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
Abstract
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
https://www.rfc-editor.org/info/rfc8591.
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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.
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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
algorithms.
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
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 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 4.2.1.6.
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
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.
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.
Campbell & Housley Standards Track [Page 14]
RFC 8591 S/MIME for SIP Messaging April 2019
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
Campbell & Housley Standards Track [Page 15]
RFC 8591 S/MIME for SIP Messaging April 2019
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
document.
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.)
Campbell & Housley Standards Track [Page 16]
RFC 8591 S/MIME for SIP Messaging April 2019
10. Examples
The following sections show examples of S/MIME messages in SIP and
MSRP. The examples include the tags "[start-hex]" and "[end-hex]" to
denote binary content shown in hexadecimal. The tags are not part of
the actual message and do not count towards the Content-Length header
field values.
In all of these examples, the cleartext message is the string
"Watson, come here - I want to see you." followed by a newline
character.
The cast of characters includes Alice, with a SIP AoR of
"
[email protected]", and Bob, with a SIP AoR of "
[email protected]".
Appendix A shows the detailed content of each S/MIME body.
10.1. Signed Message in SIP including the Sender's Certificate
Figure 1 shows a message signed by Alice. This body uses the
"application/pkcs7-mime" media type with an smime-type parameter
value of "signed-data".
The S/MIME body includes Alice's signing certificate. Even though
the original message content is fairly short and only minimal SIP
header fields are included, the total message size approaches the
maximum allowed for the SIP MESSAGE method unless the UAC has advance
knowledge that all SIP hops will use congestion-controlled transport
protocols. A message that included all the SIP header fields that
are commonly in use in some SIP deployments would likely exceed the
limit.
Campbell & Housley Standards Track [Page 17]
RFC 8591 S/MIME for SIP Messaging April 2019
MESSAGE sip:
[email protected] SIP/2.0
Via: SIP/2.0/TCP alice-pc.example.com;branch=z9hG4bK776sgdkfie
Max-Forwards: 70
From: sip:
[email protected];tag=49597
To: sip:
[email protected]
Call-ID:
[email protected]
CSeq: 1 MESSAGE
Content-Transfer-Encoding: binary
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name="smime.p7m"
Content-Disposition: attachment; filename="smime.p7m"
Content-Length: 762
[start-hex]
308202f606092a864886f70d010702a08202e7308202e3020101310d300b0609
608648016503040201305306092a864886f70d010701a0460444436f6e74656e
742d547970653a20746578742f706c61696e0d0a0d0a576174736f6e2c20636f
6d652068657265202d20492077616e7420746f2073656520796f752e0d0aa082
016b308201673082010da003020102020900b8793ec0e4c21530300a06082a86
48ce3d040302302631143012060355040a0c0b6578616d706c652e636f6d310e
300c06035504030c05416c696365301e170d3137313231393233313230355a17
0d3138313231393233313230355a302631143012060355040a0c0b6578616d70
6c652e636f6d310e300c06035504030c05416c6963653059301306072a8648ce
3d020106082a8648ce3d03010703420004d87b54729f2c22feebd9ddba0efa40
642297a6093887a4dae7990b23f87fa7ed99db8cf5a314f2ee64106ef1ed61db
fc0a4b91c953cbd022a751b914807bb794a324302230200603551d1104193017
86157369703a616c696365406578616d706c652e636f6d300a06082a8648ce3d
040302034800304502207879be1c27f846276fdf15e333e53c6f17a757388a02
cb7b8ae481c1641ae7a9022100ff99cd9c94076c82b02fea3b1350179a4b7752
e16fa30a3f9ab29650b0e2818931820109308201050201013033302631143012
060355040a0c0b6578616d706c652e636f6d310e300c06035504030c05416c69
6365020900b8793ec0e4c21530300b0609608648016503040201a06930180609
2a864886f70d010903310b06092a864886f70d010701301c06092a864886f70d
010905310f170d3139303132363036313335345a302f06092a864886f70d0109
0431220420ef778fc940d5e6dc2576f47a599b3126195a9f1a227adaf35fa22c
050d8d195a300a06082a8648ce3d04030204473045022005fdc2b55b0f444a46
be468dfc7ef3b7de30019ef0952a223e8521890b35bb4e02210090e43a9d9846
cf2af8159c5c0ef48848fa2f39f998b1bb99b52a6fc6c776f2c8
[end-hex]
Figure 1: Signed Message in SIP
Campbell & Housley Standards Track [Page 18]
RFC 8591 S/MIME for SIP Messaging April 2019
10.2. Signed Message in SIP with No Certificate
Figure 2 shows the same message from Alice without the embedded
certificate. The shorter total message length may be more
manageable.
MESSAGE sip:
[email protected] SIP/2.0
Via: SIP/2.0/TCP alice-pc.example.com;branch=z9hG4bK776sgdkfie
Max-Forwards: 70
From: sip:
[email protected];tag=49597
To: sip:
[email protected]
Call-ID:
[email protected]
CSeq: 1 MESSAGE
Content-Transfer-Encoding: binary
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name="smime.p7m"
Content-Disposition: attachment; filename="smime.p7m"
Content-Length: 395
[start-hex]
3082018706092a864886f70d010702a082017830820174020101310d300b0609
608648016503040201305306092a864886f70d010701a0460444436f6e74656e
742d547970653a20746578742f706c61696e0d0a0d0a576174736f6e2c20636f
6d652068657265202d20492077616e7420746f2073656520796f752e0d0a3182
0109308201050201013033302631143012060355040a0c0b6578616d706c652e
636f6d310e300c06035504030c05416c696365020900b8793ec0e4c21530300b
0609608648016503040201a069301806092a864886f70d010903310b06092a86
4886f70d010701301c06092a864886f70d010905310f170d3139303132363036
313335345a302f06092a864886f70d01090431220420ef778fc940d5e6dc2576
f47a599b3126195a9f1a227adaf35fa22c050d8d195a300a06082a8648ce3d04
03020447304502203607275592d30c8c5a931041a01804d60c638ac9a8080918
87172a0887c8d4aa022100cd9e14bd21817336e9052fe590af2e2bcde16dd3e9
48d0f5f78a969e26382682
[end-hex]
Figure 2: Signed Message in SIP with No Certificate Included
Campbell & Housley Standards Track [Page 19]
RFC 8591 S/MIME for SIP Messaging April 2019
10.3. MSRP Signed and Encrypted Message in a Single Chunk
Figure 3 shows a signed and encrypted message from Bob to Alice sent
via MSRP.
