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.
Meta-Signatures does something similiar.
Perhaps as a diagnostic, a simple checksum of
the body could be placed within the signature to confirm the body has
been altered, and could be a reason the signature has failed. I like
the idea of dropping the body hash into the signature header, but
this seems to demand two separate signatures and this would be bad.
Nope. Only one signature is done. I can elaborate more if you
require.
--ewh