Internet Engineering Task Force (IETF) J. Schaad Request for Comments: 8551 August Cellars Obsoletes: 5751 B. Ramsdell Category: Standards Track Brute Squad Labs, Inc. ISSN: 2070-1721 S. Turner sn3rd April 2019
Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification
Abstract
This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751.
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/rfc8551.
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Copyright Notice
Copyright (c) 2019 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.
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
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RFC 8551 S/MIME 4.0 Message Specification April 2019
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a consistent way to send and receive secure MIME data. Based on the popular Internet MIME standard, S/MIME provides the following cryptographic security services for electronic messaging applications: authentication, message integrity, and non-repudiation of origin (using digital signatures), and data confidentiality (using encryption). As a supplementary service, S/MIME provides message compression.
S/MIME can be used by traditional mail user agents (MUAs) to add cryptographic security services to mail that is sent, and to interpret cryptographic security services in mail that is received. However, S/MIME is not restricted to mail; it can be used with any transport mechanism that transports MIME data, such as HTTP or SIP. As such, S/MIME takes advantage of the object-based features of MIME and allows secure messages to be exchanged in mixed-transport systems.
Further, S/MIME can be used in automated message transfer agents that use cryptographic security services that do not require any human intervention, such as the signing of software-generated documents and the encryption of FAX messages sent over the Internet.
This document defines version 4.0 of the S/MIME Message Specification. As such, this document obsoletes version 3.2 of the S/MIME Message Specification [RFC5751].
This specification contains a number of references to documents that have been obsoleted or replaced. This is intentional, as the updated documents often do not have the same information or protocol requirements in them.
This document describes a protocol for adding cryptographic signature and encryption services to MIME data. The MIME standard [MIME-SPEC] provides a general structure for the content of Internet messages and allows extensions for new applications based on content-type.
This specification defines how to create a MIME body part that has been cryptographically enhanced according to the Cryptographic Message Syntax (CMS) [CMS], which is derived from PKCS #7 [RFC2315]. This specification also defines the application/pkcs7-mime media type, which can be used to transport those body parts.
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This document also discusses how to use the multipart/signed media type defined in [RFC1847] to transport S/MIME signed messages. multipart/signed is used in conjunction with the application/pkcs7-signature media type, which is used to transport a detached S/MIME signature.
In order to create S/MIME messages, an S/MIME agent MUST follow the specifications in this document, as well as the specifications listed in [CMS], [RFC3370], [RFC4056], [RFC3560], and [RFC5754].
Throughout this specification, there are requirements and recommendations made for how receiving agents handle incoming messages. There are separate requirements and recommendations for how sending agents create outgoing messages. In general, the best strategy is to follow the Robustness Principle (be liberal in what you receive and conservative in what you send). Most of the requirements are placed on the handling of incoming messages, while the recommendations are mostly on the creation of outgoing messages.
The separation for requirements on receiving agents and sending agents also derives from the likelihood that there will be S/MIME systems that involve software other than traditional Internet mail clients. S/MIME can be used with any system that transports MIME data. An automated process that sends an encrypted message might not be able to receive an encrypted message at all, for example. Thus, the requirements and recommendations for the two types of agents are listed separately when appropriate.
For the purposes of this specification, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in ITU-T Recommendations X.680, X.681, X.682, and X.683 [ASN.1].
BER: Basic Encoding Rules for ASN.1, as defined in ITU-T Recommendation X.690 [X.690].
Certificate: A type that binds an entity's name to a public key with a digital signature.
DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T Recommendation X.690 [X.690].
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7-bit data: Text data with lines less than 998 characters long, where none of the characters have the 8th bit set, and there are no NULL characters. <CR> and <LF> occur only as part of a <CR><LF> end-of-line delimiter.
8-bit data: Text data with lines less than 998 characters, and where none of the characters are NULL characters. <CR> and <LF> occur only as part of a <CR><LF> end-of-line delimiter.
Binary data: Arbitrary data.
Transfer encoding: A reversible transformation made on data so 8-bit or binary data can be sent via a channel that only transmits 7-bit data.
Receiving agent: Software that interprets and processes S/MIME CMS objects, MIME body parts that contain CMS content types, or both.
Sending agent: Software that creates S/MIME CMS content types, MIME body parts that contain CMS content types, or both.
S/MIME agent: User software that is a receiving agent, a sending agent, or both.
Data integrity service: A security service that protects against unauthorized changes to data by ensuring that changes to the data are detectable [RFC4949].
Data confidentiality: The property that data is not disclosed to system entities unless they have been authorized to know the data [RFC4949].
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.
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We define the additional requirement levels:
SHOULD+ This term means the same as SHOULD. However, the authors expect that a requirement marked as SHOULD+ will be promoted at some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, the authors expect that a requirement marked as SHOULD- will be demoted to a MAY in a future version of this document.
MUST- This term means the same as MUST. However, the authors expect that this requirement will no longer be a MUST in a future document. Although its status will be determined at a later time, it is reasonable to expect that if a future revision of a document alters the status of a MUST- requirement, it will remain at least a SHOULD or a SHOULD-.
The term "RSA" in this document almost always refers to the PKCS #1 v1.5 RSA [RFC2313] signature or encryption algorithms even when not qualified as such. There are a couple of places where it refers to the general RSA cryptographic operation; these can be determined from the context where it is used.
This section describes the changes made between S/MIME v3 and S/MIME v3.1. Note that the requirement levels indicated by the capitalized key words ("MUST", "SHOULD", etc.) may have changed in later versions of S/MIME.
- The RSA public key algorithm was changed to a MUST implement. The key wrap algorithm and the Diffie-Hellman (DH) algorithm [RFC2631] were changed to a SHOULD implement.
- The AES symmetric encryption algorithm has been included as a SHOULD implement.
- The RSA public key algorithm was changed to a MUST implement signature algorithm.
- Ambiguous language about the use of "empty" SignedData messages to transmit certificates was clarified to reflect that transmission of Certificate Revocation Lists is also allowed.
- The use of binary encoding for some MIME entities is now explicitly discussed.
- Header protection through the use of the message/rfc822 media type has been added.
- Use of the CompressedData CMS type is allowed, along with required media type and file extension additions.
This section describes the changes made between S/MIME v3.1 and S/MIME v3.2. Note that the requirement levels indicated by the capitalized key words ("MUST", "SHOULD", etc.) may have changed in later versions of S/MIME. Note that the section numbers listed here (e.g., 3.4.3.2) are from [RFC5751].
- Made editorial changes, e.g., replaced "MIME type" with "media type", "content-type" with "Content-Type".
- Moved "Conventions Used in This Document" to Section 1.3. Added definitions for SHOULD+, SHOULD-, and MUST-.
- Section 1.1 and Appendix A: Added references to RFCs for RSASSA-PSS, RSAES-OAEP, and SHA2 CMS algorithms. Added CMS Multiple Signers Clarification to CMS reference.
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- Section 1.2: Updated references to ASN.1 to X.680, and BER and DER to X.690.
- Section 1.4: Added references to S/MIME v3.1 RFCs.
- Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5 made SHOULD-.
- Section 2.2 (signature algorithms): RSA with SHA-256 added as MUST; DSA with SHA-256 added as SHOULD+; RSA with SHA-1, DSA with SHA-1, and RSA with MD5 changed to SHOULD-; and RSASSA-PSS with SHA-256 added as SHOULD+. Also added note about what S/MIME v3.1 clients support.
- Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES- OAEP added as SHOULD+. Elaborated on requirements for key wrap algorithm.
- Section 2.5.1: Added requirement that receiving agents MUST support both GeneralizedTime and UTCTime.
- Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with "sha256WithRSAEncryption", replaced "DES-3EDE-CBC" with "AES-128 CBC", and deleted the RC5 example.
RFC 8551 S/MIME 4.0 Message Specification April 2019
- Section 4.1: Updated RSA and DSA key size discussion. Moved last four sentences to security considerations. Updated reference to randomness requirements for security.
- Section 5: Added IANA registration templates to update media type registry to point to this document as opposed to RFC 2311.
