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RFC 8591

SIP-Based Messaging with S/MIME

Pages: 39
Proposed Standard
Updates:  326134284975
Part 1 of 2 – Pages 1 to 16
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Internet Engineering Task Force (IETF)                       B. Campbell
Request for Comments: 8591                             Standard Velocity
Updates: 3261, 3428, 4975                                     R. Housley
Category: Standards Track                                 Vigil Security
ISSN: 2070-1721                                               April 2019

                    SIP-Based Messaging with S/MIME


Mobile messaging applications used with the Session Initiation Protocol (SIP) commonly use some combination of the SIP MESSAGE method and the Message Session Relay Protocol (MSRP). While these provide mechanisms for hop-by-hop security, neither natively provides end-to-end protection. This document offers guidance on how to provide end-to-end authentication, integrity protection, and confidentiality using the Secure/Multipurpose Internet Mail Extensions (S/MIME). It updates and provides clarifications for RFCs 3261, 3428, and 4975. 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
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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
   ( 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.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Problem Statement and Scope . . . . . . . . . . . . . . . . . 5 4. Applicability of S/MIME . . . . . . . . . . . . . . . . . . . 6 4.1. Signed Messages . . . . . . . . . . . . . . . . . . . . . 6 4.2. Encrypted Messages . . . . . . . . . . . . . . . . . . . 7 4.3. Signed and Encrypted Messages . . . . . . . . . . . . . . 9 4.4. Certificate Handling . . . . . . . . . . . . . . . . . . 9 4.4.1. Subject Alternative Name . . . . . . . . . . . . . . 9 4.4.2. Certificate Validation . . . . . . . . . . . . . . . 9 5. Transfer Encoding . . . . . . . . . . . . . . . . . . . . . . 9 6. User Agent Capabilities . . . . . . . . . . . . . . . . . . . 10 7. Using S/MIME with the SIP MESSAGE Method . . . . . . . . . . 11 7.1. Size Limit . . . . . . . . . . . . . . . . . . . . . . . 11 7.2. SIP User Agent Capabilities . . . . . . . . . . . . . . . 11 7.3. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 12 8. Using S/MIME with MSRP . . . . . . . . . . . . . . . . . . . 12 8.1. Chunking . . . . . . . . . . . . . . . . . . . . . . . . 12 8.2. Streamed Data . . . . . . . . . . . . . . . . . . . . . . 13 8.3. Indicating Support for S/MIME . . . . . . . . . . . . . . 14 8.4. MSRP URIs . . . . . . . . . . . . . . . . . . . . . . . . 14 8.5. Failure Cases . . . . . . . . . . . . . . . . . . . . . . 15 9. S/MIME Interaction with Other SIP Messaging Features . . . . 15 9.1. Common Profile for Instant Messaging . . . . . . . . . . 15 9.2. Instant Message Disposition Notifications . . . . . . . . 16 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17 10.1. Signed Message in SIP including the Sender's Certificate 17 10.2. Signed Message in SIP with No Certificate . . . . . . . 19 10.3. MSRP Signed and Encrypted Message in a Single Chunk . . 20 10.4. MSRP Signed and Encrypted Message Sent in Multiple Chunks . . . . . . . . . . . . . . . . . . . . . . . . . 21 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 12. Security Considerations . . . . . . . . . . . . . . . . . . . 23 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 13.1. Normative References . . . . . . . . . . . . . . . . . . 25 13.2. Informative References . . . . . . . . . . . . . . . . . 28 Appendix A. Message Details . . . . . . . . . . . . . . . . . . 30 A.1. Signed Message . . . . . . . . . . . . . . . . . . . . . 30 A.2. Short Signed Message . . . . . . . . . . . . . . . . . . 32 A.3. Signed and Encrypted Message . . . . . . . . . . . . . . 33 A.3.1. Signed Message prior to Encryption . . . . . . . . . 33 A.3.2. Encrypted Message . . . . . . . . . . . . . . . . . . 35 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
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1. Introduction