MSRP dsdfoe38sd SEND
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bobpc.example.org:8888/9di4eae923wzd;tcp
Message-ID: 456so39s
Byte-Range: 1-1940/1940
Content-Disposition: attachment; filename="smime.p7m"
Content-Type: application/pkcs7-mime; smime-type=auth-enveloped-data;
name="smime.p7m"
[start-hex]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 & Housley Standards Track [Page 20]
RFC 8591 S/MIME for SIP Messaging April 2019
1e93770d8f4f22dea61d73083c32c4038c1eb3dd3383a89a8795e241c2ed7cb6
80758c041069489860fc9f490e85236072548b3249698f99953acf1ec658b7aa
85e554c449701a6d4b039ed103dc458df4b29cb04b8cedd540c84348da79c186
56d5188f9f3a9e4b9b840c70664b90296c60b7ac984e918d48a09dbddfb281fc
862510db59d9fa9dc93f10f9c6d7bef72931d184cad7ac13c1a5295fc89fe3bb
7eb8e02085a828c5a138786e607ade4f5e8d4115909209ba878a79305a5316c2
2229e42b886d06481c8473f9d51269e2af6341bce20f768e860d7784ed46150e
04ff50cd209c5b127511369fe06bc4aa9a72d8f1fe4fcf0866d664b365ffa86e
8c1b43e7a9212aecc16ca350a28efae25fac054dd934bfe7e5fa4f753aa41596
8c7ebec439e0ac0270b4874a068d22484c09d9e8abe17f1372b4b2f65f1148e8
933eda92e5d1774564963b391c3bbd9f1c27ffe36f832e05155fc39ee6652fa7
b4188975ec5c67b32c9f213c8ac6b8e132a5a7c3bf74f016405cd8c201d10521
93e186d44358de388d73211ba2f1792f3cfeb9bbde7211d26f56ab06e11ccc9c
cde2b88cd8373773eafc37fd85b7a7a2bcaec752e617d6e01c02b86e9d9a40f3
20462c5d66f8351716dcd6014bdf30a60f75fc0631c920845ed8c0bad35ddf19
84f2241cd3b529dc1028845f8089543df4f1441ede36b1bf31af5afc8c2b708d
50b645d4e7db88648c3eefe14765158fb0e8d3bb53ddcbe26d7124c6e1d992f8
3230aa953376ee8c68109568e8571f0c9bbda48f4df306fe747f371175148f31
832767cd766cf07b450cbf62cad2a7bd71f1f88233f116a1a7f3caf12f34bcf4
0d21e79ffc9827221b68b080ff03ad782d6d6d07871676f798943e54f13fd75c
89c0b4263bf10f56243f9e72ef3b3899a539d9a3ac5be2b69400a3cf8d196c5c
ed697b2ed803b987a5ee85c5095b48da7a5b03b47e2b9fe4cd4bc3098e864e0c
e7d467da99cd7f3a9e947b5eea77f7a6be16c8c7e9e0decc1ff132559c234321
7b9c2950386e85d2942121086cdfa19658195be6d7f86bca9881b695082964f1
2e7cf801025d6792c6882409414d703321ec83abd698d68956118713a0ff1272
acbc9a6d148900c74c16921df9b38f29ec46d4f10060fffe5e36bbbacaf2d1ba
d7dd057ed3e30ebcd69083f9d3a2a26ef90b751d6a1adfa0590db19da107cf3e
a8db0410f6ffc6e1aef19cd23d985a921976352d
[end-hex]
-------dsdfoe38sd$
Figure 3: Signed and Encrypted Message in MSRP
10.4. MSRP Signed and Encrypted Message Sent in Multiple Chunks
Figure 4 shows the same message as in Figure 3 except that the
message is broken into two chunks. The S/MIME operations were
performed prior to breaking the message into chunks.
MSRP d93kswow SEND
To-Path: msrp://alicepc.example.com:7777/iau39soe2843z;tcp
From-Path: msrp://bobpc.example.org:8888/9di4eae923wzd;tcp
Message-ID: 12339sdqwer
Byte-Range: 1-960/1940
Content-Disposition: attachment; filename="smime.p7m"
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name="smime.p7m"
Campbell & Housley Standards Track [Page 21]
RFC 8591 S/MIME for SIP Messaging April 2019
[start-hex]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[end-hex]
-------d93kswow+
MSRP op2nc9a SEND
To-Path: msrp://alicepc.example.com:8888/9di4eae923wzd;tcp
From-Path: msrp://bobpc.example.org:7654/iau39soe2843z;tcp
Message-ID: 12339sdqwer
Byte-Range: 961-1940/1940
Content-Disposition: attachment; filename="smime.p7m"
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name="smime.p7m"
Campbell & Housley Standards Track [Page 22]
RFC 8591 S/MIME for SIP Messaging April 2019
[start-hex]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[end-hex]
-------op2nc9a$
Figure 4: Signed, Encrypted, and Chunked MSRP Message
11. IANA Considerations
This document has no IANA actions.
12. Security Considerations
The security considerations for S/MIME [RFC8550] [RFC8551] and
elliptic curves in CMS [RFC5753] apply. The S/MIME-related security
considerations for SIP [RFC3261], SIP MESSAGE [RFC3428], and MSRP
[RFC4975] apply.
Campbell & Housley Standards Track [Page 23]
RFC 8591 S/MIME for SIP Messaging April 2019
The security considerations for algorithms recommended in this
document also apply; see [RFC3565], [RFC5480], [RFC5753], [RFC5754],
[RFC7748], [RFC8032], [RFC8418], and [RFC8419].
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 between this work and the techniques discussed
in [RFC8224] and [RTP-Sec] are out of scope for this document.
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. Some UAs may extract the
sender identity from SIP AoRs in other header fields -- for example,
P-Asserted-Identity [RFC3325]. In general, the UAS should compare
the certificate to the identity that it relies upon -- for example,
for display to the end user or comparison against message-filtering
rules.