This section describes the changes made between S/MIME v3.2 and S/MIME v4.0.
- Added the use of AuthEnvelopedData, including defining and registering an smime-type value (Sections 2.4.4 and 3.4).
- Updated the content-encryption algorithms (Sections 2.7 and 2.7.1.2): added AES-256 Galois/Counter Mode (GCM), added ChaCha20-Poly1305, removed mention of AES-192 Cipher Block Chaining (CBC), and marked tripleDES as historic.
- Updated the set of signature algorithms (Section 2.2): added the Edwards-curve Digital Signature Algorithm (EdDSA), added the Elliptic Curve Digital Signature Algorithm (ECDSA), and marked DSA as historic.
- Updated the set of digest algorithms (Section 2.1): added SHA-512, and marked SHA-1 as historic.
- Updated the size of keys to be used for RSA encryption and RSA signing (Section 4).
- Created Appendix B, which discusses considerations for dealing with historic email messages.
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CMS allows for a wide variety of options in content, attributes, and algorithm support. This section puts forth a number of support requirements and recommendations in order to achieve a base level of interoperability among all S/MIME implementations. [RFC3370] and [RFC5754] provide additional details regarding the use of the cryptographic algorithms. [ESS] provides additional details regarding the use of additional attributes.
The algorithms here are used for digesting the body of the message and are not the same as the digest algorithms used as part of the signature algorithms. The result of this is placed in the message-digest attribute of the signed attributes. It is RECOMMENDED that the algorithm used for digesting the body of the message be of similar strength to, or greater strength than, the signature algorithm.
Sending and receiving agents:
- MUST support SHA-256.
- MUST support SHA-512.
[RFC5754] provides the details for using these algorithms with S/MIME.
There are different sets of requirements placed on receiving and sending agents. By having the different requirements, the maximum amount of interoperability is achieved, as it allows for specialized protection of private key material but maximum signature validation.
Receiving agents:
- MUST support ECDSA with curve P-256 and SHA-256.
- MUST support EdDSA with curve25519 using PureEdDSA mode [RFC8419].
- MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS) with SHA-256.
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Sending agents:
- MUST support at least one of the following algorithms: ECDSA with curve P-256 and SHA-256, or EdDSA with curve25519 using PureEdDSA mode.
- MUST- support RSA PKCS #1 v1.5 with SHA-256.
- SHOULD support RSASSA-PSS with SHA-256.
See Section 4.1 for information on key size and algorithm references.
- MUST support Elliptic Curve Diffie-Hellman (ECDH) ephemeral-static mode for P-256, as specified in [RFC5753].
- MUST support ECDH ephemeral-static mode for X25519 using HKDF-256 ("HKDF" stands for "HMAC-based Key Derivation Function") for the KDF, as specified in [RFC8418].
- MUST- support RSA encryption, as specified in [RFC3370].
- SHOULD+ support RSA Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-OAEP), as specified in [RFC3560].
When ECDH ephemeral-static is used, a key wrap algorithm is also specified in the KeyEncryptionAlgorithmIdentifier [RFC5652]. The underlying encryption functions for the key wrap and content- encryption algorithms [RFC3370] [RFC3565] and the key sizes for the two algorithms MUST be the same (e.g., AES-128 key wrap algorithm with AES-128 content-encryption algorithm). As both 128-bit and 256-bit AES modes are mandatory to implement as content-encryption algorithms (Section 2.7), both the AES-128 and AES-256 key wrap algorithms MUST be supported when ECDH ephemeral-static is used. Recipients MAY enforce this but MUST use the weaker of the two as part of any cryptographic strength computations they might do.
Appendix B provides information on algorithm support in older versions of S/MIME.
There are several CMS content types. Of these, only the Data, SignedData, EnvelopedData, AuthEnvelopedData, and CompressedData content types are currently used for S/MIME.
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Sending agents MUST use the id-data content type identifier to identify the "inner" MIME message content. For example, when applying a digital signature to MIME data, the CMS SignedData encapContentInfo eContentType MUST include the id-data object identifier (OID), and the media type MUST be stored in the SignedData encapContentInfo eContent OCTET STRING (unless the sending agent is using multipart/signed, in which case the eContent is absent, per Section 3.5.3 of this document). As another example, when applying encryption to MIME data, the CMS EnvelopedData encryptedContentInfo contentType MUST include the id-data OID and the encrypted MIME content MUST be stored in the EnvelopedData encryptedContentInfo encryptedContent OCTET STRING.
Sending agents MUST use the SignedData content type to apply a digital signature to a message or, in a degenerate case where there is no signature information, to convey certificates. Applying a signature to a message provides authentication, message integrity, and non-repudiation of origin.
This content type is used to apply data confidentiality to a message. In order to distribute the symmetric key, a sender needs to have access to a public key for each intended message recipient to use this service.
This content type is used to apply data confidentiality and message integrity to a message. This content type does not provide authentication or non-repudiation. In order to distribute the symmetric key, a sender needs to have access to a public key for each intended message recipient to use this service.
This content type is used to apply data compression to a message. This content type does not provide authentication, message integrity, non-repudiation, or data confidentiality; it is only used to reduce the message's size.
See Section 3.7 for further guidance on the use of this type in conjunction with other CMS types.
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The SignerInfo type allows the inclusion of unsigned and signed attributes along with a signature. These attributes can be required for the processing of messages (e.g., message digest), information the signer supplied (e.g., SMIME capabilities) that should be processed, or attributes that are not relevant to the current situation (e.g., mlExpansionHistory [RFC2634] for mail viewers).
Receiving agents MUST be able to handle zero or one instance of each of the signed attributes listed here. Sending agents SHOULD generate one instance of each of the following signed attributes in each S/MIME message:
Further, receiving agents SHOULD be able to handle zero or one instance of the signingCertificate and signingCertificateV2 signed attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3 of RFC 5035 [ESS], respectively.
Sending agents SHOULD generate one instance of the signingCertificate or signingCertificateV2 signed attribute in each SignerInfo structure.
Additional attributes and values for these attributes might be defined in the future. Receiving agents SHOULD handle attributes or values that they do not recognize in a graceful manner.
Interactive sending agents that include signed attributes that are not listed here SHOULD display those attributes to the user, so that the user is aware of all of the data being signed.
The signingTime attribute is used to convey the time that a message was signed. The time of signing will most likely be created by a signer and therefore is only as trustworthy as that signer.
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Sending agents MUST encode signing time through the year 2049 as UTCTime; signing times in 2050 or later MUST be encoded as GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST interpret the year field (YY) as follows:
If YY is greater than or equal to 50, the year is interpreted as 19YY; if YY is less than 50, the year is interpreted as 20YY.
Receiving agents MUST be able to process signingTime attributes that are encoded in either UTCTime or GeneralizedTime.
The SMIMECapabilities attribute includes signature algorithms (such as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128 CBC"), authenticated symmetric algorithms (such as "AES-128 GCM"), and key encipherment algorithms (such as "rsaEncryption"). The presence of an SMIMECapability attribute containing an algorithm implies that the sender can deal with the algorithm as well as understand the ASN.1 structures associated with that algorithm. There are also several identifiers that indicate support for other optional features such as binary encoding and compression. The SMIMECapabilities attribute was designed to be flexible and extensible so that, in the future, a means of identifying other capabilities and preferences such as certificates can be added in a way that will not cause current clients to break.
If present, the SMIMECapabilities attribute MUST be a SignedAttribute. CMS defines SignedAttributes as a SET OF Attribute. The SignedAttributes in a signerInfo MUST include a single instance of the SMIMECapabilities attribute. CMS defines the ASN.1 syntax for Attribute to include attrValues SET OF AttributeValue. An SMIMECapabilities attribute MUST only include a single instance of AttributeValue. If a signature is detected as violating these requirements, the signature SHOULD be treated as failing.
The semantics of the SMIMECapabilities attribute specify a partial list as to what the client announcing the SMIMECapabilities can support. A client does not have to list every capability it supports, and it need not list all its capabilities so that the capabilities list doesn't get too long. In an SMIMECapabilities attribute, the OIDs are listed in order of their preference but SHOULD be separated logically along the lines of their categories (signature algorithms, symmetric algorithms, key encipherment algorithms, etc.).
