Network Working Group B. Campbell
Internet-Draft Independent
Updates: RFC 3261, RFC 3428, RFC 4975 R. Housley
(if approved) Vigil Security
Intended status: Standards Track November 29, 2017
Expires: June 2, 2018

Securing Session Initiation Protocol (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 RFC 3261, RFC 3428, and RFC 4975.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on June 2, 2018.

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Table of Contents

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) [CPM], [RCS] system uses the SIP Message Method for short “pager mode” messages and MSRP for large messages and for sessions of messages. The GSM Association (GMSA) rich communication services (RCS) uses CPM for messaging.

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 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 [RFC5750][RFC5751]. However at the time of this writing, S​/​MIME is not in common use for SIP and MSRP based messaging services. This document updates and clarifies RFC 3261, RFC 3428, and RFC 4975 in an attempt to make the S​/​MIME for SIP and MSRP easier to implement and deploy in an interoperable fashion.

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.

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 application. 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 system would require considerable work around user enrollment, certificate and key generation and management, multiparty 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” send 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, but may be discussed in a future version.

Cryptographic algorithm requirements in this document are intended 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 version 3.2 [RFC5751] 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 is used to encrypt an encapsulated body part.

4.1. Signed Messages

While both SIP and MSRP require support for the multipart/signed format, this document recommends the use of application/pkcs7-mime 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 Delivery notification requests 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 [RFC5751] 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 [RFC5751] for the application/pkcs7-mime media type with smime-type=signed-data.

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 algorithms.

Sending and receiving UAs MUST support the Elliptic Curve Digital Signature Algorithm (ECDSA) using the NIST P256 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][I-D.ietf-curdle-cms-eddsa-signatures]. For convenience, the ECDSA with SHA-256 algorithm identifier, the object identifier for the well-known NIST P256 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 P256 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  ::=  { 1 3 101 112 }

4.2. Encrypted Messages

When generating an encrypted message, sending UAs MUST follow the conventions specified in [RFC5751] for the application/pkcs7-mime media type with smime-type=enveloped-data. When decrypting a received message, receiving UAs MUST follow the conventions specified in [RFC5751] for the application/pkcs7-mime media type with smime-type=enveloped-data.

Sending and receiving UAs MUST support the AES-128-CBC for content encryption [RFC3565]. For convenience, the AES-128-CBC algorithm identifier is repeated here:

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

Sending and receiving UAs MAY support other content encryption algorithms.

Sending and receiving UAs MUST support the AES-128-WRAP 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 algorithms.

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 KEK identifier [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) using the NIST P256 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 Elliptic Curve Diffie-Hellman (ECDH) using curve25519 (X25519) [RFC7748][I-D.ietf-curdle-cms-ecdh-new-curves]. For convenience, the ECDH using the ANSI-X9.63-KDF with SHA-256 algorithm identifier and the X25519 algorithm identifier are repeated here:

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

     id-X25519 OBJECT IDENTIFIER ::= { 1 3 101 110 }

4.3. Signed and Encrypted Messages

When generating a signed and encrypted message, sending UAs MUST sign the message first, and then encrypt it.

4.4. Certificate Handling

Sending and receiving UAs MUST follow the S​/​MIME certificate handling procedures [RFC5750], with a few exceptions detailed below.

4.4.1. Subject Alternative Name

The subject alternative name extension is used as the preferred means to convey the SIP URI of a message signer. 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.

Open Issue: Should we consider other means of linking the identity to the certificate other than a SIP URI? For example, a specially constructed domain name for a cert issued via an ACME service? One approach might to be to say to use a SIP URI in the absence of other mechanisms.

4.4.2. Certificate Validation

When validating a certificate, receiving UAs MUST support the Elliptic Curve Digital Signature Algorithm (ECDSA) using the NIST P256 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. 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 entity.

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.

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 are supported, the UA includes a separate instance of the media-type string, appropriately parameterized, for each.

For example, a UA that can only received 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.

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 RFC 3261 and RFC 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 via SIP MESSAGE should be kept as small as possible.

[RFC3261] says that a SignedData message MUST contain a certificate to be used to validate the signature. In order to reduce the message size, this document updates that 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 already has the certificate in its keychain, or has some other method of accessing the certificate.

7.2. User Agent Capabilities

SIP user agents (UA) can 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 response to a SIP request that contained an unsupported media type in the body.

UAs might be able to use the user agent 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

[RFC3261] requires that the recipient of a SIP request that includes a body part of an unsupported media type and a Content-Disposition header “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 [RFC3428] to say that a UAC that receives a SIP MESSAGE request with an unsupported media type MUST return a 415 Unsupported Media Type” response.

[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 User Agent Server (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 the message was encrypted to the wrong certificate. However, it MAY return a 2XX 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.

The sending user agent may break a message into chunks, which the receiving user agent 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. MRSP requires the sending user agent to scan data sent in a specific chunk to be sent ensure that the end-line does not accidentally occur as part of the sent 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 operation, so that each fragment appears as a distinct S​/​MIME separate message in MSRP. Such mechanisms are beyond the scope for this document.

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 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 [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 Addresses of Record (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 answer. Thus, the the sender and recipient roles may reverse between one message and another in a given session.

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 since the recipient may not immediately view the message, and the user agent 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 use of “im:” URIs as subject alternative names in certificates is for future study.

The Common Profile for Instant Messages 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, Intant Message Delivery Notifications (IMDN)[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 and RCS based service include application servers that may need to insert time stamps 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 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.

If such clients need to provide 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 document.

9.2. Instant Message Delivery Notifications

The Instant Message Delivery Notification (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 IMDN. IMDN 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, IMDN 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.)

10. Examples

Examples will be added in a future version of this document.

11. IANA Considerations

This document makes no requests of the IANA.

