Cryptographic Message Syntax (CMS) Multiple Signer Clarification
Clarifies how to handle multiple digital signatures in signed messages
Housley, R., "Cryptographic Message Syntax (CMS) Multiple Signer Clarification", RFC 4853, DOI 10.17487/RFC4853, April 2007, <https://www.rfc-editor.org/info/rfc4853>.
@misc{rfc4853,
series = {Request for Comments},
number = 4853,
howpublished = {RFC 4853},
publisher = {RFC Editor},
doi = {10.17487/RFC4853},
url = {https://www.rfc-editor.org/info/rfc4853},
author = {R. Housley},
title = {{Cryptographic Message Syntax (CMS) Multiple Signer Clarification}},
pagetotal = 5,
year = 2007,
month = apr,
} RFC text
Contents
Network Working Group R. Housley
Request for Comments: 4853 Vigil Security
Updates: 3852 April 2007
Category: Standards Track
Cryptographic Message Syntax (CMS)
Multiple Signer Clarification
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document updates the Cryptographic Message Syntax (CMS), which
is published in RFC 3852. This document clarifies the proper
handling of the SignedData protected content type when more than one
digital signature is present.
1. Introduction
This document updates the Cryptographic Message Syntax [CMS]. The
CMS SignedData protected content type allows multiple digital
signatures, but the specification is unclear about the appropriate
processing by a recipient of such a signed content. This document
provides replacement text for a few paragraphs, making it clear that
the protected content is validly signed by a given signer, if any of
the digital signatures from that signer are valid.
This property is especially important in two cases. First, when the
recipients do not all implement the same digital signature algorithm,
a signer can sign the content with several different digital
signature algorithms so that each of the recipients can find an
acceptable signature. For example, if some recipients support RSA
and some recipients support ECDSA, then the signer can generate two
signatures, one with RSA and one with ECDSA, so that each recipient
will be able to validate one of the signatures. Second, when a
community is transitioning one-way hash functions or digital
signature algorithms, a signer can sign the content with the older
and the newer signature algorithms so that each recipient can find an
acceptable signature, regardless of their state in the transition.
For example, consider a transition from RSA with SHA-1 to RSA with
SHA-256. The signer can generate two signatures, one with SHA-1 and
one with SHA-256, so that each recipient will be able to validate at
least one of the RSA signatures.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [STDWORDS].
3. Update to RFC 3852, Section 5: Signed-data Content Type
RFC 3852, section 5, the next to the last paragraph says:
| A recipient independently computes the message digest. This message
| digest and the signer's public key are used to verify the signature
| value. The signer's public key is referenced either by an issuer
| distinguished name along with an issuer-specific serial number or by
| a subject key identifier that uniquely identifies the certificate
| containing the public key. The signer's certificate can be included
| in the SignedData certificates field.
This block of text is replaced with:
| A recipient independently computes the message digest. This message
| digest and the signer's public key are used to verify the signature
| value. The signer's public key is referenced either by an issuer
| distinguished name along with an issuer-specific serial number or by
| a subject key identifier that uniquely identifies the certificate
| containing the public key. The signer's certificate can be included
| in the SignedData certificates field.
|
| When more than one signature is present, the successful validation
| of one signature associated with a given signer is usually treated
| as a successful signature by that signer. However, there are some
| application environments where other rules are needed. An
| application that employs a rule other than one valid signature for
| each signer must specify those rules. Also, where simple matching of
| the signer identifier is not sufficient to determine whether the
| signatures were generated by the same signer, the application
| specification must describe how to determine which signatures were
| generated by the same signer. Support of different communities of
| recipients is the primary reason that signers choose to include more
| than one signature. For example, the signed-data content type might
| include signatures generated with the RSA signature algorithm and
| with the ECDSA signature algorithm. This allows recipients to
| verify the signature associated with one algorithm or the other.
4. Update to RFC 3852, Section 5.1: SignedData Type
RFC 3852, section 5.1, the next to the last paragraph says:
| signerInfos is a collection of per-signer information. There MAY
| be any number of elements in the collection, including zero. The
| details of the SignerInfo type are discussed in section 5.3.
| Since each signer can employ a digital signature technique and
| future specifications could update the syntax, all implementations
| MUST gracefully handle unimplemented versions of SignerInfo.
| Further, since all implementations will not support every possible
| signature algorithm, all implementations MUST gracefully handle
| unimplemented signature algorithms when they are encountered.
This block of text is replaced with:
| signerInfos is a collection of per-signer information. There MAY
| be any number of elements in the collection, including zero. When
| the collection represents more than one signature, the successful
| validation of one of signature from a given signer ought to be
| treated as a successful signature by that signer. However,
| there are some application environments where other rules are
| needed. The details of the SignerInfo type are discussed in
| section 5.3. Since each signer can employ a different digital
| signature technique, and future specifications could update the
| syntax, all implementations MUST gracefully handle unimplemented
| versions of SignerInfo. Further, since all implementations will
| not support every possible signature algorithm, all
| implementations MUST gracefully handle unimplemented signature
| algorithms when they are encountered.
6. Security Considerations
The replacement text will reduce the likelihood of interoperability
errors during the transition from MD5 and SHA-1 to stronger one-way
hash functions, or to better signature algorithms.
7. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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On this page
§1The Multiple-Signer Problem
Cryptographic Message Syntax (CMS) is a standard format for signed, encrypted, and authenticated messages. The SignedData content type in CMS allows one signer to attach multiple digital signatures to the same content, but the original specification in RFC 3852 did not make clear how recipients should interpret this.
This document updates RFC 3852 to clarify that when a single signer provides multiple signatures, recipients should treat the content as validly signed by that signer if any one of those signatures is valid. The application does not need to verify all of them.
Imagine you sign a letter, but you write your name with three different pens in case the reader can only see some colors. The letter is still just signed once by you, even though your name appears three times.
§2Why Signers Use Multiple Signatures
There are two main practical reasons for a signer to create multiple signatures on the same content. First, different recipients may support different signature algorithms. For example, some recipients might only support RSA, while others only support ECDSA. A signer can generate both an RSA signature and an ECDSA signature so that each recipient can verify the one it understands.
Second, communities sometimes transition from older to newer algorithms or hash functions. During the migration from SHA-1 to SHA-256, for example, a signer can produce both a SHA-1-based signature and a SHA-256-based signature. This way, recipients that still only understand the older algorithm can verify one signature, while those that have upgraded can verify the other.
Some people read only English, some only Spanish. If you want everyone to understand your letter, you might write it in both languages. Your message is still one, just available in two forms.
§3What This Clarification Means
The updated specification makes two key points. First, when more than one signature from a given signer is present, successful validation of just one of them counts as successful signing by that signer. Second, applications that require different rules (for example, requiring all signatures to be valid) must explicitly state those rules in their own specification.
The clarification also notes that when determining whether multiple signatures come from the same signer, simple matching of the signer's identifier is usually sufficient, but some applications may need more sophisticated matching rules.
The rule is simple: if you see a valid signature from someone, the message is signed by that person, even if you also see other signatures from them that don't work or that you don't recognize.