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The structure of the SMIMECapabilities attribute is to facilitate simple table lookups and binary comparisons in order to determine matches. For instance, the encoding for the SMIMECapability for sha256WithRSAEncryption includes rather than omits the NULL parameter. Because of the requirement for identical encoding, individuals documenting algorithms to be used in the SMIMECapabilities attribute SHOULD explicitly document the correct byte sequence for the common cases.
For any capability, the associated parameters for the OID MUST specify all of the parameters necessary to differentiate between two instances of the same algorithm.
The same OID that is used to identify an algorithm SHOULD also be used in the SMIMECapability for that algorithm. There are cases where a single OID can correspond to multiple algorithms. In these cases, a single algorithm MUST be assigned to the SMIMECapability using that OID. Additional OIDs from the smimeCapabilities OID tree are then allocated for the other algorithms usages. For instance, in an earlier specification, rsaEncryption was ambiguous because it could refer to either a signature algorithm or a key encipherment algorithm. In the event that an OID is ambiguous, it needs to be arbitrated by the maintainer of the registered SMIMECapabilities list as to which type of algorithm will use the OID, and a new OID MUST be allocated under the smimeCapabilities OID to satisfy the other use of the OID.
The registered SMIMECapabilities list specifies the parameters for OIDs that need them, most notably key lengths in the case of variable-length symmetric ciphers. In the event that there are no differentiating parameters for a particular OID, the parameters MUST be omitted and MUST NOT be encoded as NULL. Additional values for the SMIMECapabilities attribute might be defined in the future. Receiving agents MUST handle an SMIMECapabilities object that has values that it does not recognize in a graceful manner.
Section 2.7.1 explains a strategy for caching capabilities.
The encryption key preference attribute allows the signer to unambiguously describe which of the signer's certificates has the signer's preferred encryption key. This attribute is designed to enhance behavior for interoperating with those clients that use separate keys for encryption and signing. This attribute is used to convey to anyone viewing the attribute which of the listed certificates is appropriate for encrypting a session key for future encrypted messages.
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If present, the SMIMEEncryptionKeyPreference attribute MUST be a SignedAttribute. CMS defines SignedAttributes as a SET OF Attribute. The SignedAttributes in a signerInfo MUST include a single instance of the SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax for Attribute to include attrValues SET OF AttributeValue. An SMIMEEncryptionKeyPreference attribute MUST only include a single instance of AttributeValue. If a signature is detected as violating these requirements, the signature SHOULD be treated as failing.
The sending agent SHOULD include the referenced certificate in the set of certificates included in the signed message if this attribute is used. The certificate MAY be omitted if it has been previously made available to the receiving agent. Sending agents SHOULD use this attribute if the commonly used or preferred encryption certificate is not the same as the certificate used to sign the message.
Receiving agents SHOULD store the preference data if the signature on the message is valid and the signing time is greater than the currently stored value. (As with the SMIMECapabilities, the clock skew SHOULD be checked and the data not used if the skew is too great.) Receiving agents SHOULD respect the sender's encryption key preference attribute if possible. This, however, represents only a preference, and the receiving agent can use any certificate in replying to the sender that is valid.
Section 2.7.1 explains a strategy for caching preference data.
2.5.3.1. Selection of Recipient Key Management Certificate
In order to determine the key management certificate to be used when sending a future CMS EnvelopedData message for a particular recipient, the following steps SHOULD be followed:
- If an SMIMEEncryptionKeyPreference attribute is found in a SignedData object received from the desired recipient, this identifies the X.509 certificate that SHOULD be used as the X.509 key management certificate for the recipient.
- If an SMIMEEncryptionKeyPreference attribute is not found in a SignedData object received from the desired recipient, the set of X.509 certificates SHOULD be searched for an X.509 certificate with the same subject name as the signer of an X.509 certificate that can be used for key management.
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- Or, use some other method of determining the user's key management key. If an X.509 key management certificate is not found, then encryption cannot be done with the signer of the message. If multiple X.509 key management certificates are found, the S/MIME agent can make an arbitrary choice between them.
S/MIME v4.0 implementations MUST support both issuerAndSerialNumber and subjectKeyIdentifier. Messages that use the subjectKeyIdentifier choice cannot be read by S/MIME v2 clients.
It is important to understand that some certificates use a value for subjectKeyIdentifier that is not suitable for uniquely identifying a certificate. Implementations MUST be prepared for multiple certificates for potentially different entities to have the same value for subjectKeyIdentifier and MUST be prepared to try each matching certificate during signature verification before indicating an error condition.
When a sending agent creates an encrypted message, it has to decide which type of encryption to use. The decision process involves using information garnered from the capabilities lists included in messages received from the recipient, as well as out-of-band information such as private agreements, user preferences, legal restrictions, and so on.
Section 2.5.2 defines a method by which a sending agent can optionally announce, among other things, its decrypting capabilities in its order of preference. The following method for processing and remembering the encryption capabilities attribute in incoming signed messages SHOULD be used.
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- If the receiving agent has not yet created a list of capabilities for the sender's public key, then, after verifying the signature on the incoming message and checking the timestamp, the receiving agent SHOULD create a new list containing at least the signing time and the symmetric capabilities.
- If such a list already exists, the receiving agent SHOULD verify that the signing time in the incoming message is greater than the signing time stored in the list and that the signature is valid. If so, the receiving agent SHOULD update both the signing time and capabilities in the list. Values of the signing time that lie far in the future (that is, a greater discrepancy than any reasonable clock skew), or a capabilities list in messages whose signature could not be verified, MUST NOT be accepted.
The list of capabilities SHOULD be stored for future use in creating messages.
Before sending a message, the sending agent MUST decide whether it is willing to use weak encryption for the particular data in the message. If the sending agent decides that weak encryption is unacceptable for this data, then the sending agent MUST NOT use a weak algorithm. The decision to use or not use weak encryption overrides any other decision in this section about which encryption algorithm to use.
Sections 2.7.1.1 and 2.7.1.2 describe the decisions a sending agent SHOULD use when choosing which type of encryption will be applied to a message. These rules are ordered, so the sending agent SHOULD make its decision in the order given.
If the sending agent has received a set of capabilities from the recipient for the message the agent is about to encrypt, then the sending agent SHOULD use that information by selecting the first capability in the list (that is, the capability most preferred by the intended recipient) that the sending agent knows how to encrypt. The sending agent SHOULD use one of the capabilities in the list if the agent reasonably expects the recipient to be able to decrypt the message.
2.7.1.2. Rule 2: Unknown Capabilities, Unknown Version of S/MIME
If the following two conditions are met, the sending agent SHOULD use AES-256 GCM, as AES-256 GCM is a stronger algorithm and is required by S/MIME v4.0:
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- The sending agent has no knowledge of the encryption capabilities of the recipient.
- The sending agent has no knowledge of the version of S/MIME used or supported by the recipient.
If the sending agent chooses not to use AES-256 GCM in this step, given the presumption is that a client implementing AES-GCM would do both AES-256 and AES-128, it SHOULD use AES-128 CBC.
Algorithms such as RC2 are considered to be weak encryption algorithms. Algorithms such as TripleDES are not state of the art and are considered to be weaker algorithms than AES. A sending agent that is controlled by a human SHOULD allow a human sender to determine the risks of sending data using a weaker encryption algorithm before sending the data, and possibly allow the human to use a stronger encryption algorithm such as AES GCM or AES CBC even if there is a possibility that the recipient will not be able to process that algorithm.
If a sending agent is composing an encrypted message to a group of recipients where the encryption capabilities of some of the recipients do not overlap, the sending agent is forced to send more than one message. Please note that if the sending agent chooses to send a message encrypted with a strong algorithm and then send the same message encrypted with a weak algorithm, someone watching the communications channel could learn the contents of the strongly encrypted message simply by decrypting the weakly encrypted message.