Several mobile messaging systems use the Session Initiation Protocol (SIP) [RFC3261], typically as some combination of the SIP MESSAGE method [RFC3428] and the Message Session Relay Protocol (MSRP) [RFC4975]. For example, Voice over LTE (VoLTE) uses the SIP MESSAGE method to send Short Message Service (SMS) messages. The Open Mobile Alliance (OMA) Converged IP Messaging (CPM) system [CPM] uses the SIP MESSAGE method for short "pager mode" messages and uses MSRP for large messages and for sessions of messages. The Global System for Mobile Communications Association (GSMA) Rich Communication Services (RCS) uses CPM for messaging [RCS]. At the same time, organizations increasingly depend on mobile messaging systems to send notifications to their customers. Many of these notifications are security sensitive. For example, such notifications are commonly used for notice of financial transactions, notice of login or password change attempts, and the sending of two-factor authentication codes. Both SIP and MSRP can be used to transport any content using Multipurpose Internet Mail Extensions (MIME) formats. The SIP MESSAGE method is typically limited to short messages (under 1300 octets for the MESSAGE request). MSRP can carry arbitrarily large messages and can break large messages into chunks. While both SIP and MSRP provide mechanisms for hop-by-hop security, neither provides native end-to-end protection. Instead, they depend on S/MIME [RFC8550] [RFC8551]. However, at the time of this writing, S/MIME is not in common use for SIP-based and MSRP-based messaging services. This document updates and clarifies RFCs 3261, 3428, and 4975 in an attempt to make S/MIME for SIP and MSRP easier to implement and deploy in an interoperable fashion. This document updates RFCs 3261, 3428, and 4975 to update the cryptographic algorithm recommendations and the handling of S/MIME data objects. It updates RFC 3261 to allow S/MIME signed messages to be sent without embedded certificates in some situations. Finally, it updates RFCs 3261, 3428, and 4975 to clarify error-reporting requirements for certain situations.

2. Terminology

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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3. Problem Statement and Scope

This document discusses the use of S/MIME with SIP-based messaging. Other standardized messaging protocols exist, such as the Extensible Messaging and Presence Protocol (XMPP) [RFC6121]. Likewise, other end-to-end protection formats exist, such as JSON Web Signatures [RFC7515] and JSON Web Encryption [RFC7516]. This document focuses on SIP-based messaging because its use is becoming more common in mobile environments. It focuses on S/MIME, since several mobile operating systems already have S/MIME libraries installed. While there may also be value in specifying end-to-end security for other messaging and security mechanisms, it is out of scope for this document. MSRP sessions are negotiated using the Session Description Protocol (SDP) [RFC4566] offer/answer mechanism [RFC3264] or similar mechanisms. This document assumes that SIP is used for the offer/answer exchange. However, the techniques should be adaptable to other signaling protocols. [RFC3261], [RFC3428], and [RFC4975] already describe the use of S/MIME. [RFC3853] updates SIP to support the Advanced Encryption Standard (AES). In aggregate, that guidance is incomplete, contains inconsistencies, and is still out of date in terms of supported and recommended algorithms. The guidance in RFC 3261 is based on an implicit assumption that S/MIME is being used to secure signaling applications. That advice is not entirely appropriate for messaging applications. For example, it assumes that message decryption always happens before the SIP transaction completes. This document offers normative updates and clarifications to the use of S/MIME with the SIP MESSAGE method and MSRP. It does not attempt to define a complete secure messaging system. Such a system would require considerable work around user enrollment, certificate and key generation and management, multi-party chats, device management, etc. While nothing herein should preclude those efforts, they are out of scope for this document. This document primarily covers the sending of single messages -- for example, "pager-mode messages" sent using the SIP MESSAGE method and "large messages" sent in MSRP. Techniques to use a common signing or encryption key across a session of messages are out of scope for this document.
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   Cryptographic algorithm requirements in this document are intended to
   supplement those already specified for SIP and MSRP.

