller: For Standards Track RFCs, list the "IESG". For others, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included.
Specification Document(s): Reference to the document or documents that specify the parameter, preferably including URIs that can be used to retrieve copies of the documents. An indication of the relevant sections may also be included but is not required.
o Claim Name: (RESERVED) o Claim Description: This registration reserves the key value 0. o JWT Claim Name: N/A o Claim Key: 0 o Claim Value Type(s): N/A o Change Controller: IESG o Specification Document(s): [RFC8392]
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o Claim Name: iss o Claim Description: Issuer o JWT Claim Name: iss o Claim Key: 1 o Claim Value Type(s): text string o Change Controller: IESG o Specification Document(s): Section 3.1.1 of [RFC8392]
o Claim Name: sub o Claim Description: Subject o JWT Claim Name: sub o Claim Key: 2 o Claim Value Type(s): text string o Change Controller: IESG o Specification Document(s): Section 3.1.2 of [RFC8392]
o Claim Name: aud o Claim Description: Audience o JWT Claim Name: aud o Claim Key: 3 o Claim Value Type(s): text string o Change Controller: IESG o Specification Document(s): Section 3.1.3 of [RFC8392]
o Claim Name: exp o Claim Description: Expiration Time o JWT Claim Name: exp o Claim Key: 4 o Claim Value Type(s): integer or floating-point number o Change Controller: IESG o Specification Document(s): Section 3.1.4 of [RFC8392]
o Claim Name: nbf o Claim Description: Not Before o JWT Claim Name: nbf o Claim Key: 5 o Claim Value Type(s): integer or floating-point number o Change Controller: IESG o Specification Document(s): Section 3.1.5 of [RFC8392]
o Claim Name: iat o Claim Description: Issued At o JWT Claim Name: iat o Claim Key: 6 o Claim Value Type(s): integer or floating-point number o Change Controller: IESG o Specification Document(s): Section 3.1.6 of [RFC8392]
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o Claim Name: cti o Claim Description: CWT ID o JWT Claim Name: jti o Claim Key: 7 o Claim Value Type(s): byte string o Change Controller: IESG o Specification Document(s): Section 3.1.7 of [RFC8392]
IANA has registered the "application/cwt" media type in the "Media Types" registry [IANA.MediaTypes] in the manner described in RFC 6838 [RFC6838], which can be used to indicate that the content is a CWT.
o Type name: application o Subtype name: cwt o Required parameters: N/A o Optional parameters: N/A o Encoding considerations: binary o Security considerations: See the Security Considerations section of [RFC8392] o Interoperability considerations: N/A o Published specification: [RFC8392] o Applications that use this media type: IoT applications sending security tokens over HTTP(S), CoAP(S), and other transports. o Fragment identifier considerations: N/A o Additional information:
Magic number(s): N/A File extension(s): N/A Macintosh file type code(s): N/A
o Person & email address to contact for further information: IESG, iesg@ietf.org o Intended usage: COMMON o Restrictions on usage: none o Author: Michael B. Jones, mbj@microsoft.com o Change controller: IESG o Provisional registration? No
IANA has registered the CoAP Content-Format ID for the "application/ cwt" media type in the "CoAP Content-Formats" registry [IANA.CoAP.Content-Formats].
o CBOR Tag: 61 o Data Item: CBOR Web Token (CWT) o Semantics: CBOR Web Token (CWT), as defined in [RFC8392] o Reference: [RFC8392] o Point of Contact: Michael B. Jones, mbj@microsoft.com
This appendix includes a set of CWT examples that show how the CWT Claims Set can be protected. There are examples that are signed, MACed, encrypted, and that use nested signing and encryption. To make the examples easier to read, they are presented both as hex strings and in the extended CBOR diagnostic notation described in Section 6 of [RFC7049].
Where a byte string is to carry an embedded CBOR-encoded item, the diagnostic notation for this CBOR data item can be enclosed in '<<' and '>>' to notate the byte string resulting from encoding the data item, e.g., h'63666F6F' translates to <<"foo">>.
The CWT Claims Set used for the different examples displays usage of all the defined claims. For signed and MACed examples, the CWT Claims Set is the CBOR encoding as a byte string.
This section shows a signed CWT with a single recipient and a full CWT Claims Set.
The signature is generated using the private key listed in Appendix A.2.3, and it can be validated using the public key from Appendix A.2.3. Line breaks are for display purposes only.
This section shows an encrypted CWT with a single recipient and a full CWT Claims Set.
The encryption is done with AES-CCM mode using the 128-bit symmetric key from Appendix A.2.1 with a 64-bit tag and 13-byte nonce, i.e., COSE AES-CCM-16-64-128. Line breaks are for display purposes only.
This section shows a Nested CWT, signed and then encrypted, with a single recipient and a full CWT Claims Set.
The signature is generated using the private ECDSA key from Appendix A.2.3, and it can be validated using the public ECDSA parts from Appendix A.2.3. The encryption is done with AES-CCM mode using the 128-bit symmetric key from Appendix A.2.1 with a 64-bit tag and 13-byte nonce, i.e., COSE AES-CCM-16-64-128. The content type is set to CWT to indicate that there are multiple layers of COSE protection before finding the CWT Claims Set. The decrypted ciphertext will be a COSE_sign1 structure. In this example, it is the same one as in Appendix A.3, i.e., a Signed CWT Claims Set. Note that there is no limitation to the number of layers; this is an example with two layers. Line breaks are for display purposes only.
Figure 19: MACed CWT with a Floating-Point Value in CBOR Diagnostic Notation
Acknowledgements
This specification is based on JSON Web Token (JWT) [RFC7519], the authors of which also include Nat Sakimura and John Bradley. It also incorporates suggestions made by many people, including Carsten Bormann, Alissa Cooper, Esko Dijk, Benjamin Kaduk, Warren Kumari, Carlos Martinez, Alexey Melnikov, Kathleen Moriarty, Eric Rescorla, Dan Romascanu, Adam Roach, Kyle Rose, Jim Schaad, Ludwig Seitz, and Goeran Selander.