This section describes the S/MIME message formats and how they are created. S/MIME messages are a combination of MIME bodies and CMS content types. Several media types as well as several CMS content types are used. The data to be secured is always a canonical MIME entity. The MIME entity and other data, such as certificates and algorithm identifiers, are given to CMS processing facilities that produce a CMS object. Finally, the CMS object is wrapped in MIME. The "Enhanced Security Services for S/MIME" documents [ESS] provide descriptions of how nested, secured S/MIME messages are formatted. ESS provides a description of how a triple-wrapped S/MIME message is formatted using multipart/signed and application/pkcs7-mime for the signatures.
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S/MIME provides one format for enveloped-only data, several formats for signed-only data, and several formats for signed and enveloped data. Several formats are required to accommodate several environments -- in particular, for signed messages. The criteria for choosing among these formats are also described.
Anyone reading this section is expected to understand MIME as described in [MIME-SPEC] and [RFC1847].
3.1. Preparing the MIME Entity for Signing, Enveloping, or Compressing
S/MIME is used to secure MIME entities. A MIME message is composed of a MIME header and a MIME body. A body can consist of a single MIME entity or a tree of MIME entities (rooted with a multipart). S/MIME can be used to secure either a single MIME entity or a tree of MIME entities. These entities can be in locations other than the root. S/MIME can be applied multiple times to different entities in a single message. A MIME entity that is the whole message includes only the MIME message headers and MIME body and does not include the rfc822 header. Note that S/MIME can also be used to secure MIME entities used in applications other than Internet mail. For cases where protection of the rfc822 header is required, the use of the message/rfc822 media type is explained later in this section.
The MIME entity that is secured and described in this section can be thought of as the "inside" MIME entity. That is, it is the "innermost" object in what is possibly a larger MIME message. Processing "outside" MIME entities into CMS EnvelopedData, CompressedData, and AuthEnvelopedData content types is described in Sections 3.2 and 3.5. Other documents define additional CMS content types; those documents should be consulted for processing those CMS content types.
The procedure for preparing a MIME entity is given in [MIME-SPEC]. The same procedure is used here with some additional restrictions when signing. The description of the procedures from [MIME-SPEC] is repeated here, but it is suggested that the reader refer to those documents for the exact procedures. This section also describes additional requirements.
A single procedure is used for creating MIME entities that are to have any combination of signing, enveloping, and compressing applied. Some additional steps are recommended to defend against known corruptions that can occur during mail transport that are of particular importance for clear-signing using the multipart/signed format. It is recommended that these additional steps be performed on enveloped messages, or signed and enveloped messages, so that the messages can be forwarded to any environment without modification.
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These steps are descriptive rather than prescriptive. The implementer is free to use any procedure as long as the result is the same.
Step 1. The MIME entity is prepared according to local conventions.
Step 2. The leaf parts of the MIME entity are converted to canonical form.
Step 3. Appropriate transfer encoding is applied to the leaves of the MIME entity.
When an S/MIME message is received, the security services on the message are processed, and the result is the MIME entity. That MIME entity is typically passed to a MIME-capable user agent where it is further decoded and presented to the user or receiving application.
In order to protect outer, non-content-related message header fields (for instance, the "Subject", "To", "From", and "Cc" fields), the sending client MAY wrap a full MIME message in a message/rfc822 wrapper in order to apply S/MIME security services to these header fields. It is up to the receiving client to decide how to present this "inner" header along with the unprotected "outer" header. Given the security difference between headers, it is RECOMMENDED that the receiving client provide a distinction between header fields, depending on where they are located.
When an S/MIME message is received, if the top-level protected MIME entity has a Content-Type of message/rfc822, it can be assumed that the intent was to provide header protection. This entity SHOULD be presented as the top-level message, taking into account header-merging issues as previously discussed.
Each MIME entity MUST be converted to a canonical form that is uniquely and unambiguously representable in the environment where the signature is created and the environment where the signature will be verified. MIME entities MUST be canonicalized for enveloping and compressing as well as signing.
The exact details of canonicalization depend on the actual media type and subtype of an entity and are not described here. Instead, the standard for the particular media type SHOULD be consulted. For example, canonicalization of type text/plain is different from canonicalization of audio/basic. Other than text types, most types have only one representation, regardless of computing platform or environment, that can be considered their canonical representation.
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In general, canonicalization will be performed by the non-security part of the sending agent rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is often represented differently in different environments. MIME entities of major type "text" MUST have both their line endings and character set canonicalized. The line ending MUST be the pair of characters <CR><LF>, and the charset SHOULD be a registered charset [CHARSETS]. The details of the canonicalization are specified in [MIME-SPEC].
Note that some charsets such as ISO-2022 have multiple representations for the same characters. When preparing such text for signing, the canonical representation specified for the charset MUST be used.
When generating any of the secured MIME entities below, except the signing using the multipart/signed format, no transfer encoding is required at all. S/MIME implementations MUST be able to deal with binary MIME objects. If no Content-Transfer-Encoding header field is present, the transfer encoding is presumed to be 7BIT.
As a rule, S/MIME implementations SHOULD use transfer encoding as described in Section 3.1.3 for all MIME entities they secure. The reason for securing only 7-bit MIME entities, even for enveloped data that is not exposed to the transport, is that it allows the MIME entity to be handled in any environment without changing it. For example, a trusted gateway might remove the envelope, but not the signature, of a message, and then forward the signed message on to the end recipient so that they can verify the signatures directly. If the transport internal to the site is not 8-bit clean, such as on a wide-area network with a single mail gateway, verifying the signature will not be possible unless the original MIME entity was only 7-bit data.
In the case where S/MIME implementations can determine that all intended recipients are capable of handling inner (all but the outermost) binary MIME objects, implementations SHOULD use binary encoding as opposed to a 7-bit-safe transfer encoding for the inner entities. The use of a 7-bit-safe encoding (such as base64) unnecessarily expands the message size. Implementations MAY determine that recipient implementations are capable of handling inner binary MIME entities by (1) interpreting the id-cap-preferBinaryInside SMIMECapabilities attribute, (2) prior agreement, or (3) other means.
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If one or more intended recipients are unable to handle inner binary MIME objects or if this capability is unknown for any of the intended recipients, S/MIME implementations SHOULD use transfer encoding as described in Section 3.1.3 for all MIME entities they secure.
3.1.3. Transfer Encoding for Signing Using multipart/signed
If a multipart/signed entity is ever to be transmitted over the standard Internet SMTP infrastructure or other transport that is constrained to 7-bit text, it MUST have transfer encoding applied so that it is represented as 7-bit text. MIME entities that are already 7-bit data need no transfer encoding. Entities such as 8-bit text and binary data can be encoded with quoted-printable or base64 transfer encoding.
The primary reason for the 7-bit requirement is that the Internet mail transport infrastructure cannot guarantee transport of 8-bit or binary data. Even though many segments of the transport infrastructure now handle 8-bit and even binary data, it is sometimes not possible to know whether the transport path is 8-bit clean. If a mail message with 8-bit data were to encounter a message transfer agent that cannot transmit 8-bit or binary data, the agent has three options, none of which are acceptable for a clear-signed message:
- The agent could change the transfer encoding; this would invalidate the signature.
- The agent could transmit the data anyway, which would most likely result in the 8th bit being corrupted; this too would invalidate the signature.
- The agent could return the message to the sender.
[RFC1847] prohibits an agent from changing the transfer encoding of the first part of a multipart/signed message. If a compliant agent that cannot transmit 8-bit or binary data encountered a multipart/signed message with 8-bit or binary data in the first part, it would have to return the message to the sender as undeliverable.
This example shows a multipart/mixed message with full transfer encoding. This message contains a text part and an attachment. The sample message text includes characters that are not ASCII and thus need to be transfer encoded. Though not shown here, the end of each line is <CR><LF>. The line ending of the MIME headers, the text, and the transfer-encoded parts all MUST be <CR><LF>.
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Note that this example is not an example of an S/MIME message.
