...
On a new topic: Another concern I have is that there's a DoS attack
when the attacker sends a really big bogus signture and the client
has to spend time figuring this out. We had a long discussion about
this on the SMIME list. We added a security consideration in
draft-ietf-smime-3851bis as follows:
Receiving agents that validate signatures and sending agents that
encrypt messages, need to be cautious of cryptographic processing
usage when validating signatures and encrypting messages using keys
larger than those mandated in this specification. An attacker could
send certificates with keys which would result in excessive
cryptographic processing, for example keys larger than those mandated
in this specification, which could swamp the processing element.
Agents which use such keys without first validating the certificate
to a trust anchor are advised to have some sort of cryptographic
resource management system to prevent such attacks.