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 vulnerable to malicious or
compromised intermediaries. End-to-end protection prevents
modification by intermediaries. However, neither provides much
protection unless the recipient knows to expect messages from a
particular sender to be signed and refuses to accept unsigned
messages that appear to be from that source.
Mobile messaging is typically an online application; online
certificate revocation checks should usually be feasible.
S/MIME does not normally protect the SIP or MSRP headers. While it
normally does protect the CPIM header, certain CPIM header fields may
not be protected if the sender excludes them from the encrypted or
signed part of the message. (See Section 9.1.) Certain messaging
services -- for example, those based on RCS -- may include
intermediaries that attach metadata to user-generated messages in the
form of SIP, MSRP, or CPIM header fields. This metadata could
possibly reveal information to third parties that the sender might
Campbell & Housley Standards Track [Page 24]
RFC 8591 S/MIME for SIP Messaging April 2019
prefer not to send as cleartext. Implementors and operators should
consider whether inserted metadata may create privacy leaks. Such an
analysis is beyond the scope of this document.
MSRP messages broken into chunks must be reassembled by the recipient
prior to decrypting or validation of signatures. (See Section 8.1.)
Section 14.5 of [RFC4975] describes a potential denial-of-service
attack where the attacker puts large values in the Byte-Range header
field. Implementations should sanity-check these values before
allocating memory space for reassembly.
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.
An attack on S/MIME implementations of HTML and multipart/mixed
messages is highlighted in [Efail]. To avoid this attack, clients
MUST ensure that a text/html content type is a complete HTML
document. Clients SHOULD treat each of the different pieces of the
multipart/mixed construct as coming from different origins. Clients
MUST treat each encrypted or signed piece of a MIME message as being
from different origins both from unprotected content and from each
other.
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,
<
https://www.rfc-editor.org/info/rfc2119>.
[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,
<
https://www.rfc-editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<
https://www.rfc-editor.org/info/rfc3264>.
Campbell & Housley Standards Track [Page 25]
RFC 8591 S/MIME for SIP Messaging April 2019
[RFC3428] Campbell, B., Ed., 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,
<
https://www.rfc-editor.org/info/rfc3428>.
[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,
<
https://www.rfc-editor.org/info/rfc3565>.
[RFC3853] Peterson, J., "S/MIME Advanced Encryption Standard (AES)
Requirement for the Session Initiation Protocol (SIP)",
RFC 3853, DOI 10.17487/RFC3853, July 2004,
<
https://www.rfc-editor.org/info/rfc3853>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <
https://www.rfc-editor.org/info/rfc4566>.
[RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC4975, September 2007,
<
https://www.rfc-editor.org/info/rfc4975>.
[RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, DOI 10.17487/RFC5084, November 2007,
<
https://www.rfc-editor.org/info/rfc5084>.
[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,
<
https://www.rfc-editor.org/info/rfc5280>.
[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,
<
https://www.rfc-editor.org/info/rfc5480>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<
https://www.rfc-editor.org/info/rfc5652>.
Campbell & Housley Standards Track [Page 26]
RFC 8591 S/MIME for SIP Messaging April 2019
[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, <
https://www.rfc-editor.org/info/rfc5753>.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
2010, <
https://www.rfc-editor.org/info/rfc5754>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <
https://www.rfc-editor.org/info/rfc8174>.
[RFC8418] Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
Agreement Algorithm with X25519 and X448 in the
Cryptographic Message Syntax (CMS)", RFC 8418,
DOI 10.17487/RFC8418, August 2018,
<
https://www.rfc-editor.org/info/rfc8418>.
[RFC8419] Housley, R., "Use of Edwards-Curve Digital Signature
Algorithm (EdDSA) Signatures in the Cryptographic Message
Syntax (CMS)", RFC 8419, DOI 10.17487/RFC8419, August
2018, <
https://www.rfc-editor.org/info/rfc8419>.
[RFC8550] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Certificate Handling", RFC 8550, DOI 10.17487/RFC8550,
April 2019, <
https://www.rfc-editor.org/info/rfc8550>.
[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <
https://www.rfc-editor.org/info/rfc8551>.
[X680] ITU-T, "Information technology -- Abstract Syntax Notation
One (ASN.1): Specification of basic notation",
ITU-T Recommendation X.680, ISO/IEC 8824-1, August 2015,
<
https://www.itu.int/rec/T-REC-X.680>.
[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, ISO/IEC 8825-1, August
2015, <
https://www.itu.int/rec/T-REC-X.690/>.
Campbell & Housley Standards Track [Page 27]
RFC 8591 S/MIME for SIP Messaging April 2019
13.2. Informative References
[CPM] Open Mobile Alliance, "OMA Converged IP Messaging System
Description, Candidate Version 2.2", September 2017.
[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", August 2018,
<
https://www.usenix.org/system/files/conference/
usenixsecurity18/sec18-poddebniak.pdf>.
[RCS] GSMA, "RCS Universal Profile Service Definition Document,
Version 2.2", May 2018,
<
https://www.gsma.com/futurenetworks/wp-
content/uploads/2018/05/
Universal-Profile-RCC.71-v2.2.pdf>.
[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,
<
https://www.rfc-editor.org/info/rfc3325>.
[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,
<
https://www.rfc-editor.org/info/rfc3840>.
[RFC3860] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
<
https://www.rfc-editor.org/info/rfc3860>.
[RFC3862] Klyne, G. and D. Atkins, "Common Presence and Instant
Messaging (CPIM): Message Format", RFC 3862,
DOI 10.17487/RFC3862, August 2004,
<
https://www.rfc-editor.org/info/rfc3862>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<
https://www.rfc-editor.org/info/rfc4648>.
[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,
<
https://www.rfc-editor.org/info/rfc4976>.
Campbell & Housley Standards Track [Page 28]
RFC 8591 S/MIME for SIP Messaging April 2019
[RFC5438] Burger, E. and H. Khartabil, "Instant Message Disposition
Notification (IMDN)", RFC 5438, DOI 10.17487/RFC5438,
February 2009, <
https://www.rfc-editor.org/info/rfc5438>.