4. Applicability of S/MIME

The Cryptographic Message Syntax (CMS) [RFC5652] is an encapsulation syntax that is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. The CMS supports a variety of architectures for certificate-based key management, especially the one defined by the IETF PKIX (Public Key Infrastructure using X.509) Working Group [RFC5280]. The CMS values are generated using ASN.1 [X680], using the Basic Encoding Rules (BER) and Distinguished Encoding Rules (DER) [X690]. The S/MIME Message Specification [RFC8551] defines MIME body parts based on the CMS. In this document, the application/pkcs7-mime media type is used to digitally sign an encapsulated body part, and it is also used to encrypt an encapsulated body part.

4.1. Signed Messages

While both SIP and MSRP require support for the multipart/signed format, the use of application/pkcs7-mime is RECOMMENDED for most signed messages. Experience with the use of S/MIME in electronic mail has shown that multipart/signed bodies are at greater risk of "helpful" tampering by intermediaries, a common cause of signature validation failure. This risk is also present for messaging applications; for example, intermediaries might insert Instant Message Disposition Notification (IMDN) requests [RFC5438] into messages. (See Section 9.2.) The application/pkcs7-mime format is also more compact, which can be important for messaging applications, especially when using the SIP MESSAGE method. (See Section 7.1.) The use of multipart/signed may still make sense if the message needs to be readable by receiving agents that do not support S/MIME. When generating a signed message, sending User Agents (UAs) SHOULD follow the conventions specified in [RFC8551] for the application/pkcs7-mime media type with smime-type=signed-data. When validating a signed message, receiving UAs MUST follow the conventions specified in [RFC8551] for the application/pkcs7-mime media type with smime-type=signed-data.
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   Sending and receiving UAs MUST support the SHA-256 message digest
   algorithm [RFC5754].  For convenience, the SHA-256 algorithm
   identifier is repeated here:

      id-sha256 OBJECT IDENTIFIER ::= {
        joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
        csor(3) nistalgorithm(4) hashalgs(2) 1 }

   Sending and receiving UAs MAY support other message digest

   Sending and receiving UAs MUST support the Elliptic Curve Digital
   Signature Algorithm (ECDSA) using the NIST P-256 elliptic curve and
   the SHA-256 message digest algorithm [RFC5480] [RFC5753].  Sending
   and receiving UAs SHOULD support the Edwards-curve Digital Signature
   Algorithm (EdDSA) with curve25519 (Ed25519) [RFC8032] [RFC8419].  For
   convenience, the ECDSA with SHA-256 algorithm identifier, the object
   identifier for the well-known NIST P-256 elliptic curve, and the
   Ed25519 algorithm identifier are repeated here:

      ecdsa-with-SHA256 OBJECT IDENTIFIER ::= {
        iso(1) member-body(2) us(840) ansi-X9-62(10045) signatures(4)
        ecdsa-with-SHA2(3) 2 }

      -- Note: The NIST P-256 elliptic curve is also known as secp256r1.

      secp256r1 OBJECT IDENTIFIER ::= {
        iso(1) member-body(2) us(840) ansi-X9-62(10045) curves(3)
        prime(1) 7 }

      id-Ed25519  OBJECT IDENTIFIER  ::=  {
        iso(1) identified-organization(3) thawte(101) 112 }

4.2. Encrypted Messages

When generating an encrypted message, sending UAs MUST follow the conventions specified in [RFC8551] for the application/pkcs7-mime media type with smime-type=auth-enveloped-data. When decrypting a received message, receiving UAs MUST follow the conventions specified in [RFC8551] for the application/pkcs7-mime media type with smime-type=auth-enveloped-data.
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   Sending and receiving UAs MUST support the AES-128-GCM algorithm for
   content encryption [RFC5084].  For convenience, the AES-128-GCM
   algorithm identifier is repeated here:

      id-aes128-GCM OBJECT IDENTIFIER ::=  {
        joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
        csor(3) nistAlgorithm(4) aes(1) 6 }

   Sending and receiving UAs MAY support other content-authenticated
   encryption algorithms.