It's generally a good idea to encode lines that begin with From=20because some mail transport agents will insert a greater-than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20 trailing whitespace that occurs on lines in order to ensure =20 that the message signature is not invalidated when passing =20 a gateway that modifies such whitespace (like BITNET). =20
The application/pkcs7-mime media type is used to carry CMS content types, including EnvelopedData, SignedData, and CompressedData. The details of constructing these entities are described in subsequent sections. This section describes the general characteristics of the application/pkcs7-mime media type.
The carried CMS object always contains a MIME entity that is prepared as described in Section 3.1 if the eContentType is id-data. Other contents MAY be carried when the eContentType contains different values. See [ESS] for an example of this with signed receipts.
Since CMS content types are binary data, in most cases base64 transfer encoding is appropriate -- in particular, when used with SMTP transport. The transfer encoding used depends on the transport
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through which the object is to be sent and is not a characteristic of the media type.
Note that this discussion refers to the transfer encoding of the CMS object or "outside" MIME entity. It is completely distinct from, and unrelated to, the transfer encoding of the MIME entity secured by the CMS object -- the "inside" object, which is described in Section 3.1.
Because there are several types of application/pkcs7-mime objects, a sending agent SHOULD do as much as possible to help a receiving agent know about the contents of the object without forcing the receiving agent to decode the ASN.1 for the object. The Content-Type header field of all application/pkcs7-mime objects SHOULD include the optional "smime-type" parameter, as described in the following sections.
For application/pkcs7-mime, sending agents SHOULD emit the optional "name" parameter to the Content-Type field for compatibility with older systems. Sending agents SHOULD also emit the optional Content-Disposition field [RFC2183] with the "filename" parameter. If a sending agent emits the above parameters, the value of the parameters SHOULD be a filename with the appropriate extension:
In addition, the filename SHOULD be limited to eight characters followed by a three-letter extension. The eight-character filename base can be any distinct name; the use of the filename base "smime" SHOULD be used to indicate that the MIME entity is associated with S/MIME.
Including a filename serves two purposes. It facilitates easier use of S/MIME objects as files on disk. It also can convey type information across gateways. When a MIME entity of type application/pkcs7-mime (for example) arrives at a gateway that has no special knowledge of S/MIME, it will default the entity's media type to application/octet-stream and treat it as a generic attachment, thus losing the type information. However, the suggested filename
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for an attachment is often carried across a gateway. This often allows the receiving systems to determine the appropriate application to hand the attachment off to -- in this case, a standalone S/MIME processing application. Note that this mechanism is provided as a convenience for implementations in certain environments. A proper S/MIME implementation MUST use the media types and MUST NOT rely on the file extensions.
The application/pkcs7-mime content type defines the optional "smime-type" parameter. The intent of this parameter is to convey details about the security applied (signed or enveloped) along with information about the contained content. This specification defines the following smime-types.
In order for consistency to be obtained with future specifications, the following guidelines SHOULD be followed when assigning a new smime-type parameter.
1. If both signing and encryption can be applied to the content, then three values for smime-type SHOULD be assigned: "signed-*", "authEnv-*", and "enveloped-*". If one operation can be assigned, then this can be omitted. Thus, since "certs-only" can only be signed, "signed-" is omitted.
2. A common string for a content OID SHOULD be assigned. We use "data" for the id-data content OID when MIME is the inner content.
3. If no common string is assigned, then the common string of "OID.<oid>" is recommended (for example, "OID.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).
It is explicitly intended that this field be a suitable hint for mail client applications to indicate whether a message is "signed", "authEnveloped", or "enveloped" without having to tunnel into the CMS payload.
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A registry for additional smime-type parameter values has been defined in [RFC7114].
This section describes the format for enveloping a MIME entity without signing it. It is important to note that sending enveloped but not signed messages does not provide for data integrity. The "enveloped-only" structure does not support authenticated symmetric algorithms. Use the "authenticated enveloped" structure for these algorithms. Thus, it is possible to replace ciphertext in such a way that the processed message will still be valid, but the meaning can be altered.
Step 1. The MIME entity to be enveloped is prepared according to Section 3.1.
Step 2. The MIME entity and other required data are processed into a CMS object of type EnvelopedData. In addition to encrypting a copy of the content-encryption key (CEK) for each recipient, a copy of the CEK SHOULD be encrypted for the originator and included in the EnvelopedData (see [RFC5652], Section 6).
Step 3. The EnvelopedData object is wrapped in a CMS ContentInfo object.
Step 4. The ContentInfo object is inserted into an application/pkcs7-mime MIME entity.
The smime-type parameter for enveloped-only messages is "enveloped-data". The file extension for this type of message is ".p7m".
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3.4. Creating an Authenticated Enveloped-Only Message
This section describes the format for enveloping a MIME entity without signing it. Authenticated enveloped messages provide confidentiality and data integrity. It is important to note that sending authenticated enveloped messages does not provide for proof of origination when using S/MIME. It is possible for a third party to replace ciphertext in such a way that the processed message will still be valid, but the meaning can be altered. However, this is substantially more difficult than it is for an enveloped-only message, as the algorithm does provide a level of authentication. Any recipient for whom the message is encrypted can replace it without detection.
Step 1. The MIME entity to be enveloped is prepared according to Section 3.1.
Step 2. The MIME entity and other required data are processed into a CMS object of type AuthEnvelopedData. In addition to encrypting a copy of the CEK for each recipient, a copy of the CEK SHOULD be encrypted for the originator and included in the AuthEnvelopedData (see [RFC5083]).
Step 3. The AuthEnvelopedData object is wrapped in a CMS ContentInfo object.
Step 4. The ContentInfo object is inserted into an application/pkcs7-mime MIME entity.
The smime-type parameter for authenticated enveloped-only messages is "authEnveloped-data". The file extension for this type of message is ".p7m".
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There are no hard-and-fast rules as to when a particular signed-only format is chosen. It depends on the capabilities of all the receivers and the relative importance of receivers with S/MIME facilities being able to verify the signature versus the importance of receivers without S/MIME software being able to view the message.
Messages signed using the multipart/signed format can always be viewed by the receiver whether or not they have S/MIME software. They can also be viewed whether they are using a MIME-native user agent or they have messages translated by a gateway. In this context, "be viewed" means the ability to process the message essentially as if it were not a signed message, including any other MIME structure the message might have.
Messages signed using the SignedData format cannot be viewed by a recipient unless they have S/MIME facilities. However, the SignedData format protects the message content from being changed by benign intermediate agents. Such agents might do line wrapping or content-transfer encoding changes that would break the signature.
3.5.2. Signing Using application/pkcs7-mime with SignedData
This signing format uses the application/pkcs7-mime media type. The steps to create this format are as follows:
Step 1. The MIME entity is prepared according to Section 3.1.
Step 2. The MIME entity and other required data are processed into a CMS object of type SignedData.
Step 3. The SignedData object is wrapped in a CMS ContentInfo object.
Step 4. The ContentInfo object is inserted into an application/pkcs7-mime MIME entity.
The smime-type parameter for messages using application/pkcs7-mime with SignedData is "signed-data". The file extension for this type of message is ".p7m".
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This format is a clear-signing format. Recipients without any S/MIME or CMS processing facilities are able to view the message. It makes use of the multipart/signed media type described in [RFC1847]. The multipart/signed media type has two parts. The first part contains the MIME entity that is signed; the second part contains the "detached signature" CMS SignedData object in which the encapContentInfo eContent field is absent.
3.5.3.1. The application/pkcs7-signature Media Type
This media type always contains a CMS ContentInfo containing a single CMS object of type SignedData. The SignedData encapContentInfo eContent field MUST be absent. The signerInfos field contains the signatures for the MIME entity.
The file extension for signed-only messages using application/pkcs7-signature is ".p7s".
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Step 1. The MIME entity to be signed is prepared according to Section 3.1, taking special care for clear-signing.
Step 2. The MIME entity is presented to CMS processing in order to obtain an object of type SignedData in which the encapContentInfo eContent field is absent.
Step 3. The MIME entity is inserted into the first part of a multipart/signed message with no processing other than that described in Section 3.1.
Step 4. Transfer encoding is applied to the "detached signature" CMS SignedData object, and it is inserted into a MIME entity of type application/pkcs7-signature.