12. Security Considerations

The security considerations from S​/​MIME [RFC5750][RFC5751] and elliptic curves in CMS [RFC5753] apply. The S​/​MIME related security considerations from SIP [RFC3261][RFC3853], SIP MESSAGE [RFC3428], and MSRP [RFC4975] apply.

This document assumes that end-entity certificate validation is provided by a chain of trust to a certification authority (CA), using a public key infrastructure. The security considerations from [RFC5280] apply. However, other validations methods may be possible; for example sending a signed fingerprint for the end-entity in SDP. The relationship of this work and the techniques discussed in [RFC4474], [I-D.ietf-stir-rfc4474bis], and [I-D.ietf-sipbrandy-rtpsec] are for future study.

When matching an end-entity certificate to the sender or recipient identity, the respective SIP AoRs are used. Typically these will match the SIP From and To header fields. Matching SIP AoRs from other header fields, for example, P-Asserted-Identity [RFC3325], is for future study.

The secure notification use case discussed in Section 1 has significant vulnerabilities when used in an insecure environment. For example, “phishing” messages could be used to trick users into revealing credentials. Eavesdroppers could learn confirmation codes from unprotected two-factor authentication messages. Unsolicited messages sent by impersonators could tarnish the reputation of an organization. While hop-by-hop protection can mitigate some of those risks, it still leaves messages vulnerabile to malicious or compromised intermediaries.

Mobile messaging is typically an online application; online certificate revocation checks should usually be feasible.

Certain messaging services, for example those based on CPM and RCS, may include intermediaries that attach metadata to user generated messages. In certain cases this metadata may reveal information to third parties that would have otherwise been encrypted. Implementors and operators should consider whether this metadata may create privacy leaks. Such an analysis is beyond the scope of this document.

13. References

13.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, DOI 10.17487/RFC3428, December 2002.
[RFC3565] Schaad, J., "Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC 3565, DOI 10.17487/RFC3565, July 2003.
[RFC3853] Peterson, J., "S​/​MIME Advanced Encryption Standard (AES) Requirement for the Session Initiation Protocol (SIP)", RFC 3853, DOI 10.17487/RFC3853, July 2004.
[RFC4566] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006.
[RFC4975] Campbell, B., Mahy, R. and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, DOI 10.17487/RFC4975, September 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R. and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, DOI 10.17487/RFC5480, March 2009.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S​/​MIME) Version 3.2 Certificate Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet Mail Extensions (S​/​MIME) Version 3.2 Message Specification", RFC 5751, DOI 10.17487/RFC5751, January 2010.
[RFC5753] Turner, S. and D. Brown, "Use of Elliptic Curve Cryptography (ECC) Algorithms in Cryptographic Message Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753, January 2010.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January 2010.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation One (ASN.1): Specification of basic notation", ITU-T Recommendation X.680, 2015.
[X690] ITU-T, "Information Technology -- ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)", ITU-T Recommendation X.690, 2015.

13.2. Informative References

[CPM] Open Mobile Alliance, "OMA Converged IP Messaging System Description, Candidate Version 2.2", September 2017.
[I-D.ietf-curdle-cms-ecdh-new-curves] Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key Agreement Algorithm with X25519 and X448 in the Cryptographic Message Syntax (CMS)", Internet-Draft draft-ietf-curdle-cms-ecdh-new-curves-10, August 2017.
[I-D.ietf-curdle-cms-eddsa-signatures] Housley, R., "Use of EdDSA Signatures in the Cryptographic Message Syntax (CMS)", Internet-Draft draft-ietf-curdle-cms-eddsa-signatures-08, October 2017.
[I-D.ietf-sipbrandy-rtpsec] Peterson, J., Rescorla, E., Barnes, R. and R. Housley, "Best Practices for Securing RTP Media Signaled with SIP", Internet-Draft draft-ietf-sipbrandy-rtpsec-03, October 2017.
[I-D.ietf-stir-rfc4474bis] Peterson, J., Jennings, C., Rescorla, E. and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", Internet-Draft draft-ietf-stir-rfc4474bis-16, February 2017.
[RCS] GSMA, "RCS Universal Profile Service Definition Document, Version 2.0", June 2017.
[RFC3325] Jennings, C., Peterson, J. and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks", RFC 3325, DOI 10.17487/RFC3325, November 2002.
[RFC3840] Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, DOI 10.17487/RFC3840, August 2004.
[RFC3860] Peterson, J., "Common Profile for Instant Messaging (CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004.
[RFC3862] Klyne, G. and D. Atkins, "Common Presence and Instant Messaging (CPIM): Message Format", RFC 3862, DOI 10.17487/RFC3862, August 2004.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 4474, DOI 10.17487/RFC4474, August 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[RFC4976] Jennings, C., Mahy, R. and A. Roach, "Relay Extensions for the Message Sessions Relay Protocol (MSRP)", RFC 4976, DOI 10.17487/RFC4976, September 2007.
[RFC5438] Burger, E. and H. Khartabil, "Instant Message Disposition Notification (IMDN)", RFC 5438, DOI 10.17487/RFC5438, February 2009.
[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence", RFC 6121, DOI 10.17487/RFC6121, March 2011.
[RFC7515] Jones, M., Bradley, J. and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 2015.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, May 2015.
[RFC7701] Niemi, A., Garcia-Martin, M. and G. Sandbakken, "Multi-party Chat Using the Message Session Relay Protocol (MSRP)", RFC 7701, DOI 10.17487/RFC7701, December 2015.
[RFC7748] Langley, A., Hamburg, M. and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017.

Authors' Addresses

Ben Campbell Independent 204 Touchdown Dr Irving, TX, 75063 US EMail:
Russ Housley Vigil Security 918 Spring Knoll Drive Herndon, VA, 20170 US EMail: