[ietf-dkim] Interesting Dupe Signatures

[hector]

John Levine wrote:

> Assuming you make the generally useful optimization to do 

 > the body hash only once, it's hard to see a significant
 > load from an extra signature check.

+1.

But a body hash is calculated per signature because it might have a 
different c14n parsing.  So the ideal optimization would be to reduce 
the body hashing to unique c= c14n types. Given a ridiculous multiple 
signatures case:

   DKIM-Signature: .... c=relaxed/relaxed;   // dupe
   DKIM-Signature: .... c=relaxed/relaxed;   // dupe
   DKIM-Signature: .... c=relaxed/relaxed;   // dupe
   DKIM-Signature: .... c=simple/relaxed;    // other signer
   DKIM-Signature: .... c=relaxed/relaxed;   // other signer

A reduction consideration with body hash calculations per signature 
c14n type has some merit when mail size is a consideration.  In this 
case, a minimum of two calculations are necessary.

--
HLS