Step 5. The MIME entity of the application/pkcs7-signature is inserted into the second part of the multipart/signed entity.
The multipart/signed Content-Type has two required parameters: the protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note that quotation marks are required around the protocol parameter because MIME requires that the "/" character in the parameter value MUST be quoted.
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The micalg parameter allows for one-pass processing when the signature is being verified. The value of the micalg parameter is dependent on the message digest algorithm(s) used in the calculation of the Message Integrity Check. If multiple message digest algorithms are used, they MUST be separated by commas per [RFC1847]. The values to be placed in the micalg parameter SHOULD be from the following:
Algorithm Value Used ----------------------------------------------------------- MD5* md5 SHA-1* sha-1 SHA-224 sha-224 SHA-256 sha-256 SHA-384 sha-384 SHA-512 sha-512 Any other (defined separately in the algorithm profile or "unknown" if not defined)
*Note: MD5 and SHA-1 are historical and no longer considered secure. See Appendix B for details.
(Historical note: Some early implementations of S/MIME emitted and expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.) Receiving agents SHOULD be able to recover gracefully from a micalg parameter value that they do not recognize. Future values for this parameter will be taken from the IANA "Hash Function Textual Names" registry.
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This is some sample content. ------=_NextBoundary____Fri,_06_Sep_2002_00:25:21 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s
This section describes the format for compressing a MIME entity. Please note that versions of S/MIME prior to version 3.1 did not specify any use of CompressedData and will not recognize it. The use of a capability to indicate the ability to receive CompressedData is described in [RFC3274] and is the preferred method for compatibility.
Step 1. The MIME entity to be compressed is prepared according to Section 3.1.
Step 2. The MIME entity and other required data are processed into a CMS object of type CompressedData.
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Step 3. The CompressedData object is wrapped in a CMS ContentInfo object.
Step 4. The ContentInfo object is inserted into an application/pkcs7-mime MIME entity.
The smime-type parameter for compressed-only messages is "compressed-data". The file extension for this type of message is ".p7z".
The signed-only, enveloped-only, and compressed-only MIME formats can be nested. This works because these formats are all MIME entities that encapsulate other MIME entities.
An S/MIME implementation MUST be able to receive and process arbitrarily nested S/MIME within reasonable resource limits of the recipient computer.
It is possible to apply any of the signing, encrypting, and compressing operations in any order. It is up to the implementer and the user to choose. When signing first, the signatories are then securely obscured by the enveloping. When enveloping first, the signatories are exposed, but it is possible to verify signatures without removing the enveloping. This can be useful in an environment where automatic signature verification is desired, as no private key material is required to verify a signature.
There are security ramifications related to choosing whether to sign first or encrypt first. A recipient of a message that is encrypted and then signed can validate that the encrypted block was unaltered but cannot determine any relationship between the signer and the unencrypted contents of the message. A recipient of a message that is signed and then encrypted can assume that the signed message itself has not been altered but that a careful attacker could have changed the unauthenticated portions of the encrypted message.
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When using compression, keep the following guidelines in mind:
- Compression of encrypted data that is transferred as binary data is discouraged, since it will not yield significant compression. Encrypted data that is transferred as base64-encoded data could benefit as well.
- If a lossy compression algorithm is used with signing, you will need to compress first, then sign.
The certificate management message or MIME entity is used to transport certificates and/or Certificate Revocation Lists (CRLs), such as in response to a registration request.
Step 1. The certificates and/or CRLs are made available to the CMS generating process that creates a CMS object of type SignedData. The SignedData encapContentInfo eContent field MUST be absent, and the signerInfos field MUST be empty.
Step 2. The SignedData object is wrapped in a CMS ContentInfo object.
Step 3. The ContentInfo object is enclosed in an application/pkcs7-mime MIME entity.
The smime-type parameter for a certificate management message is "certs-only". The file extension for this type of message is ".p7c".
A sending agent that signs messages MUST have a certificate for the signature so that a receiving agent can verify the signature. There are many ways of getting certificates, such as through an exchange with a certification authority, through a hardware token or diskette, and so on.
S/MIME v2 [SMIMEv2] specified a method for "registering" public keys with certificate authorities using an application/pkcs10 body part. Since that time, the IETF PKIX Working Group has developed other methods for requesting certificates. However, S/MIME v4.0 does not require a particular certificate request mechanism.
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Because S/MIME takes into account interoperation in non-MIME environments, several different mechanisms are employed to carry the type information, and it becomes a bit difficult to identify S/MIME messages. The following table lists criteria for determining whether or not a message is an S/MIME message. A message is considered an S/MIME message if it matches any of the criteria listed below.
The file suffix in the table below comes from the "name" parameter in the Content-Type header field or the "filename" parameter in the Content-Disposition header field. The MIME parameters that carry the file suffix are not listed below.
Media Type Parameters File Suffix --------------------------------------------------------------------- application/pkcs7-mime N/A N/A
A receiving agent MUST provide some certificate retrieval mechanism in order to gain access to certificates for recipients of digital envelopes. This specification does not cover how S/MIME agents handle certificates -- only what they do after a certificate has been validated or rejected. S/MIME certificate issues are covered in [RFC5750].
At a minimum, for initial S/MIME deployment, a user agent could automatically generate a message to an intended recipient requesting that recipient's certificate in a signed return message. Receiving and sending agents SHOULD also provide a mechanism to allow a user to "store and protect" certificates for correspondents in such a way as to guarantee their later retrieval.
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All key pairs MUST be generated from a good source of non-deterministic random input [RFC4086], and the private key MUST be protected in a secure fashion.
An S/MIME user agent MUST NOT generate asymmetric keys less than 2048 bits for use with an RSA signature algorithm.
For 2048-bit through 4096-bit RSA with SHA-256, see [RFC5754] and [FIPS186-4]. The first reference provides the signature algorithm's OID, and the second provides the signature algorithm's definition.
For RSASSA-PSS with SHA-256, see [RFC4056]. For RSAES-OAEP, see [RFC3560].
This section (1) updates the media type registrations for application/pkcs7-mime and application/pkcs7-signature to refer to this document as opposed to RFC 5751, (2) adds authEnveloped-data to the list of values for smime-type, and (3) updates references from RFC 5751 to this document in general.
Note that other documents can define additional media types for S/MIME.
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Applications that use this media type: Security applications
Fragment identifier considerations: N/A
Additional information: Deprecated alias names for this type: N/A Magic number(s): N/A File extensions(s): See Section 3.2.1 of this document Macintosh file type code(s): N/A
Person & email address to contact for further information: The IESG <iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: NONE
Author: Sean Turner
Change Controller: LAMPS working group delegated from the IESG
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Applications that use this media type: Security applications
Fragment identifier considerations: N/A
Additional information: Deprecated alias names for this type: N/A Magic number(s): N/A File extensions(s): See Section 3.2.1 of this document Macintosh file type code(s): N/A
Person & email address to contact for further information: The IESG <iesg@ietf.org>
Intended usage: COMMON
Restrictions on usage: N/A
Author: Sean Turner
Change Controller: LAMPS working group delegated from the IESG
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Cryptographic algorithms will be broken or weakened over time. Implementers and users need to check that the cryptographic algorithms listed in this document continue to provide the expected level of security. The IETF from time to time may issue documents dealing with the current state of the art. For example:
- The Million Message Attack described in RFC 3218 [RFC3218].
- The Diffie-Hellman "small-subgroup" attacks described in RFC 2785 [RFC2785].
- The attacks against hash algorithms described in RFC 4270 [RFC4270].
This specification uses Public-Key Cryptography technologies. It is assumed that the private key is protected to ensure that it is not accessed or altered by unauthorized parties.
It is impossible for most people or software to estimate the value of a message's content. Further, it is impossible for most people or software to estimate the actual cost of recovering an encrypted message's content that is encrypted with a key of a particular size. Further, it is quite difficult to determine the cost of a failed decryption if a recipient cannot process a message's content. Thus, choosing between different key sizes (or choosing whether to just use plaintext) is also impossible for most people or software. However, decisions based on these criteria are made all the time, and therefore this specification gives a framework for using those estimates in choosing algorithms.
