Scott,
I don't know if you've considered this approach - rather than the
recipient generating stamps using a computationally expensive method,
why doesn't it just generate a unique stamp using a simple method
(e.g. a random or sequential number), and then encrypt it using it's
private key ?
Private key encryption is not overly expensive, but has the attribute
that it is infeasible for the sender to "guess" cipher text that would
result in a stamp that the recipient consider valid.
When the recipient receives a stamp, it simply decrypts it using its
public key, and compares it against its database of valid stamps.
The "down" side of this is that stamps would be larger - e.g. 512 bits
for a 512 bit RSA key pair. However, I think that this increase in
traffic would be fairly neglibable considering the size of average
emails.
Dave
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