draft-ietf-smime-cms-mult-sign-02.txt   draft-ietf-smime-cms-mult-sign-03.txt 
S/MIME Working Group R. Housley S/MIME Working Group R. Housley
Updates: 3852 (once approved) Vigil Security Updates: 3852 (once approved) Vigil Security
Expires June 2007 November 2006 Expires August 2007 February 2007
Cryptographic Message Syntax (CMS) Cryptographic Message Syntax (CMS)
Multiple Signer Clarification Multiple Signer Clarification
<draft-ietf-smime-cms-mult-sign-02.txt> <draft-ietf-smime-cms-mult-sign-03.txt>
Status of this Memo Status of this Memo
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Abstract Abstract
This document updates the Cryptographic Message Syntax (CMS), which This document updates the Cryptographic Message Syntax (CMS), which
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handling of the SignedData protected content type when more than one handling of the SignedData protected content type when more than one
digital signature is present. digital signature is present.
1. Introduction 1. Introduction
This document updates the Cryptographic Message Syntax (CMS) [CMS]. This document updates the Cryptographic Message Syntax (CMS) [CMS].
The CMS SignedData protected content type allows multiple digital The CMS SignedData protected content type allows multiple digital
signatures, but the specification is unclear about the appropriate signatures, but the specification is unclear about the appropriate
processing by a recipient of such a signed content. This document processing by a recipient of such a signed content. This document
provides replacement text for a few paragraphs, making it clear that provides replacement text for a few paragraphs, making it clear that
the protected content is valid if any of the digital signatures for a the protected content is validly signed by a given signer, if any of
particular signer is valid. the digital signatures from that signer is valid.
This property is especially important in two cases. First, when the This property is especially important in two cases. First, when the
recipients do not all implement the same digital signature algorithm, recipients do not all implement the same digital signature algorithm,
a signer can sign the content with several different digital a signer can sign the content with several different digital
signature algorithms so that each of the recipients can find an signature algorithms so that each of the recipients can find an
acceptable signature. For example, if some recipients support RSA acceptable signature. For example, if some recipients support RSA
and some recipients support ECDSA, then the signer can generate two and some recipients support ECDSA, then the signer can generate two
signatures, one with RSA and one with ECDSA, so that each recipient signatures, one with RSA and one with ECDSA, so that each recipient
will be able to validate one of the signature. Second, when a will be able to validate one of the signatures. Second, when a
community is transitioning one-way hash functions or digital community is transitioning one-way hash functions or digital
signature algorithms, a signer can sign the content with the older signature algorithms, a signer can sign the content with the older
and the newer signature algorithms so that each recipient can find an and the newer signature algorithms so that each recipient can find an
acceptable signature, regardless of their state in the transition. acceptable signature, regardless of their state in the transition.
For example, consider a transition from RSA with SHA-1 to RSA with 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 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 one with SHA-256, so that each recipient will be able to validate at
least one of the RSA signatures. least one of the RSA signatures.
2. Terminology 2. Terminology
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| A recipient independently computes the message digest. This message | A recipient independently computes the message digest. This message
| digest and the signer's public key are used to verify the signature | 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 | value. The signer's public key is referenced either by an issuer
| distinguished name along with an issuer-specific serial number or by | distinguished name along with an issuer-specific serial number or by
| a subject key identifier that uniquely identifies the certificate | a subject key identifier that uniquely identifies the certificate
| containing the public key. The signer's certificate can be included | containing the public key. The signer's certificate can be included
| in the SignedData certificates field. | in the SignedData certificates field.
| |
| When more than one signature is present, the successful validation | When more than one signature is present, the successful validation
| of one signature associated with each signer is usually treated | of one signature associated with a given signer is usually treated
| as a successful validation of the signed-data content type. However, | as a successful signature by that signer. However, there are some
| there are some application environments where other rules are needed. | application environments where other rules are needed. An
| An application that employs a rule other than one valid signature for | application that employs a rule other than one valid signature for
| each signer must specify those rules. Also, where simple matching of | each signer must specify those rules. Also, where simple matching of
| the signer identifier is not sufficient to determine whether the | the signer identifier is not sufficient to determine whether the
| signatures were generated by the same signer, the application | signatures were generated by the same signer, the application
| specification must describe how to determine which signatures were | specification must describe how to determine which signatures were
| generated by the same signer. Support of different communities of | generated by the same signer. Support of different communities of
| recipients is the primary reason that signers choose to include more | recipients is the primary reason that signers choose to include more
| than one signature. For example, the signed-data content type might | than one signature. For example, the signed-data content type might
| include signatures generated with the RSA signature algorithm and | include signatures generated with the RSA signature algorithm and
| with the ECDSA signature algorithm. This allows recipients to | with the ECDSA signature algorithm. This allows recipients to
| verify the signature associated with one algorithm or the other. | verify the signature associated with one algorithm or the other.
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| MUST gracefully handle unimplemented versions of SignerInfo. | MUST gracefully handle unimplemented versions of SignerInfo.
| Further, since all implementations will not support every possible | Further, since all implementations will not support every possible
| signature algorithm, all implementations MUST gracefully handle | signature algorithm, all implementations MUST gracefully handle
| unimplemented signature algorithms when they are encountered. | unimplemented signature algorithms when they are encountered.
This block of text is replaced with: This block of text is replaced with:
| signerInfos is a collection of per-signer information. There MAY | signerInfos is a collection of per-signer information. There MAY
| be any number of elements in the collection, including zero. When | be any number of elements in the collection, including zero. When
| the collection represents more than one signature, the successful | the collection represents more than one signature, the successful
| validation of one of signature from each signer ought to be | validation of one of signature from a given signer ought to be
| treated as a successful validation of the signed-data content | treated as a successful signature by that signer. However,
| type. However, there are some application environments where | there are some application environments where other rules are
| other rules are needed. The details of the SignerInfo type are | needed. The details of the SignerInfo type are discussed in
| discussed in section 5.3. Since each signer can employ a | section 5.3. Since each signer can employ a different digital
| different digital signature technique and future specifications | signature technique and future specifications could update the
| could update the syntax, all implementations MUST gracefully | syntax, all implementations MUST gracefully handle unimplemented
| handle unimplemented versions of SignerInfo. Further, since all | versions of SignerInfo. Further, since all implementations will
| implementations will not support every possible signature | not support every possible signature algorithm, all
| algorithm, all implementations MUST gracefully handle | implementations MUST gracefully handle unimplemented signature
| unimplemented signature algorithms when they are encountered. | algorithms when they are encountered.
6. Security Considerations 6. Security Considerations
The replacement text will reduce the likelihood of interoperability The replacement text will reduce the likelihood of interoperability
errors during the transition from MD5 and SHA-1 to stronger one-way errors during the transition from MD5 and SHA-1 to stronger one-way
hash functions, or to better signature algorithms. hash functions, or to better signature algorithms.
7. Normative References 7. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", [CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
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Russell Housley Russell Housley
Vigil Security, LLC Vigil Security, LLC
918 Spring Knoll Drive 918 Spring Knoll Drive
Herndon, VA 20170 Herndon, VA 20170
USA USA
EMail: housley(at)vigilsec.com EMail: housley(at)vigilsec.com
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