3 ms·

> a DKIM-signed message proves the mailhost was authorized That is what I wrote, I may have not have explained it correctly though, I was in a bit of a rush ea

by LeonM 3y ago

> a DKIM-signed message proves the mailhost was authorized That is what I wrote, I may have not have explained it correctly though, I was in a bit of a rush earlier today. DKIM signs (a selection of) the headers, and the message body. The sender address (the 'from' header) is typically included in the signed subset of headers. The domain (the part after the '@' symbol) authorizes the sender (the owner of the private key) to send email on behalf of the domain by publishing the public key (the DKIM DNS record). So, if the signature is correct, then you know the message body, and the headers included in the signature (such as the 'from' header) are authentic (not tampered with since the host set added the signature). You also know that the sender is authorized to send email on behalf of the domain. However, the sending host is free to place any arbitrary data in the headers, including the 'from' header, and sign it. There is absolute nothing that will prevent a malicious host from placing an arbitrary name before the '@' sign, except for integrity. So just because 'jane' was in the 'from' header, it doesn't mean that Jane actually wrote the message. > and at least some of the headers could easily prove who sent the message. No, none of them do. As I explained above, the sender is free to set the header values to any value. So just because there is a name in 'from' header, doesn't mean that that person actually wrote the email, or that it originated from one of that person's devices. If you publish the DKIM DNS record for some third party email service, then that service could very well send email as you@yourdomain.tld. Should that somehow "easily prove" that you send that email? No amount of email headers are going to protect you from that kind of malicious behavior, that's just not how email works. > You should probably stop "consulting on email security" if you don't understand [...] Please refrain from insults like that. Reading your comment mocking my profession like that really ruined my evening.

rainsford 3y ago
You are technically correct (the best kind of correct), but in practice the nuance you're referring to does not make as much difference as you think. Yes, email senders could send email from whatever user they want or change the body from what the user wrote, but in practice of course they aren't doing that for the kind of email providers like Gmail that most people use. If the deniability of a DKIM signed email reduces to "maybe Gmail spoofed emails from me", you don't have much of an argument. Yes, in a technical cryptographic sense, DKIM does not prove you sent the email, but when combined with how most providers like Gmail work, it makes it very likely you did send the email in every practical sense. This isn't just a nitpicking nuance. In theory, rotating and publishing DKIM keys makes no difference to deniability since DKIM doesn't technically provide non-repudiation at the individual user level. In practice though, a service like Gmail implementing DKIM rotation and key publishing would make emails for a huge number of users more deniable going forward.
- disruptiveink 3y ago
  Yes, this is the point. Most (nearly all?) of the publicly available providers a user is likely to use won't allow you to use their MTAs and set From headers in a way that would impersonate someone else on that domain, provided you're authenticated as yourself. So pointing out DKIM only authenticates the domain only weakens the argument from "UserX at Gmail sent this email" to "UserX at Gmail sent this email provided no one found a suitable, currently unknown exploit at Gmail or performed an inside job". For a journalist and most juries, provided the absence of any reasonable suspicion or evidence of weaknesses / foul play at Gmail, both statements have equal strength.

by LeonM 3y ago

rainsford 3y ago

You are technically correct (the best kind of correct), but in practice the nuance you're referring to does not make as much difference as you think. Yes, email senders could send email from whatever user they want or change the body from what the user wrote, but in practice of course they aren't doing that for the kind of email providers like Gmail that most people use. If the deniability of a DKIM signed email reduces to "maybe Gmail spoofed emails from me", you don't have much of an argument. Yes, in a technical cryptographic sense, DKIM does not prove you sent the email, but when combined with how most providers like Gmail work, it makes it very likely you did send the email in every practical sense. This isn't just a nitpicking nuance. In theory, rotating and publishing DKIM keys makes no difference to deniability since DKIM doesn't technically provide non-repudiation at the individual user level. In practice though, a service like Gmail implementing DKIM rotation and key publishing would make emails for a huge number of users more deniable going forward.

disruptiveink 3y ago
Yes, this is the point. Most (nearly all?) of the publicly available providers a user is likely to use won't allow you to use their MTAs and set From headers in a way that would impersonate someone else on that domain, provided you're authenticated as yourself. So pointing out DKIM only authenticates the domain only weakens the argument from "UserX at Gmail sent this email" to "UserX at Gmail sent this email provided no one found a suitable, currently unknown exploit at Gmail or performed an inside job". For a journalist and most juries, provided the absence of any reasonable suspicion or evidence of weaknesses / foul play at Gmail, both statements have equal strength.