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> it's a grave privacy violation. I'm missing something here. DKIM mostly proves an email from person@from.me was sent by a server @from.me controls. There i

> it's a grave privacy violation. I'm missing something here. DKIM mostly proves an email from person@from.me was sent by a server @from.me controls. There is also a bloody great audit trail inside of the email with together with SPF can do a pretty good job of proving the same thing. I'm struggling to see how an email sent to me, that presumably was always intended to be readable by me could suddenly become a privacy violation because it's signed. That is doubly so if I won't receive it if it isn't validly signed when it is received, which is often the case.

A1kmm 2y ago
I'm not sure privacy violation is necessarily the right term to help people understand why long-term non repudiation is an undesirable property for some people. It comes down to if a third party gets access to your emails (e.g. through a server compromise), should they be able to prove to a fourth party that the emails are legitimately yours, vs completely faked? Non repudiation through strong DKIM keys enables this. Example: Third party is a ransomware gang who releases your emails because you didn't pay a ransom after your email server was compromised. Fourth party is a journalist who doesn't trust the ransomware gang, but also wants to publish juicy stories about your company if there is one, but doesn't want to risk their reputation / a defamation case if the ransomware gang just invented the emails.
- tptacek 2y ago
  Non-repudiation is virtually always undesirable in general-purpose messaging systems. Revealing to a stranger whether a message is valid is a concession to that stranger, not a benefit to the email's owner. This property is called "deniability" and most secure messaging systems go way out of their way to have it.
tptacek 2y ago
It's nobody else's business whether the emails in your inbox are valid or not, and that's basically all non-deniable DKIM signatures do: durable secure verifiable DKIM signatures are "security feature" with real-world value exclusively for attackers.
- singleshot_ 2y ago
  If you’re under Rule 26 discovery or disclosure requirements, it absolutely might be my business whether emails in your client are valid, but I suppose you could classify opposing counsel as an “attacker,” so you’re not wrong.
  - tptacek 2y ago
    Note, just to rub this in: you don't even get the verification you're looking for, because DKIM verifies domains and not users.
    - singleshot_ 2y ago
      I get the verification the email actually came from the domain (i.e., someone, who has the credentials of an insider) vs. an email that was totally spoofed, from the outside, without any creds of the purported sender org. (Right?) Hands on keyboard? You're right, absolutely not. But I can learn something useful via DKIM nevertheless.

A1kmm 2y ago

I'm not sure privacy violation is necessarily the right term to help people understand why long-term non repudiation is an undesirable property for some people. It comes down to if a third party gets access to your emails (e.g. through a server compromise), should they be able to prove to a fourth party that the emails are legitimately yours, vs completely faked? Non repudiation through strong DKIM keys enables this. Example: Third party is a ransomware gang who releases your emails because you didn't pay a ransom after your email server was compromised. Fourth party is a journalist who doesn't trust the ransomware gang, but also wants to publish juicy stories about your company if there is one, but doesn't want to risk their reputation / a defamation case if the ransomware gang just invented the emails.

tptacek 2y ago
Non-repudiation is virtually always undesirable in general-purpose messaging systems. Revealing to a stranger whether a message is valid is a concession to that stranger, not a benefit to the email's owner. This property is called "deniability" and most secure messaging systems go way out of their way to have it.

tptacek 2y ago

It's nobody else's business whether the emails in your inbox are valid or not, and that's basically all non-deniable DKIM signatures do: durable secure verifiable DKIM signatures are "security feature" with real-world value exclusively for attackers.

singleshot_ 2y ago
If you’re under Rule 26 discovery or disclosure requirements, it absolutely might be my business whether emails in your client are valid, but I suppose you could classify opposing counsel as an “attacker,” so you’re not wrong.
- tptacek 2y ago
  Note, just to rub this in: you don't even get the verification you're looking for, because DKIM verifies domains and not users.
  - singleshot_ 2y ago
    I get the verification the email actually came from the domain (i.e., someone, who has the credentials of an insider) vs. an email that was totally spoofed, from the outside, without any creds of the purported sender org. (Right?) Hands on keyboard? You're right, absolutely not. But I can learn something useful via DKIM nevertheless.