[ietf-dkim] Re: DoS and Replay protection for message signatures

[Douglas Otis]

On Fri, 2005-08-05 at 13:20 -0500, Earl Hood wrote:
> On August 5, 2005 at 09:02, Douglas Otis wrote:
> 
> > > You could potentially save even a little more if the data that
> > > is signed is completely in the message headers.  For example, if
> > > a separate hash of the body is computed and placed in the
> > > DKIM-Signature field, the cryptographic signature would be limited
> > > to header only data while still protected the integrity of the
> > > body.
> ...
> > > The separate hash of the body also allows for limited verification
> > > of a message when the body data is not available.
> > 
> > This sounds like a good idea, but how would you sign the hash used to  
> > develop the signature?
> 
> The hash is signed just regular data.  The hash would be the SHA-1
> (or maybe other cryptographic hash algorithm) of the body base64
> encoded.  This value is placed in DKIM-Signature for signing.

Let me try to restate this idea to see if I understand it.  You are
suggesting that there be two hash operations.  One first done for the
message body where this hash value is then placed into the header.  The
second would be for the headers which is verified by the signature.

-Doug