[RFC6121] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 6121, DOI 10.17487/RFC6121, March 2011,
<
https://www.rfc-editor.org/info/rfc6121>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <
https://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<
https://www.rfc-editor.org/info/rfc7516>.
[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,
<
https://www.rfc-editor.org/info/rfc7701>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <
https://www.rfc-editor.org/info/rfc7748>.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
<
https://www.rfc-editor.org/info/rfc8032>.
[RFC8224] Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 8224,
DOI 10.17487/RFC8224, February 2018,
<
https://www.rfc-editor.org/info/rfc8224>.
[RTP-Sec] Peterson, J., Barnes, R., and R. Housley, "Best Practices
for Securing RTP Media Signaled with SIP", Work in
Progress, draft-ietf-sipbrandy-rtpsec-08, April 2019.
Campbell & Housley Standards Track [Page 29]
RFC 8591 S/MIME for SIP Messaging April 2019
Appendix A. Message Details
The following section shows the detailed content of the S/MIME bodies
used in Section 10.
A.1. Signed Message
Figure 5 shows the details of the message signed by Alice used in the
example in Section 10.1.
CMS_ContentInfo:
contentType: pkcs7-signedData (1.2.840.113549.1.7.2)
d.signedData:
version: 1
digestAlgorithms:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
encapContentInfo:
eContentType: pkcs7-data (1.2.840.113549.1.7.1)
eContent:
0000 - 43 6f 6e 74 65 6e 74 2d-54 79 70 65 3a 20 74 Content-Type: t
000f - 65 78 74 2f 70 6c 61 69-6e 0d 0a 0d 0a 57 61 ext/plain....Wa
001e - 74 73 6f 6e 2c 20 63 6f-6d 65 20 68 65 72 65 tson, come here
002d - 20 2d 20 49 20 77 61 6e-74 20 74 6f 20 73 65 - I want to se
003c - 65 20 79 6f 75 2e 0d 0a- e you...
certificates:
d.certificate:
cert_info:
version: 2
serialNumber: 13292724773353297200
signature:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
issuer: O=example.com, CN=Alice
validity:
notBefore: Dec 19 23:12:05 2017 GMT
notAfter: Dec 19 23:12:05 2018 GMT
subject: O=example.com, CN=Alice
key:
algor:
algorithm: id-ecPublicKey (1.2.840.10045.2.1)
parameter: OBJECT:prime256v1 (1.2.840.10045.3.1.7)
public_key: (0 unused bits)
0000 - 04 d8 7b 54 72 9f 2c 22-fe eb d9 dd ba 0e ..{Tr.,"......
000e - fa 40 64 22 97 a6 09 38-87 a4 da e7 99 0b .@d"...8......
001c - 23 f8 7f a7 ed 99 db 8c-f5 a3 14 f2 ee 64 #............d
002a - 10 6e f1 ed 61 db fc 0a-4b 91 c9 53 cb d0 .n..a...K..S..
0038 - 22 a7 51 b9 14 80 7b b7-94 ".Q...{..
Campbell & Housley Standards Track [Page 30]
RFC 8591 S/MIME for SIP Messaging April 2019
issuerUID: <ABSENT>
subjectUID: <ABSENT>
extensions:
object: X509v3 Subject Alternative Name (2.5.29.17)
critical: BOOL ABSENT
value:
0000 - 30 17 86 15 73 69 70 3a-61 6c 69 63 65 0...sip:alice
000d - 40 65 78 61 6d 70 6c 65-2e 63 6f 6d @example.com
sig_alg:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
signature: (0 unused bits)
0000 - 30 45 02 20 78 79 be 1c-27 f8 46 27 6f df 15 0E. xy..'.F'o..
000f - e3 33 e5 3c 6f 17 a7 57-38 8a 02 cb 7b 8a e4 .3.<o..W8...{..
001e - 81 c1 64 1a e7 a9 02 21-00 ff 99 cd 9c 94 07 ..d....!.......
002d - 6c 82 b0 2f ea 3b 13 50-17 9a 4b 77 52 e1 6f l../.;.P..KwR.o
003c - a3 0a 3f 9a b2 96 50 b0-e2 81 89 ..?...P....
crls:
<ABSENT>
signerInfos:
version: 1
d.issuerAndSerialNumber:
issuer: O=example.com, CN=Alice
serialNumber: 13292724773353297200
digestAlgorithm:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
signedAttrs:
object: contentType (1.2.840.113549.1.9.3)
set:
OBJECT:pkcs7-data (1.2.840.113549.1.7.1)
object: signingTime (1.2.840.113549.1.9.5)
set:
UTCTIME:Jan 24 23:52:56 2019 GMT
object: messageDigest (1.2.840.113549.1.9.4)
set:
OCTET STRING:
0000 - ef 77 8f c9 40 d5 e6 dc-25 76 f4 7a 59 .w..@...%v.zY
000d - 9b 31 26 19 5a 9f 1a 22-7a da f3 5f a2 .1&.Z.."z.._.
001a - 2c 05 0d 8d 19 5a ,....Z
signatureAlgorithm:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
Campbell & Housley Standards Track [Page 31]
RFC 8591 S/MIME for SIP Messaging April 2019
signature:
0000 - 30 45 02 20 58 79 cc 62-85 e0 86 06 19 d3 bf 0E. Xy.b.......
000f - 53 d4 67 9f 03 73 d7 45-20 cf 56 10 c2 55 5b S.g..s.E .V..U[
001e - 7b ec 61 d4 72 dc 02 21-00 83 aa 53 44 28 4d {.a.r..!...SD(M
002d - 4c ef de 31 07 9c f9 71-bd 69 5d 6e c8 71 e9 L..1...q.i]n.q.
003c - a4 60 ec 2e 12 65 2b 77-a4 62 4d .`...e+w.bM
unsignedAttrs:
<ABSENT>
Figure 5: Signed Message
A.2. Short Signed Message
Figure 6 shows the message signed by Alice with no embedded
certificate, as used in the example in Section 10.2.
