Multiple Signatures in CMS

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Multiple Signatures in CMS

• Russ Housley
• IETF 66, Montreal, Canada

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Goal

• Change the processing in CMS to ensure that it
  accommodates transition from one signature
  algorithm to another one
• S/MIME Mechanism the originator generates two
  signatures (one with the old algorithm and one
  with the new algorithm), and the recipient
  considers the message valid if either one of the
  signatures validates

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ASN.1 Syntax Reminder (1 of 2)

• SignedData SEQUENCE
• version CMSVersion,
• digestAlgorithms DigestAlgorithmIdentifier
  s,
• encapContentInfo EncapsulatedContentInfo,
• certificates 0 IMPLICIT CertificateSet
  OPTIONAL,
• crls 1 IMPLICIT RevocationInfoChoices
  OPTIONAL,
• signerInfos SignerInfos
• SignerInfos SET OF SignerInfo

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ASN.1 Syntax Reminder (2 of 2)

• SignerInfo SEQUENCE
• version CMSVersion,
• sid SignerIdentifier,
• digestAlgorithm DigestAlgorithmIdentifier,
  
• signedAttrs 0 IMPLICIT SignedAttributes
  OPTIONAL,
• signatureAlgorithm SignatureAlgorithmIdent
  ifier,
• signature SignatureValue,
• unsignedAttrs 1 IMPLICIT
  UnsignedAttributes OPTIONAL
• SignerIdentifier CHOICE
• issuerAndSerialNumber IssuerAndSerialNumbe
  r,
• subjectKeyIdentifier 0
  SubjectKeyIdentifier

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One Signature
Fred RSA with SHA-1
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Two Signatures
Fred RSA with SHA-1
Fred RSA with SHA-256
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Four Signatures
Fred RSA with SHA-1
Fred RSA with SHA-256
Gary ECDSA with SHA-256
Gary RSA with SHA-256
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Five Signatures
Earl DSA with SHA-1
Fred RSA with SHA-256
Gary ECDSA with SHA-256
Harry RSA with SHA-1
Irene RSA with SHA-256
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draft-ietf-smime-cms-mult-sign-00

• The text in the current draft is not quite
  right.It says
• When more than one signature is present, the
  successful validation
• of any one of these signatures ought to be
  treated as a successful
• validation of the signed-data content type.
  The primary reason
• is that signers may include separate signatures
  for different
• communities of recipients. 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 one algorithm or the other.

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Proposed Way Forward

• Proposed replacement text
• When more than one signature is present, the
  successful validation
• of one of signature associated with each signer
  ought to be treated
• as a successful validation of the signed-data
  content type. However,
• there are some application environments where
  all of the included
• signatures must be valid. 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
• one algorithm or the other.

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Discussion!Questions?