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The choice of 2048 bits as an RSA asymmetric key size in this specification is based on the desire to provide at least 100 bits of security. The key sizes that must be supported to conform to this specification seem appropriate for the Internet, based on [RFC3766]. Of course, there are environments, such as financial and medical systems, that may select different key sizes. For this reason, an implementation MAY support key sizes beyond those recommended in this specification.
Receiving agents that validate signatures and sending agents that encrypt messages need to be cautious of cryptographic processing usage when validating signatures and encrypting messages using keys larger than those mandated in this specification. An attacker could send certificates with keys that would result in excessive cryptographic processing -- for example, keys larger than those mandated in this specification, as such keys could swamp the processing element. Agents that use such keys without first validating the certificate to a trust anchor are advised to have some sort of cryptographic resource management system to prevent such attacks.
Some cryptographic algorithms such as RC2 offer little actual security over sending plaintext. Other algorithms such as TripleDES provide security but are no longer considered to be state of the art. S/MIME requires the use of current state-of-the-art algorithms such as AES and provides the ability to announce cryptographic capabilities to parties with whom you communicate. This allows the sender to create messages that can use the strongest common encryption algorithm. Using algorithms such as RC2 is never recommended unless the only alternative is no cryptography.
RSA and DSA keys of less than 2048 bits are now considered by many experts to be cryptographically insecure (due to advances in computing power) and should no longer be used to protect messages. Such keys were previously considered secure, so processing previously received signed and encrypted mail will often result in the use of weak keys. Implementations that wish to support previous versions of S/MIME or process old messages need to consider the security risks that result from smaller key sizes (e.g., spoofed messages) versus the costs of denial of service. If an implementation supports verification of digital signatures generated with RSA and DSA keys of less than 1024 bits, it MUST warn the user. Implementers should consider providing different warnings for newly received messages and previously stored messages. Server implementations (e.g., secure mail list servers) where user warnings are not appropriate SHOULD reject messages with weak signatures.
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Implementers SHOULD be aware that multiple active key pairs can be associated with a single individual. For example, one key pair can be used to support confidentiality, while a different key pair can be used for digital signatures.
If a sending agent is sending the same message using different strengths of cryptography, an attacker watching the communications channel might be able to determine the contents of the strongly encrypted message by decrypting the weakly encrypted version. In other words, a sender SHOULD NOT send a copy of a message using weaker cryptography than they would use for the original of the message.
Modification of the ciphertext in EnvelopedData can go undetected if authentication is not also used, which is the case when sending EnvelopedData without wrapping it in SignedData or enclosing SignedData within it. This is one of the reasons for moving from EnvelopedData to AuthEnvelopedData, as the authenticated encryption algorithms provide the authentication without needing the SignedData layer.
If an implementation is concerned about compliance with National Institute of Standards and Technology (NIST) key size recommendations, then see [SP800-57].
If messaging environments make use of the fact that a message is signed to change the behavior of message processing (examples would be running rules or UI display hints), without first verifying that the message is actually signed and knowing the state of the signature, this can lead to incorrect handling of the message. Visual indicators on messages may need to have the signature validation code checked periodically if the indicator is supposed to give information on the current status of a message.
Many people assume that the use of an authenticated encryption algorithm is all that is needed for the sender of the message to be authenticated. In almost all cases, this is not a correct statement. There are a number of preconditions that need to hold for an authenticated encryption algorithm to provide this service:
- The starting key must be bound to a single entity. The use of a group key only would allow for the statement that a message was sent by one of the entities that held the key but will not identify a specific entity.
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- The message must have exactly one sender and one recipient. Having more than one recipient would allow for the second recipient to create a message that the first recipient would believe is from the sender by stripping the second recipient from the message.
- A direct path needs to exist from the starting key to the key used as the CEK. That path needs to guarantee that no third party could have seen the resulting CEK. This means that one needs to be using an algorithm that is called a "Direct Encryption" or a "Direct Key Agreement" algorithm in other contexts. This means that the starting key is (1) used directly as the CEK or (2) used to create a secret that is then transformed into the CEK via a KDF step.
S/MIME implementations almost universally use ephemeral-static rather than static-static key agreement and do not use a shared secret for encryption. This means that the first precondition is not met. [RFC6278] defines how to use static-static key agreement with CMS, so the first precondition can be met. Currently, all S/MIME key agreement methods derive a key-encryption key (KEK) and wrap a CEK. This violates the third precondition above. New key agreement algorithms that directly created the CEK without creating an intervening KEK would need to be defined.
Even when all of the preconditions are met and origination of a message is established by the use of an authenticated encryption algorithm, users need to be aware that there is no way to prove this to a third party. This is because either of the parties can successfully create the message (or just alter the content) based on the fact that the CEK is going to be known to both parties. Thus, the origination is always built on a presumption that "I did not send this message to myself."
All of the authenticated encryption algorithms in this document use counter mode for the encryption portion of the algorithm. This means that the length of the plaintext will always be known, as the ciphertext length and the plaintext length are always the same. This information can enable passive observers to infer information based solely on the length of the message. Applications for which this is a concern need to provide some type of padding so that the length of the message does not provide this information.
When compression is used with encryption, it has the potential to provide an additional layer of security. However, care needs to be taken when designing a protocol that relies on using compression, so as not to create a compression oracle. Compression oracle attacks require an adaptive input to the process and attack the unknown
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content of a message based on the length of the compressed output. This means that no attack on the encryption key is necessarily required.
A recent paper on S/MIME and OpenPGP email security [Efail] has pointed out a number of problems with the current S/MIME specifications and how people have implemented mail clients. Due to the nature of how CBC mode operates, the modes allow for malleability of plaintexts. This malleability allows for attackers to make changes in the ciphertext and, if parts of the plaintext are known, create arbitrary blocks of plaintext. These changes can be made without the weak integrity check in CBC mode being triggered. This type of attack can be prevented by the use of an Authenticated Encryption with Associated Data (AEAD) algorithm with a more robust integrity check on the decryption process. It is therefore recommended that mail systems migrate to using AES-GCM as quickly as possible and that the decrypted content not be acted on prior to finishing the integrity check.
The other attack that is highlighted in [Efail] is due to an error in how mail clients deal with HTML and multipart/mixed messages. Clients MUST require that a text/html content type be a complete HTML document (per [RFC1866]). Clients SHOULD treat each of the different pieces of the multipart/mixed construct as being of different origins. Clients MUST treat each encrypted or signed piece of a MIME message as being of different origins both from unprotected content and from each other.
[FIPS186-4] National Institute of Standards and Technology (NIST), "Digital Signature Standard (DSS)", Federal Information Processing Standards Publication 186-4, DOI 10.6028/NIST.FIPS.186-4, July 2013, <https://nvlpubs.nist.gov/nistpubs/fips/ nist.fips.186-4.pdf>.
[Efail] Poddebniak, D., Dresen, C., Muller, J., Ising, F., Schinzel, S., Friedberger, S., Somorovsky, J., and J. Schwenk, "Efail: Breaking S/MIME and OpenPGP Email Encryption using Exfiltration Channels", UsenixSecurity 2018, August 2018, <https://www.usenix.org/system/files/conference/ usenixsecurity18/sec18-poddebniak.pdf>.
[FIPS186-2] National Institute of Standards and Technology (NIST), "Digital Signature Standard (DSS) (also with Change Notice 1)", Federal Information Processing Standards Publication 186-2, January 2000, <https://csrc.nist.gov/publications/detail/fips/186/2/ archive/2000-01-27>.
[RFC6268] Schaad, J. and S. Turner, "Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI 10.17487/RFC6268, July 2011, <https://www.rfc-editor.org/info/rfc6268>.
[RFC7905] Langley, A., Chang, W., Mavrogiannopoulos, N., Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS)", RFC 7905, DOI 10.17487/RFC7905, June 2016, <https://www.rfc-editor.org/info/rfc7905>.
[SP800-56A] National Institute of Standards and Technology (NIST), "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, <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ NIST.SP.800-56Ar2.pdf>.
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[SP800-57] National Institute of Standards and Technology (NIST), "Recommendation for Key Management - Part 1: General", NIST Special Publication 800-57 Revision 4, DOI 10.6028/NIST.SP.800-57pt1r4, January 2016, <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/ NIST.SP.800-57pt1r4.pdf>.
[TripleDES] Tuchman, W., "Hellman Presents No Shortcut Solutions to the DES", IEEE Spectrum v. 16, n. 7, pp. 40-41, DOI 10.1109/MSPEC.1979.6368160, July 1979.
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Note: The ASN.1 module contained herein is unchanged from RFC 5751 [SMIMEv2] and RFC 3851 [SMIMEv3.1], with the exception of a change to the preferBinaryInside ASN.1 comment in RFC 3851 [SMIMEv3.1]. If a module is needed that is compatible with current ASN.1 standards, one can be found in [RFC6268]. This module uses the 1988 version of ASN.1.
-- S/MIME Capabilities provides a method of broadcasting the -- symmetric capabilities understood. Algorithms SHOULD be ordered -- by preference and grouped by type.
-- The preferBinaryInside OID indicates an ability to receive -- messages with binary encoding inside the CMS wrapper. -- The preferBinaryInside attribute's value field is ABSENT.
-- -- Other Signed Attributes -- -- signingTime OBJECT IDENTIFIER ::= -- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) -- 5} -- See [CMS] for a description of how to encode the attribute -- value.
SMIMECapabilitiesParametersForRC2CBC ::= INTEGER -- (RC2 Key Length (number of bits))
END
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Over the course of updating the S/MIME specifications, the set of recommended algorithms has been modified each time the documents have been updated. This means that if a user has historic emails and their user agent has been updated to only support the current set of recommended algorithms, some of those old emails will no longer be accessible. It is strongly suggested that user agents implement some of the following algorithms for dealing with historic emails.
This appendix contains a number of references to documents that have been obsoleted or replaced. This is intentional, as the updated documents often do not have the same information in them.
The following algorithms have been called out for some level of support by previous S/MIME specifications:
- SHA-1 was dropped in [SMIMEv4]. SHA-1 is no longer considered to be secure, as it is no longer collision resistant. The IETF statement on SHA-1 can be found in [RFC6194], but it is out of date relative to the most recent advances.
- MD5 was dropped in [SMIMEv4]. MD5 is no longer considered to be secure, as it is no longer collision resistant. Details can be found in [RFC6151].
There are a number of problems with validating signatures on sufficiently historic messages. For this reason, it is strongly suggested that user agents treat these signatures differently from those on current messages. These problems include the following:
- Certification authorities are not required to keep certificates on a CRL beyond one update after a certificate has expired. This means that unless CRLs are cached as part of the message it is not always possible to check to see if a certificate has been revoked. The same problems exist with Online Certificate Status Protocol (OCSP) responses, as they may be based on a CRL rather than on the certificate database.
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- RSA and DSA keys of less than 2048 bits are now considered by many experts to be cryptographically insecure (due to advances in computing power). Such keys were previously considered secure, so the processing of historic signed messages will often result in the use of weak keys. Implementations that wish to support previous versions of S/MIME or process old messages need to consider the security risks that result from smaller key sizes (e.g., spoofed messages) versus the costs of denial of service.
[SMIMEv3.1] set the lower limit on suggested key sizes for creating and validation at 1024 bits. Prior to that, the lower bound on key sizes was 512 bits.
- Hash functions used to validate signatures on historic messages may no longer be considered to be secure (see below). While there are not currently any known practical pre-image or second pre-image attacks against MD5 or SHA-1, the fact that they are no longer considered to be collision resistant implies that the security levels of the signatures are generally considered suspect. If a message is known to be historic and it has been in the possession of the client for some time, then it might still be considered to be secure.
- The previous two issues apply to the certificates used to validate the binding of the public key to the identity that signed the message as well.
The following algorithms have been called out for some level of support by previous S/MIME specifications:
- RSA with MD5 was dropped in [SMIMEv4]. MD5 is no longer considered to be secure, as it is no longer collision resistant. Details can be found in [RFC6151].
- RSA and DSA with SHA-1 were dropped in [SMIMEv4]. SHA-1 is no longer considered to be secure, as it is no longer collision resistant. The IETF statement on SHA-1 can be found in [RFC6194], but it is out of date relative to the most recent advances.
- DSA with SHA-256 was dropped in [SMIMEv4]. DSA has been replaced by elliptic curve versions.
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As requirements for "mandatory to implement" have changed over time, some issues have been created that can cause interoperability problems:
- S/MIME v2 clients are only required to verify digital signatures using the rsaEncryption algorithm with SHA-1 or MD5 and might not implement id-dsa-with-sha1 or id-dsa at all.
- S/MIME v3 clients might only implement signing or signature verification using id-dsa-with-sha1 and might also use id-dsa as an AlgorithmIdentifier in this field.
- Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1 and rsaEncryption and might not implement sha256WithRSAEncryption.
NOTE: Receiving clients SHOULD recognize id-dsa as equivalent to id-dsa-with-sha1.
For 512-bit RSA with SHA-1, see [RFC3370] and [FIPS186-2] without Change Notice 1; for 512-bit RSA with SHA-256, see [RFC5754] and [FIPS186-2] without Change Notice 1; and for 1024-bit through 2048-bit RSA with SHA-256, see [RFC5754] and [FIPS186-2] with Change Notice 1. The first reference provides the signature algorithm's OID, and the second provides the signature algorithm's definition.
For 512-bit DSA with SHA-1, see [RFC3370] and [FIPS186-2] without Change Notice 1; for 512-bit DSA with SHA-256, see [RFC5754] and [FIPS186-2] without Change Notice 1; for 1024-bit DSA with SHA-1, see [RFC3370] and [FIPS186-2] with Change Notice 1; and for 1024-bit and above DSA with SHA-256, see [RFC5754] and [FIPS186-4]. The first reference provides the signature algorithm's OID, and the second provides the signature algorithm's definition.
The following algorithms have been called out for some level of support by previous S/MIME specifications:
- RC2/40 [RFC2268] was dropped in [SMIMEv3.2]. The algorithm is known to be insecure and, if supported, should only be used to decrypt existing email.
- DES EDE3 CBC [TripleDES], also known as "tripleDES", was dropped in [SMIMEv4]. This algorithm is removed from the list of supported algorithms because (1) it has a 64-bit block size and (2) it offers less than 128 bits of security. This algorithm should be supported only to decrypt existing email; it should not be used to encrypt new emails.
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The following algorithms have been called out for some level of support by previous S/MIME specifications:
- DH ephemeral-static mode, as specified in [RFC3370] and [SP800-57], was dropped in [SMIMEv4].
- RSA key sizes have been increased over time. Decrypting old mail with smaller key sizes is reasonable; however, new mail should use the updated key sizes.
For 1024-bit DH, see [RFC3370]. For 1024-bit and larger DH, see [SP800-56A]; regardless, use the KDF, which is from X9.42, specified in [RFC3370].
Appendix C. Moving S/MIME v2 Message Specification to Historic Status
The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 [SMIMEv3.2] specifications are backward compatible with the S/MIME v2 Message Specification [SMIMEv2], with the exception of the algorithms (dropped RC2/40 requirement and added DSA and RSASSA-PSS requirements). Therefore, RFC 2311 [SMIMEv2] was moved to Historic status.
Acknowledgements
Many thanks go out to the other authors of the S/MIME version 2 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence Lundblade, and Lisa Repka. Without v2, there wouldn't be a v3, v3.1, v3.2, or v4.0.
Some of the examples in this document were copied from [RFC4134]. Thanks go to the people who wrote and verified the examples in that document.
A number of the members of the S/MIME Working Group have also worked very hard and contributed to this document. Any list of people is doomed to omission, and for that I apologize. In alphabetical order, the following people stand out in my mind because they made direct contributions to this document:
Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway, and John Pawling.
The version 4 update to the S/MIME documents was done under the auspices of the LAMPS Working Group.
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