CMS_ContentInfo:
contentType: pkcs7-signedData (1.2.840.113549.1.7.2)
d.signedData:
version: 1
digestAlgorithms:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
encapContentInfo:
eContentType: pkcs7-data (1.2.840.113549.1.7.1)
eContent:
0000 - 43 6f 6e 74 65 6e 74 2d-54 79 70 65 3a 20 74 Content-Type: t
000f - 65 78 74 2f 70 6c 61 69-6e 0d 0a 0d 0a 57 61 ext/plain....Wa
001e - 74 73 6f 6e 2c 20 63 6f-6d 65 20 68 65 72 65 tson, come here
002d - 20 2d 20 49 20 77 61 6e-74 20 74 6f 20 73 65 - I want to se
003c - 65 20 79 6f 75 2e 0d 0a- e you...
certificates:
<ABSENT>
crls:
<ABSENT>
signerInfos:
version: 1
d.issuerAndSerialNumber:
issuer: O=example.com, CN=Alice
serialNumber: 13292724773353297200
digestAlgorithm:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
signedAttrs:
object: contentType (1.2.840.113549.1.9.3)
set:
OBJECT:pkcs7-data (1.2.840.113549.1.7.1)
Campbell & Housley Standards Track [Page 32]
RFC 8591 S/MIME for SIP Messaging April 2019
object: signingTime (1.2.840.113549.1.9.5)
set:
UTCTIME:Jan 24 23:52:56 2019 GMT
object: messageDigest (1.2.840.113549.1.9.4)
set:
OCTET STRING:
0000 - ef 77 8f c9 40 d5 e6 dc-25 76 f4 7a 59 .w..@...%v.zY
000d - 9b 31 26 19 5a 9f 1a 22-7a da f3 5f a2 .1&.Z.."z.._.
001a - 2c 05 0d 8d 19 5a ,....Z
signatureAlgorithm:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
signature:
0000 - 30 44 02 20 1c 51 6e ed-9c 10 10 a2 87 e1 11 0D. .Qn........
000f - 6b af 76 1d f1 c4 e6 48-da ea 17 89 bc e2 8a k.v....H.......
001e - 9d 8a f4 a4 ae f9 02 20-72 7f 5e 4b cc e2 0b ....... r.^K...
002d - cf 3c af 07 c8 1c 11 64-f0 21 e7 70 e0 f6 a0 .<.....d.!.p...
003c - 96 2e 0a 7b 19 b7 42 ad-cb 34 ...{..B..4
unsignedAttrs:
<ABSENT>
Figure 6: Signed Message without Embedded Certificate
A.3. Signed and Encrypted Message
The following sections show details for the message signed by Bob and
encrypted to Alice, as used in the examples in Sections 10.3
and 10.4.
A.3.1. Signed Message prior to Encryption
CMS_ContentInfo:
contentType: pkcs7-signedData (1.2.840.113549.1.7.2)
d.signedData:
version: 1
digestAlgorithms:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
encapContentInfo:
eContentType: pkcs7-data (1.2.840.113549.1.7.1)
eContent:
0000 - 43 6f 6e 74 65 6e 74 2d-54 79 70 65 3a 20 74 Content-Type: t
000f - 65 78 74 2f 70 6c 61 69-6e 0d 0a 0d 0a 57 61 ext/plain....Wa
001e - 74 73 6f 6e 2c 20 63 6f-6d 65 20 68 65 72 65 tson, come here
002d - 20 2d 20 49 20 77 61 6e-74 20 74 6f 20 73 65 - I want to se
003c - 65 20 79 6f 75 2e 0d 0a- e you...
Campbell & Housley Standards Track [Page 33]
RFC 8591 S/MIME for SIP Messaging April 2019
certificates:
d.certificate:
cert_info:
version: 2
serialNumber: 11914627415941064473
signature:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
issuer: O=example.org, CN=Bob
validity:
notBefore: Dec 20 23:07:49 2017 GMT
notAfter: Dec 20 23:07:49 2018 GMT
subject: O=example.org, CN=Bob
key:
algor:
algorithm: id-ecPublicKey (1.2.840.10045.2.1)
parameter: OBJECT:prime256v1 (1.2.840.10045.3.1.7)
public_key: (0 unused bits)
0000 - 04 86 4f ff fc 53 f1 a8-76 ca 69 b1 7e 27 ..O..S..v.i.~'
000e - 48 7a 07 9c 71 52 ae 1b-13 7e 39 3b af 1a Hz..qR...~9;..
001c - ae bd 12 74 3c 7d 41 43-a2 fd 8a 37 0f 02 ...t<}AC...7..
002a - ba 9d 03 b7 30 1f 1d a6-4e 30 55 94 bb 6f ....0...N0U..o
0038 - 95 cb 71 fa 48 b6 d0 a3-83 ..q.H....
issuerUID: <ABSENT>
subjectUID: <ABSENT>
extensions:
object: X509v3 Subject Alternative Name (2.5.29.17)
critical: TRUE
value:
0000 - 30 15 86 13 73 69 70 3a-62 6f 62 40 65 0...sip:bob@e
000d - 78 61 6d 70 6c 65 2e 6f-72 67 xample.org
sig_alg:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
signature: (0 unused bits)
0000 - 30 45 02 21 00 b2 24 8c-92 40 28 22 38 9e c9 0E.!..$..@("8..
000f - 25 7f 64 cc fd 10 6f ba-0b 96 c1 19 07 30 34 %.d...o......04
001e - d5 1b 10 2f 73 39 6c 02-20 15 8e b1 51 f0 85 .../s9l. ...Q..
002d - b9 bd 2e 04 cf 27 8f 0d-52 2e 6b b6 fe 4f 36 .....'..R.k..O6
003c - f7 4c 77 10 b1 5a 4f 47-9d e4 0d .Lw..ZOG...
crls:
<ABSENT>
signerInfos:
version: 1
d.issuerAndSerialNumber:
issuer: O=example.org, CN=Bob
serialNumber: 11914627415941064473
Campbell & Housley Standards Track [Page 34]
RFC 8591 S/MIME for SIP Messaging April 2019
digestAlgorithm:
algorithm: sha256 (2.16.840.1.101.3.4.2.1)
parameter: <ABSENT>
signedAttrs:
object: contentType (1.2.840.113549.1.9.3)
set:
OBJECT:pkcs7-data (1.2.840.113549.1.7.1)
object: signingTime (1.2.840.113549.1.9.5)
set:
UTCTIME:Jan 24 23:52:56 2019 GMT
object: messageDigest (1.2.840.113549.1.9.4)
set:
OCTET STRING:
0000 - ef 77 8f c9 40 d5 e6 dc-25 76 f4 7a 59 .w..@...%v.zY
000d - 9b 31 26 19 5a 9f 1a 22-7a da f3 5f a2 .1&.Z.."z.._.
001a - 2c 05 0d 8d 19 5a ,....Z
signatureAlgorithm:
algorithm: ecdsa-with-SHA256 (1.2.840.10045.4.3.2)
parameter: <ABSENT>
signature:
0000 - 30 45 02 21 00 f7 88 ed-44 6a b7 0f ff 2c 1f 0E.!....Dj...,.
000f - fa 4c 03 74 fd 08 77 fd-61 ee 91 7c 31 45 b3 .L.t..w.a..|1E.
001e - 89 a6 76 15 c7 46 fa 02-20 77 94 ad c5 7f 00 ..v..F.. w.....
002d - 61 c7 84 b9 61 23 cc 6e-54 bb 82 82 65 b6 d4 a...a#.nT...e..
003c - cc 12 99 76 a6 b1 fc 6d-bc 28 d6 ...v...m.(.
unsignedAttrs:
<ABSENT>
Figure 7: Message Signed by Bob prior to Encryption
A.3.2. Encrypted Message
CMS_ContentInfo:
contentType: pkcs7-authEnvelopedData (1.2.840.113549.1.9.16.1.23)
d.authEnvelopedData:
version: 0
originatorInfo: <ABSENT>
recipientInfos:
d.ktri:
version: <ABSENT>
d.issuerAndSerialNumber:
issuer: O=example.com, CN=Alice
serialNumber: 9508519069068149774
keyEncryptionAlgorithm:
algorithm: rsaEncryption (1.2.840.113549.1.1.1)
parameter: NULL
Campbell & Housley Standards Track [Page 35]
RFC 8591 S/MIME for SIP Messaging April 2019
encryptedKey:
0000 - 75 9a 61 b4 dd f1 f1 af-24 66 80 05 63 5e 47 u.a.....$f..c^G
000f - 61 10 fa 27 23 c1 b9 e4-54 84 b6 d3 3e 83 87 a..'#...T...>..
001e - de 96 7d c5 e0 ca fb 35-57 1a 56 a1 97 5c b5 ..}....5W.V..\.
002d - 50 e7 be 31 c1 31 da 80-fb 73 10 24 84 5b ab P..1.1...s.$.[.
003c - b8 d6 4c ac 26 04 04 24-d9 33 05 61 c8 43 99 ..L.&..$.3.a.C.
004b - 94 15 dd 64 4b 3c ad 95-07 2f 71 45 13 93 c9 ...dK<.../qE...
005a - 9f 28 2c 48 83 bd 0c cc-5d d5 4b 93 14 64 e0 .(,H....].K..d.
0069 - 0a 6e 55 e5 92 c5 1a 68-de 10 62 51 6e c7 d3 .nU....h..bQn..
0078 - ca 8e 76 4b b8 ac 78 9a-88 37 77 65 ef 8d c3 ..vK..x..7we...
0087 - 6c 0a 6e d3 ec ae 52 85-ca c6 a2 9d 50 59 44 l.n...R.....PYD
0096 - 57 19 a1 bd cf 90 6e 0f-f3 7e 2c 2e f0 f4 ec W.....n..~,....
00a5 - 62 25 10 0c c0 62 e1 c7-48 96 3b bc 88 b8 e3 b%...b..H.;....
00b4 - df cf 71 40 73 72 9d d5-c7 58 3e 75 8a cf 3d
[email protected]>u..=
00c3 - 18 6f 2f a4 17 be 22 c3-7c 9a 76 c6 b4 27 29 .o/...".|.v..')
00d2 - aa d2 7f 73 ae 44 ac 98-47 4d 1e eb 48 94 8c ...s.D..GM..H..
00e1 - 12 a4 03 d0 b3 ce 08 a2-18 d6 af 45 69 24 89 ...........Ei$.
00f0 - 7c c5 c9 66 4f 6d fe b3-f1 81 41 15 8d fc 3b |..fOm....A...;
00ff - 84 09 0a a6 03 80 aa 86-51 37 e1 69 9c 5c 81 ........Q7.i.\.
010e - 97 41 67 9d 7a 3c 90 ba-79 e6 d7 d5 c8 d8 9b .Ag.z<..y......
011d - b5 4a 66 74 23 e4 3b 0b-7d 6f 78 c0 b4 ab 67 .Jft#.;.}ox...g
012c - bc 34 36 62 a6 35 fe 59-5f 11 49 c5 39 50 ca .46b.5.Y_.I.9P.
013b - c2 e0 ba 31 8c 22 7e 6f-76 a8 d9 40 40 0f d3 ...1."~ov..@@..
014a - d3 ea 1c 8e ce a0 03 dc-ce 2f 1f b0 0f 5c ea ........./...\.
0159 - 33 5d e1 30 3f cb f9 3d-8e 1c bf d6 82 f1 9b 3].0?..=.......
0168 - eb 62 4b ac d1 d7 b8 f5-80 f1 14 a1 3b 89 08 .bK.........;..
0177 - 94 fb 40 44 a5 da a7 64-b7 f8 c5 ff 92 94 94
[email protected].......
0186 - 52 b3 5a eb 96 39 b8 ad-63 c0 51 5c 95 cc c6 R.Z..9..c.Q\...
0195 - f8 23 c2 20 10 67 ea 22-62 41 3f ef 39 7d 48 .#. .g."bA?.9}H
01a4 - f7 b6 14 3f 84 2a e8 e1-a4 8c ad 3a e0 1a ba ...?.*.....:...
01b3 - a3 cf 9e e7 e3 66 20 e0-5c ca 06 11 bf ac 00 .....f .\......
01c2 - ee f1 a4 98 f2 d2 59 b9-f0 f7 da 83 ef 6f 1b ......Y......o.
01d1 - 06 1f 38 7c 2d c4 8c 8b-5d ba ca 86 23 08 f3 ..8|-...]...#..
01e0 - 2f 47 92 51 65 c9 e5 eb-b4 67 79 98 84 91 8d /G.Qe....gy....
01ef - d6 97 b4 47 f4 c4 07 98-9b 88 9b 0c 2e 95 80 ...G...........
01fe - af 78 .x
authEncryptedContentInfo:
contentType: pkcs7-data (1.2.840.113549.1.7.1)
contentEncryptionAlgorithm:
algorithm: aes-128-gcm (2.16.840.1.101.3.4.1.6)
parameter:
aes-nonce:
0000 - 4d 87 57 22 2e ac 52 94-11 7f 0c 12 M.W"..R.....
aes-ICVlen: 16
encryptedContent:
0000 - fe 2f b3 de 0b f0 69 98-c3 9b f4 a9 52 fa bf ./....i.....R..
000f - 8b 0f ee 3d 7e 2e 85 18-1a ec f1 a8 9e 1a 2e ...=~..........
001e - de cd 94 04 88 56 12 df-c6 98 43 34 d8 60 2b .....V....C4..+
Campbell & Housley Standards Track [Page 36]
RFC 8591 S/MIME for SIP Messaging April 2019
002d - 77 49 b2 50 4e 45 f5 7c-3b 06 66 26 b0 fc 74 wI.PNE.|..f&..t
003c - 62 36 1e ec 26 7c 56 01-39 be 5c d2 86 a2 af b6..&|V.9.\....
004b - 96 96 cf 51 85 22 78 e5-2c 38 18 ca b0 a6 8c ...Q."x.,8.....
005a - 59 8d e4 fc e1 4a 33 38-84 e4 de 5d df 57 ed Y....J38...].W.
0069 - d7 88 67 02 7a 31 e4 a7-c0 c0 29 91 44 c5 de ..g.z1....).D..
0078 - 6b ae 39 69 9e 70 0e 05-7e b0 f0 da d7 3b 8b k.9i.p..~......
0087 - 36 9f 42 eb 32 1b 41 53-87 81 d9 82 a1 1a 0b 6.B.2.AS.......
0096 - 39 43 ac 10 c9 7b 54 ee-b7 3b 38 ec 13 1a fc 9C...{T...8....
00a5 - 56 10 e3 73 48 72 74 d6-9c af a9 54 19 02 88 V..sHrt....T...
00b4 - 6c 64 f6 96 2d 42 eb 33-f9 04 1a 4a e1 1b 88 ld..-B.3...J...
00c3 - dc 69 58 d5 3d f5 0b 8b-b5 2a a3 5e 22 99 88 .iX.=....*.^"..
00d2 - 5d 0a ae 41 6b 86 f0 a8-8d 0e b7 a9 81 db b2 ]..Ak..........
00e1 - 83 e8 b9 4e 9d 50 bf 62-65 c2 34 8a 18 a1 69 ...N.P.be.4...i
00f0 - aa cb 5a 37 a5 29 bd a2-f9 cb 10 ef dd cf 14 ..Z7.).........
00ff - 23 10 95 d8 79 64 63 7b-d3 3f b1 3c 68 b4 cf #...ydc{.?.<h..
010e - f9 a1 90 69 60 c1 ea 23-01 d3 25 b7 a1 5c 58 ...i...#..%..\X
011d - 29 f3 ea 03 8f 24 df 6b-23 18 03 77 d3 71 31 )....$.k#..w.q1
012c - f7 5d b1 8f 41 f9 d8 5b-65 3d fa 46 bf 26 17 .]..A..[e=.F.&.
013b - 12 63 26 cc f1 cb 83 34-57 75 23 52 c8 41 7a .c&....4Wu#R.Az
014a - 09 44 84 d7 b6 4b cf 51-b2 6a 9b eb 3a 0e d4 .D...K.Q.j..:..
0159 - b9 ca f1 bd 23 c6 90 c6-54 f7 eb 9c e9 85 2e ....#...T......
0168 - 2f 6d 06 8e ef 8b a3 3b-c6 c4 dd dc a7 ae f4 /m.............
0177 - d3 57 47 37 d7 c4 dc 1e-93 77 0d 8f 4f 22 de .WG7.....w..O".
0186 - a6 1d 73 08 3c 32 c4 03-8c 1e b3 dd 33 83 a8 ..s.<2......3..
0195 - 9a 87 95 e2 41 c2 ed 7c-b6 80 75 8c 04 10 69 ....A..|..u...i
01a4 - 48 98 60 fc 9f 49 0e 85-23 60 72 54 8b 32 49 H....I..#.rT.2I
01b3 - 69 8f 99 95 3a cf 1e c6-58 b7 aa 85 e5 54 c4 i...:...X....T.
01c2 - 49 70 1a 6d 4b 03 9e d1-03 dc 45 8d f4 b2 9c Ip.mK.....E....
01d1 - b0 4b 8c ed d5 40 c8 43-48 da 79 c1 86 56 d5
[email protected].
01e0 - 18 8f 9f 3a 9e 4b 9b 84-0c 70 66 4b 90 29 6c ...:.K...pfK.)l
01ef - 60 b7 ac 98 4e 91 8d 48-a0 9d bd df b2 81 fc ....N..H.......
01fe - 86 25 10 db 59 d9 fa 9d-c9 3f 10 f9 c6 d7 be .%..Y....?.....
020d - f7 29 31 d1 84 ca d7 ac-13 c1 a5 29 5f c8 9f .)1........)_..
021c - e3 bb 7e b8 e0 20 85 a8-28 c5 a1 38 78 6e 60 ..~.....(..8xn.
022b - 7a de 4f 5e 8d 41 15 90-92 09 ba 87 8a 79 30 z.O^.A.......y0
023a - 5a 53 16 c2 22 29 e4 2b-88 6d 06 48 1c 84 73 ZS..").+.m.H..s
0249 - f9 d5 12 69 e2 af 63 41-bc e2 0f 76 8e 86 0d ...i..cA...v...
0258 - 77 84 ed 46 15 0e 04 ff-50 cd 20 9c 5b 12 75 w..F....P...[.u
0267 - 11 36 9f e0 6b c4 aa 9a-72 d8 f1 fe 4f cf 08 .6..k...r...O..
