[ietf-dkim] Proposed fingerprint tag description
Murray S. Kucherawy
msk at sendmail.com
Wed Apr 12 12:56:32 PDT 2006
Following up on prior discussion, I'd like to start working on some formal
wording for a fingerprint tag. This will be used to describe the results of
verifying a message with multiple signatures present, so that the receiver can
match up each result to its respective signature even if there's been some
This would be added to the section describing the tags present in signature
headers, namely "3.5 The DKIM-Signature header field" section:
f= Signature fingerprint (plain-text; OPTIONAL)
The fingerprint of the signature. This is a short string which is used to
distinguish between two different signatures of the same message, which may have
been added at the same time by the same signer, so that the receiver can decide
which of two or more conflicting results makes a more desirable assertion. The
fingerprint SHOULD be fairly short but MUST be unique among signature headers
added by the same signer. Thus, the concatenation of the values of "f=" and
"i=" will uniquely distinguish this signature from any others that might be
added between the originator and the receiver.
INFORMATIVE IMPLEMENTATION NOTE: It is likely sufficient to do something akin to
encoding the first four bytes of the body hash in hexadecimal, e.g.
f=44:4B:49:4D or f=444b494d.
If the "f=" tag is absent, a verifier can infer its value by using the first
four bytes of the actual (i.e. base64-decoded) "bh" tag's value when describing
its results to receivers. However, since the base64-decoded version of the "bh"
tag is not easily visible upon simple header inspection, signers SHOULD add this
tag for clarity.
This would also work even without the "bh" change to the specification; instead
the signer would just use the first few bytes of the single generated hash that
we have today.
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