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RE: S/MIME counter-signature: comment and question

1997-12-16 11:10:47
If you want to show unequivocally you sent the message then you construct a
new signing data layer with the existing data nested within, rather than add
a new signer info block to the existing signed. If you want to know the
sequence a series of parallel signatures where constructed then use time to
differentiate the signatures.

-----Original Message-----
From: Tim Dean [SMTP:t(_dot_)dean(_at_)eris(_dot_)dera(_dot_)gov(_dot_)uk]
Sent: 16 December, 1997 7:45 AM
To:   'ietf-smime(_at_)imc(_dot_)org'
Subject:      S/MIME counter-signature: comment and question

CMS-01.txt states (section 5.3) that countersigning can be done by
including a PKCS#9 countersignature attribute as an unauthenticated
attribute. Looking at CounterSignature in PKCS#9, it specifies that it is
what is signed is 'the contents octets of the DER encoding of the
encryptedDigest field of the SignerInfo value with which the attribute is
associated.'  However, it seems that the encryptedDigest attribute has
been removed from SignerInfo in CMS-01, though it is in PKCS#7.  Is this
an inconsistency or have I missed something? 

A related question: As a receiver, how do I tell who sent an S/MIME
message in the case where there are multiple signatures (or
countersignatures) on it. How can I be prevented as a sender from getting
someone to parallel-sign or counter-sign my S/MIME document, sending it
out as a mail message, and claiming it was they who sent it (e.g. by
putting their E-mail address in the From: field).  Thoughts?


Tim Dean
 (Standard Disclaimer applies)