0276 - 66 d6 64 b3 65 ff a8 6e-8c 1b 43 e7 a9 21 2a f.d.e..n..C..!*
0285 - ec c1 6c a3 50 a2 8e fa-e2 5f ac 05 4d d9 34 ..l.P...._..M.4
0294 - bf e7 e5 fa 4f 75 3a a4-15 96 8c 7e be c4 39 ....Ou:....~..9
02a3 - e0 ac 02 70 b4 87 4a 06-8d 22 48 4c 09 d9 e8 ...p..J.."HL...
02b2 - ab e1 7f 13 72 b4 b2 f6-5f 11 48 e8 93 3e da ....r..._.H..>.
02c1 - 92 e5 d1 77 45 64 96 3b-39 1c 3b bd 9f 1c 27 ...wEd..9......
02d0 - ff e3 6f 83 2e 05 15 5f-c3 9e e6 65 2f a7 b4 ..o...._...e/..
02df - 18 89 75 ec 5c 67 b3 2c-9f 21 3c 8a c6 b8 e1 ..u.\g.,.!<....
02ee - 32 a5 a7 c3 bf 74 f0 16-40 5c d8 c2 01 d1 05 2....t..@\.....
Campbell & Housley Standards Track [Page 37]
RFC 8591 S/MIME for SIP Messaging April 2019
02fd - 21 93 e1 86 d4 43 58 de-38 8d 73 21 1b a2 f1 !....CX.8.s!...
030c - 79 2f 3c fe b9 bb de 72-11 d2 6f 56 ab 06 e1 y/<....r..oV...
031b - 1c cc 9c cd e2 b8 8c d8-37 37 73 ea fc 37 fd ........77s..7.
032a - 85 b7 a7 a2 bc ae c7 52-e6 17 d6 e0 1c 02 b8 .......R.......
0339 - 6e 9d 9a 40 f3 20 46 2c-5d 66 f8 35 17 16 dc
[email protected],]f.5...
0348 - d6 01 4b df 30 a6 0f 75-fc 06 31 c9 20 84 5e ..K.0..u..1...^
0357 - d8 c0 ba d3 5d df 19 84-f2 24 1c d3 b5 29 dc ....]....$...).
0366 - 10 28 84 5f 80 89 54 3d-f4 f1 44 1e de 36 b1 .(._..T=..D..6.
0375 - bf 31 af 5a fc 8c 2b 70-8d 50 b6 45 d4 e7 db .1.Z..+p.P.E...
0384 - 88 64 8c 3e ef e1 47 65-15 8f b0 e8 d3 bb 53 .d.>..Ge......S
0393 - dd cb e2 6d 71 24 c6 e1-d9 92 f8 32 30 aa 95 ...mq$.....20..
03a2 - 33 76 ee 8c 68 10 95 68-e8 57 1f 0c 9b bd a4 3v..h..h.W.....
03b1 - 8f 4d f3 06 fe 74 7f 37-11 75 14 8f 31 83 27 .M...t.7.u..1..
03c0 - 67 cd 76 6c f0 7b 45 0c-bf 62 ca d2 a7 bd 71 g.vl.{E..b....q
03cf - f1 f8 82 33 f1 16 a1 a7-f3 ca f1 2f 34 bc f4 ...3......./4..
03de - 0d 21 e7 9f fc 98 27 22-1b 68 b0 80 ff 03 ad .!.....".h.....
03ed - 78 2d 6d 6d 07 87 16 76-f7 98 94 3e 54 f1 3f x-mm...v...>T.?
03fc - d7 5c 89 c0 b4 26 3b f1-0f 56 24 3f 9e 72 ef .\...&...V$?.r.
040b - 3b 38 99 a5 39 d9 a3 ac-5b e2 b6 94 00 a3 cf .8..9...[......
041a - 8d 19 6c 5c ed 69 7b 2e-d8 03 b9 87 a5 ee 85 ..l\.i{........
0429 - c5 09 5b 48 da 7a 5b 03-b4 7e 2b 9f e4 cd 4b ..[H.z[..~+...K
0438 - c3 09 8e 86 4e 0c e7 d4-67 da 99 cd 7f 3a 9e ....N...g....:.
0447 - 94 7b 5e ea 77 f7 a6 be-16 c8 c7 e9 e0 de cc .{^.w..........
0456 - 1f f1 32 55 9c 23 43 21-7b 9c 29 50 38 6e 85 ..2U.#C!{.)P8n.
0465 - d2 94 21 21 08 6c df a1-96 58 19 5b e6 d7 f8 ..!!.l...X.[...
0474 - 6b ca 98 81 b6 95 08 29-64 f1 2e 7c f8 01 02 k......)d..|...
0483 - 5d 67 92 c6 88 24 09 41-4d 70 33 21 ec 83 ab ]g...$.AMp3!...
0492 - d6 98 d6 89 56 11 87 13-a0 ff 12 72 ac bc 9a ....V......r...
04a1 - 6d 14 89 00 c7 4c 16 92-1d f9 b3 8f 29 ec 46 m....L......).F
04b0 - d4 f1 00 60 ff fe 5e 36-bb ba ca f2 d1 ba d7 ......^6.......
04bf - dd 05 7e d3 e3 0e bc d6-90 83 f9 d3 a2 a2 6e ..~...........n
04ce - f9 0b 75 1d 6a 1a df a0-59 0d b1 9d a1 07 cf ..u.j...Y......
04dd - 3e a8 db >..
authAttrs:
<EMPTY>
mac:
0000 - f6 ff c6 e1 ae f1 9c d2-3d 98 5a 92 19 76 35 ........=.Z..v5
000f - 2d -
unauthAttrs:
<EMPTY>
Figure 8: Message Encrypted by Bob for Alice
Campbell & Housley Standards Track [Page 38]
RFC 8591 S/MIME for SIP Messaging April 2019
Authors' Addresses
Ben Campbell
Standard Velocity, LLC
Email:
[email protected]
Russ Housley
Vigil Security, LLC
Email:
[email protected]
Campbell & Housley Standards Track [Page 39]