   Sending and receiving UAs MUST support the AES-128-WRAP algorithm for
   encryption of one AES key with another AES key [RFC3565].  For
   convenience, the AES-128-WRAP algorithm identifier is repeated here:

      id-aes128-wrap OBJECT IDENTIFIER ::=  {
        joint-iso-itu-t(2) country(16) us(840) organization(1) gov(101)
        csor(3) nistAlgorithm(4) aes(1) 5 }

   Sending and receiving UAs MAY support other key-encryption

   Symmetric key-encryption keys can be distributed before messages are
   sent.  If sending and receiving UAs support previously distributed
   key-encryption keys, then they MUST assign a KEKIdentifier [RFC5652]
   to the previously distributed symmetric key.

   Alternatively, a key agreement algorithm can be used to establish a
   single-use key-encryption key.  If sending and receiving UAs support
   key agreement, then they MUST support the Elliptic Curve
   Diffie-Hellman (ECDH) algorithm using the NIST P-256 elliptic curve
   and the ANSI-X9.63-KDF key derivation function with the SHA-256
   message digest algorithm [RFC5753].  If sending and receiving UAs
   support key agreement, then they SHOULD support the ECDH algorithm
   using curve25519 (X25519) [RFC7748] [RFC8418].  For convenience,
   (1) the identifier for the ECDH algorithm using the ANSI-X9.63-KDF
   with the SHA-256 algorithm and (2) the identifier for the X25519
   algorithm are repeated here:

      dhSinglePass-stdDH-sha256kdf-scheme OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) certicom(132)
        schemes(1) 11 1 }

      id-X25519 OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) thawte(101) 110 }
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4.3. Signed and Encrypted Messages

RFC 3261, Section 23.2 says that when a User Agent Client (UAC) sends signed and encrypted data, it "SHOULD" send an EnvelopedData object encapsulated within a SignedData message. That essentially says that one should encrypt first, then sign. This document updates RFC 3261 to say that, when sending signed and encrypted user content in a SIP MESSAGE request, the sending UAs MUST sign the message first, and then encrypt it. That is, it must send the SignedData object inside an AuthEnvelopedData object. For interoperability reasons, recipients SHOULD accept messages signed and encrypted in either order.

4.4. Certificate Handling

Sending and receiving UAs MUST follow the S/MIME certificate-handling procedures [RFC8550], with a few exceptions detailed below.

4.4.1. Subject Alternative Name

In both SIP and MSRP, the identity of the sender of a message is typically expressed as a SIP URI. The subject alternative name extension is used as the preferred means to convey the SIP URI of the subject of a certificate. Any SIP URI present MUST be encoded using the uniformResourceIdentifier CHOICE of the GeneralName type as described in [RFC5280], Section Since the SubjectAltName type is a SEQUENCE OF GeneralName, multiple URIs MAY be present. Other methods of identifying a certificate subject MAY be used.

4.4.2. Certificate Validation

When validating a certificate, receiving UAs MUST support the ECDSA using the NIST P-256 elliptic curve and the SHA-256 message digest algorithm [RFC5480]. Sending and receiving UAs MAY support other digital signature algorithms for certificate validation.

5. Transfer Encoding

SIP and MSRP UAs are always capable of receiving binary data. Inner S/MIME entities do not require base64 encoding [RFC4648]. Both SIP and MSRP provide 8-bit safe transport channels; base64 encoding is not generally needed for the outer S/MIME entities.
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   However, if there is a chance a message might cross a 7-bit transport
   (for example, gateways that convert to a 7-bit transport for
   intermediate transfer), base64 encoding may be needed for the outer

6. User Agent Capabilities

Messaging UAs may implement a subset of S/MIME capabilities. Even when implemented, some features may not be available due to configuration. For example, UAs that do not have user certificates cannot sign messages on behalf of the user or decrypt encrypted messages sent to the user. At a minimum, a UA that supports S/MIME MUST be able to validate a signed message. End-user certificates have long been a barrier to large-scale S/MIME deployment. But since UAs can validate signatures even without local certificates, the use case of organizations sending secure notifications to their users becomes a sort of "low-hanging fruit". That being said, the signed-notification use case still requires shared trust anchors. SIP and MSRP UAs advertise their level of support for S/MIME by indicating their capability to receive the "application/pkcs7-mime" media type. The fact that a UA indicates support for the "multipart/signed" media type does not necessarily imply support for S/MIME. The UA might just be able to display clear-signed content without validating the signature. UAs that wish to indicate the ability to validate signatures for clear-signed messages MUST also indicate support for "application/pkcs7-signature". A UA can indicate that it can receive all smime-types by advertising "application/pkcs7-mime" with no parameters. If a UA does not accept all smime-types, it advertises the media type with the appropriate parameters. If more than one smime-type is supported, the UA includes a separate instance of the media-type string, appropriately parameterized, for each. For example, a UA that can only receive signed-data would advertise "application/pkcs7-mime; smime-type=signed-data". SIP signaling can fork to multiple destinations for a given Address of Record (AoR). A user might have multiple UAs with different capabilities; the capabilities remembered from an interaction with one such UA might not apply to another. (See Section 7.2.)
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   UAs can also advertise or discover S/MIME using out-of-band
   mechanisms.  Such mechanisms are beyond the scope of this document.

7. Using S/MIME with the SIP MESSAGE Method

The use of S/MIME with the SIP MESSAGE method is described in Section 11.3 of [RFC3428], and for SIP in general in Section 23 of [RFC3261]. This section and its child sections offer clarifications for the use of S/MIME with the SIP MESSAGE method, along with related updates to RFCs 3261 and 3428.

7.1. Size Limit

SIP MESSAGE requests are typically limited to 1300 octets. That limit applies to the entire message, including both SIP header fields and the message content. This is due to the potential for fragmentation of larger requests sent over UDP. In general, it is hard to be sure that no proxy or other intermediary will forward a SIP request over UDP somewhere along the path. Therefore, S/MIME messages sent using the SIP MESSAGE method should be kept as small as possible. Messages that will not fit within the limit can be sent using MSRP. Section 23.2 of [RFC3261] requires that a SignedData message contain a certificate to be used to validate the signature. In order to reduce the message size, this document updates that text to say that a SignedData message sent in a SIP MESSAGE request SHOULD contain the certificate but MAY omit it if the sender has reason to believe that the recipient (1) already has the certificate in its keychain or (2) has some other method of accessing the certificate.

7.2. SIP User Agent Capabilities

SIP UAs can theoretically indicate support for S/MIME by including the appropriate media type or types in the SIP Accept header field in a response to an OPTIONS request, or in a 415 (Unsupported Media Type) response to a SIP request that contained an unsupported media type in the body. Unfortunately, this approach may not be reliable in the general case. In the case where a downstream SIP proxy forks an OPTIONS or other non-INVITE request to multiple User Agent Servers (UASs), that proxy will only forward the "best" response. If the recipient has multiple devices, the sender may only learn the capabilities of the device that sent the forwarded response. Blindly trusting this information could result in S/MIME messages being sent to UAs that do not support it, which would be at best confusing and at worst misleading to the recipient.
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   UAs might be able to use the UA capabilities framework [RFC3840] to
   indicate support.  However, doing so would require the registration
   of one or more media feature tags with IANA.

   UAs MAY use other out-of-band methods to indicate their level of
   support for S/MIME.

7.3. Failure Cases

Section 23.2 of [RFC3261] requires that the recipient of a SIP request that includes a body part of an unsupported media type and a Content-Disposition header field "handling" parameter of "required" return a 415 (Unsupported Media Type) response. Given that SIP MESSAGE exists for no reason other than to deliver content in the body, it is reasonable to treat the top-level body part as always required. However, [RFC3428] makes no such assertion. This document updates Section 11.3 of [RFC3428] to add the statement that a UAC that receives a SIP MESSAGE request with an unsupported media type MUST return a 415 response. Section 23.2 of [RFC3261] says that if a recipient receives an S/MIME body encrypted to the wrong certificate, it MUST return a SIP 493 (Undecipherable) response and SHOULD send a valid certificate in that response. This is not always possible in practice for SIP MESSAGE requests. The UAS may choose not to decrypt a message until the user is ready to read it. Messages may be delivered to a message store or sent via a store-and-forward service. This document updates RFC 3261 to say that the UAS SHOULD return a SIP 493 response if it immediately attempts to decrypt the message and determines that the message was encrypted to the wrong certificate. However, it MAY return a 200-class response if decryption is deferred.

8. Using S/MIME with MSRP

MSRP has features that interact with the use of S/MIME. In particular, the ability to send messages in chunks, the ability to send messages of unknown size, and the use of SDP to indicate media-type support create considerations for the use of S/MIME.

8.1. Chunking

MSRP allows a message to be broken into "chunks" for transmission. In this context, the term "message" refers to an entire message that one user might send to another. A chunk is a fragment of that message sent in a single MSRP SEND request. All of the chunks that make up a particular message share the same Message-ID value.
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   The sending UA may break a message into chunks, which the receiving
   UA will reassemble to form the complete message.  Intermediaries such
   as MSRP relays [RFC4976] might break chunks into smaller chunks or
   might reassemble chunks into larger ones; therefore, the message
   received by the recipient may be broken into a different number of
   chunks than were sent by the recipient.  Intermediaries might also
   cause chunks to be received in a different order than sent.

   The sender MUST apply any S/MIME operations to the whole message
   prior to breaking it into chunks.  Likewise, the receiver needs to
   reassemble the message from its chunks prior to decrypting,
   validating a signature, etc.

   MSRP chunks are framed using an end-line.  The end-line comprises
   seven hyphens, a 64-bit random value taken from the start line, and a
   continuation flag.  MSRP requires the sending UA to scan data to be
   sent in a specific chunk to ensure that the end-line does not
   accidentally occur as part of the data.  This scanning occurs on a
   chunk rather than a whole message; consequently, it must occur after
   the sender applies any S/MIME operations.

8.2. Streamed Data

MSRP allows a mode of operation where a UA sends some chunks of a message prior to knowing the full length of the message. For example, a sender might send streamed data over MSRP as a single message, even though it doesn't know the full length of that data in advance. This mode is incompatible with S/MIME, since a sending UA must apply S/MIME operations to the entire message in advance of breaking it into chunks. Therefore, when sending a message in an S/MIME format, the sender MUST include the Byte-Range header field for every chunk, including the first chunk. The Byte-Range header field MUST include the total length of the message. A higher layer could choose to break such streamed data into a series of messages prior to applying S/MIME operations, so that each fragment appears as a distinct (separate) S/MIME message in MSRP. Such mechanisms are beyond the scope of this document.
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8.3. Indicating Support for S/MIME

A UA that supports this specification MUST explicitly include the appropriate media type or types in the "accept-types" attribute in any SDP offer or answer that proposes MSRP. It MAY indicate that it requires S/MIME wrappers for all messages by putting appropriate S/MIME media types in the "accept-types" attribute and putting all other supported media types in the "accept-wrapped-types" attribute. For backwards compatibility, a sender MAY treat a peer that includes an asterisk ("*") in the "accept-types" attribute as potentially supporting S/MIME. If the peer returns an MSRP 415 (MIME type not understood) response to an attempt to send an S/MIME message, the sender should treat the peer as not supporting S/MIME for the duration of the session, as indicated in Section 7.3.1 of [RFC4975]. While these SDP attributes allow an endpoint to express support for certain media types only when wrapped in a specified envelope type, it does not allow the expression of more complex structures. For example, an endpoint can say that it supports text/plain and text/html, but only when inside an application/pkcs7 or message/cpim container, but it cannot express a requirement for the leaf types to always be contained in an application/pkcs7 container nested inside a message/cpim container. This has implications for the use of S/MIME with the message/cpim format. (See Section 9.1.) MSRP allows multiple reporting modes that provide different levels of feedback. If the sender includes a Failure-Report header field with a value of "no", it will not receive failure reports. This mode should not be used carelessly, since such a sender would never see a 415 response as described above and would have no way to learn that the recipient could not process an S/MIME body.

