RFC 4853

RFC 4853: Cryptographic Message Syntax (CMS) Multiple Signer Clarification 🦋

okay bestie, this one is a Standards Track doc from April 2007 💅 It's written by Russell Housley from Vigil Security, and it's published as RFC 4853 from the Network Working Group. It updates RFC 3852, which is the CMS spec. The title block doesn't list anything under Obsoletes, so it doesn't obsolete anything. 🎀

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Status of This Memo ✨

ok so this is an Internet standards track protocol for the Internet community, and the authors are asking for discussion and suggestions for improvements 💖 For the official standardization state and status, check the current edition of "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌸

Copyright Notice 🌈

Copyright (C) The IETF Trust (2007). 💜

Abstract 🧚

so this doc updates CMS, which is published in RFC 3852 🫶 It clarifies how you properly handle the SignedData protected content type when there's more than one digital signature on it 🔥

1. Introduction ✨

this doc updates the Cryptographic Message Syntax [CMS] 💅 The SignedData protected content type lets you have multiple digital signatures, but the spec isn't clear on how a recipient should process that signed content 😭 So this doc gives replacement text for a few paragraphs and makes it crystal clear: the protected content is validly signed by a given signer if any of that signer's digital signatures are valid ✨

this property is a big deal in two situations 👑

First, when recipients don't all support the same digital signature algorithm. The signer can sign the content with several different algorithms so that each recipient can find one they accept. For example, if some recipients support RSA and some support ECDSA, the signer can make two signatures, one with RSA and one with ECDSA, so each recipient can validate one of them 🦋

Second, when a community is transitioning between one-way hash functions or digital signature algorithms. The signer can sign with both the older and the newer algorithms, so every recipient finds an acceptable signature no matter where they are in the transition 🍓 For example, think about moving from RSA with SHA-1 to RSA with SHA-256. The signer makes two signatures, one with SHA-1 and one with SHA-256, so each recipient can 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]. 💜

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3. Update to RFC 3852, Section 5: Signed-data Content Type 🔥

okay so RFC 3852, section 5, the next to the last paragraph says this 👀

|  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.

and that block gets replaced with this 🫶

|  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.

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4. Update to RFC 3852, Section 5.1: SignedData Type 💖

same energy here: 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.

and it gets replaced with this ✨

|     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.

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6. Security Considerations 🔥

this is the serious part, bestie 😭 The replacement text reduces 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

Full Copyright Statement 💅

Copyright (C) The IETF Trust (2007).

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