Hacker News Reader

Comment by p-e-w | Hacker News Reader

I don't buy this idea:

> As a user, you probably don’t want your emails to be non-repudiable. (Other people might want to be able to prove you sent some email, but your email system ought to serve your interests, not theirs.)

Other people being able to verify that an email really was sent by me does serve my interests, because such verification establishes trust. That's like claiming signing a contract isn't in your interest, because others will be able to prove that you agreed to its terms.

There are very few legitimate situations where you don't want non-repudiation (and in most of these, you shouldn't use a standard email system anyway), while in many cases, non-repudiation is legitimately valuable – for both sender and receiver.

View on HN

I was a expert witness on a case where a dad had modified an email as proof to drop their daughter out of school. The mom was quite pissed that he did this. I used the DKIM keys to prove the email she submitted was true and his version was not the original. It was a fun case.

> Other people being able to verify that an email really was sent by me does serve my interests, because such verification establishes trust.

DKIM, the technology considered by the article, does not prove this. The D stands for domain; it asserts nothing about the user that may have sent the email. Even if the domain is yours, you might delegate to another service or two, or you may have more than one user, or some automation in place. In any case, the recipient has little, if any, way to verify that and in all likelihood doesn't care that much; if they did, they wouldn't be relying on DKIM.

> There are very few legitimate situations where you don't want non-repudiation...

As a service provider, I'd probably want this. If it gets me out from the middle of someone else's dispute and doesn't have an ill effect on the service otherwise, I'd welcome it.

But there is a chain: 1) the mail provider only allows outgoing mail with a particular Return-Path / Envelope Sender / From header if it has done authorisation checks to ensure the sender is allowed to use that address, 2) the mail provider DKIM-signs their outgoing mail.

The combination of 1 and 2 work together. Yes, the mail provider could impersonate you, or could fail to do (1) correctly, but that is a different threat model to one where having no verification at all is okay.

> Other people being able to verify that an email really was sent by me does serve my interests, because such verification establishes trust. That's like claiming signing a contract isn't in your interest, because others will be able to prove that you agreed to its terms.

Are you saying that you do want people who are not your intended recipient(s) to be able to check copies of your emails to be able to verify that you actually wrote/sent those emails? Do you have an example of when this comes up in practice?

If you mean that you do want your recipient(s) to be able to verify your emails, the article is in agreement with you. The article states:

> When you send an email, that’s between you and the recipient. Normally you don’t intend for anyone, anywhere, who happens to get a copy, to be able to verify that it was really you that sent it.

where "anyone, anywhere, who happens to get a copy" is to say "an arbitrary person other than your intended recipient(s).

Most people want messages sent to them to be non-repudiable. Otherwise they can be harassed and threatened with impunity. This sort of thing only seems like a good idea when you don't remember that a message has both a sender and a receiver.

But the sender chooses an e-mail service that sends e-mail in a way they aprove of.

The receiver has no control over that besides possibly ignoring those e-mails, therefore the e-mail services favor the interests of the sender over those of the receiver.

Then I think you'll have to use pgp or some other mechanism one layer up. E-mail ain't the way to do it, it's not built into the current protocol as it stands.

There isn't such a layer for email that is widespread (there's certified email in Italy). PGP and S/MIME crucially lack time and validity stapling.

In a lot of EU countries you can however digitally sign a container and mail that (under eIDAS those qualifying electronic signatures are legally binding, unlike random publicly trusted S/MIME signature).

But I guess if your alternative is fax, then a lot of thimngs are better than that.

Sure. I don't know if that's the case only for Gmail, but after email is received, generally, it's verified and results, at the point of receiving are stored on your email server. He's talking about verification after that initial verification. And if you want your emails to be verified, you should be using GPG/PGP/S.MIME or other signing system anyways. DKIM only verifies server, not user.

But it also verifies nobody tampered with e-mail after it left the server and that is still valuable.

As in the DKIM signature doesn't mean it was really the sender that sent it, but it does say that it wasn't changed after that.

Reply on news.ycombinator.com