8.4. MSRP URIs

MSRP URIs are ephemeral. Endpoints MUST NOT use MSRP URIs to identify certificates or insert MSRP URIs into certificate Subject Alternative Name fields. When MSRP sessions are negotiated using SIP [RFC3261], the SIP AoRs of the peers are used instead. Note that MSRP allows messages to be sent between peers in either direction. A given MSRP message might be sent from the SIP offerer to the SIP answerer. Thus, the sender and recipient roles may reverse between one message and another in a given session.
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8.5. Failure Cases

Successful delivery of an S/MIME message does not indicate that the recipient successfully decrypted the contents or validated a signature. Decryption and/or validation may not occur immediately on receipt, since the recipient may not immediately view the message, and the UA may choose not to attempt decryption or validation until the user requests it. Likewise, successful delivery of S/MIME enveloped data does not, on its own, indicate that the recipient supports the enclosed media type. If the peer only implicitly indicated support for the enclosed media type through the use of a wildcard in the "accept-types" or "accept-wrapped types" SDP attributes, it may not decrypt the message in time to send a 415 response.

9. S/MIME Interaction with Other SIP Messaging Features

9.1. Common Profile for Instant Messaging

The Common Profile for Instant Messaging (CPIM) [RFC3860] defines an abstract messaging service, with the goal of creating gateways between different messaging protocols that could relay instant messages without change. The SIP MESSAGE method and MSRP were initially designed to map to the CPIM abstractions. However, at the time of this writing, CPIM-compliant gateways have not been deployed. To the authors' knowledge, no other IM protocols have been explicitly mapped to CPIM. CPIM also defines the abstract messaging URI scheme "im:". As of the time of this writing, the "im:" scheme is not in common use. The CPIM message format [RFC3862] allows UAs to attach transport-neutral metadata to arbitrary MIME content. The format was designed as a canonicalization format to allow signed data to cross protocol-converting gateways without loss of metadata needed to verify the signature. While it has not typically been used for that purpose, it has been used for other metadata applications -- for example, IMDNs [RFC5438] and MSRP multi-party chat [RFC7701]. In the general case, a sender applies end-to-end signature and encryption operations to the entire MIME body. However, some messaging systems expect to inspect and in some cases add or modify metadata in CPIM header fields. For example, CPM-based and RCS-based services include application servers that may need to insert timestamps into chat messages and may use additional metadata to characterize the content and purpose of a message to determine application behavior. The former will cause validation failure for
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   signatures that cover CPIM metadata, while the latter is not possible
   if the metadata is encrypted.  Clients intended for use in such
   networks MAY choose to apply end-to-end signatures and encryption
   operations to only the CPIM payload, leaving the CPIM metadata
   unprotected from inspection and modification.  UAs that support
   S/MIME and CPIM SHOULD be able to validate signatures and decrypt
   enveloped data both (1) when those operations are applied to the
   entire CPIM body and (2) when they are applied to just the CPIM
   payload.  This means that the receiver needs to be flexible in its
   MIME document parsing and that it cannot make assumptions that
   S/MIME-protected body parts will always be in the same position or
   level in the message payload.

   If such clients need to encrypt or sign CPIM metadata end to end,
   they can nest a protected CPIM message format payload inside an
   unprotected CPIM message envelope.

   The use of CPIM metadata fields to identify certificates or to
   authenticate SIP or MSRP header fields is out of scope for this

9.2. Instant Message Disposition Notifications

The IMDN mechanism [RFC5438] allows both endpoints and intermediary application servers to request and to generate delivery notifications. The use of S/MIME does not impact strictly end-to-end use of IMDNs. The IMDN mechanism recommends that devices that are capable of doing so sign delivery notifications. It further requires that delivery notifications that result from encrypted messages also be encrypted. However, the IMDN mechanism allows intermediary application servers to insert notification requests into messages, to add routing information to messages, and to act on notification requests. It also allows list servers to aggregate delivery notifications. Such intermediaries will be unable to read end-to-end encrypted messages in order to interpret delivery notice requests. Intermediaries that insert information into end-to-end signed messages will cause the signature validation to fail. (See Section 9.1.)

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