[ietf-dkim] 1193 considered harmful

[Michael Thomas]

Barry Leiba wrote:
> 
> Or using the same body in multiple messages.  Suppose "Company I", say, 
> is sending a (legitimate, opted-into) mass-mailing of a 70 MB video file 
> to, say, 200,000 opted-in recipients.  Suppose also that for some reason 
> it has to batch these with different headers, so it can't just sign the 
> whole message once.  Saving the work of hashing that 70 MB video 
> multiple times would be nice.

70Mb video over SMTP? ::shudder::

>  > I suspect that the RSA signing operation overwhelms the
> 
>> SHAx cost by a very good bit on your average size of body.
> 
> 
> But that doesn't matter, because we're not RSA-signing the body, only 
> the hash.  So it's only the overhead of the hashing that matters.

But you'd assumedly need a new RSA signature per message in order
for this to make any sense. To be pedantic:

CostRSAsign = 1
CostSHAx    = .1

For message one, cost = 1.1, for message 2-n, cost = 1.0/msg -- big
whoop. These aren't accurate, just illustrative.

		Mike