Forging S/MIME signatures
Jon Udell tries his hand at S/MIME signature forgery,
revealing that PKI is not a panacea.
A digital signature proves something. The proof is strong but the something is weak (if it just demonstrates that you clicked a few things to get a persona certificate).
So if you need to prove something stronger, then you put limits on what digitally-signed content you’re willing to accept.
This can go in at least two directions (not mutually exclusive):
- higher-class certificates (where certificate authorities demand more proof, and encode that fact in the certificate). But higher quality means harder to get and less actual deployment. And higher quality means more attractive target for theft of keys.
- reputation systems. Of course, building robust reputation systems is not easy. Users may wish to have multiple sources of reputation information to fit their own definitions of good and bad behavior and how fast those judgments are made. It replays the whole DNS blacklist deployment. Some reputation systems may seem arbitrary and capricious. Others may be too slow or too tolerant. They are all lawsuit targets. Will there be too many to choose from?
For message classification, there is a predisposition to disparage machine learning and content inspection as too
probabilistic and uncertain, while viewing signatures as certain and reliable. It is not so, the uncertainty or trust is not eliminated, it’s just at a different level.
| Let’s hope Jon Udell reads it
He did :-)
| The proof is strong but the something is
| weak (if it just demonstrates that you
| clicked a few things to get a persona
| certificate).
The something is slightly more: that the certified email address is bound, by some certifying authority, to some person with access to that email account.
It ain’t much, but it’s more than we’ve got now, which is nothing.
I agree, BTW, with the observation that higher-assurance certs and reputation systems both have roles to play, but also introduce new complexity.
Meanwhile, getting Aunt Tillie to expect that her correspondents have at least passed the email ping test, in the eyes of some known third party, is a reasonable step to take.