4 ms·
> you don't need a verifiable DKIM signature to know if the message had a valid DKIM signature when it was sent. You seem to be trying to say that "the fact th
by ryan-c 2y ago
> you don't need a verifiable DKIM signature to know if the message had a valid DKIM signature when it was sent.
You seem to be trying to say that "the fact that it was delivered proves it had a valid signature when it was sent".
That presupposes that the headers indicating when it was delivered are correct, or that it was delivered at all in the first place.
I don't think you understand the attack.
I sent Thomas an email admitting to something scandalous.
A few months later, Mallory pops the server Thomas's email is stored on, and extracts his mail spool.
Mallory wants to prove to a third party that the email is authentic and not a fabrication.
I know it's real.
Thomas knows it's real.
Mallory is pretty sure it's real.
Alice, a reporter for the Daily Bugle cannot verify it is real, rather than Mallory's forgery.
I can claim it's a forgery, and point out that Mallory could have made it up and generated the signature with the published key.
Now, I may have a problem if it were a crime rather than something merely scandalous because then Bob, an FBI agent, decides to subpoena some logs and maybe prove when it was sent, but even so, logs typically don't have message content or even a hash of the message content.
- rstuart4133 2y agoI understand the attack. Consider this wrinkle: Thmoas's email provider is @gmail.com. Assume it is pretty well known the Gmail will only put email in his inbox if it is DKIM signed. (I run my own email home server. I can assure you this is true now unless you are someone like @debian.org. Unsigned email is simply dropped by most of the major players.) You send the incriminating email. It's accepted by Gmail as it's DKIM signed. You rotate your DKIM keys. Mallory now steals in the @gmail inbox. I can think of only two defences for you now. One is Google accepted the email without a valid DKIM signature - which you say is your main defence. The other is someone else sent the email by getting control of your email account / server / DKIM. I personally would find it much easier to believe you lost control of your email account than Google accepted a badly DKIM signed email from some random. I still think this is a classic example of the XKCD rubber hose comic. The cryptographers are suffering from tunnel vision. They focus on exclusively on the well known properties of their beloved cryptography. It's odd they keep doing that. Modern cryptography is mature, well understood, and for the most part unbreakable. The weakest link is invariably elsewhere.