[messaging] On Signed-Only Mails

[Bjarni Runar Einarsson]

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Hello!

Moritz Bartl <moritz at headstrong.de> wrote:
> On 11/29/2016 10:25 AM, Peter Gutmann wrote:
> >> [...] Signed-Only Mails are Useless [...]
> > Yup, and it's for exactly the reasons given there that the S/MIME WG decided
> > many years ago not to sign messages sent to the list.  Courts, similarly, rule
> > on the intent of the signer, not some attached bag of bits (see e.g. Steven
> > Mason's excellent "Electronic Signatures in Law").  So while I wouldn't go so
> > far as to call them harmful, I'd agree that they're mostly useless, unless
> > you're using one to make some special point.  Even then, if it's for legal
> > purposes, a court will look at almost everything but the signature when
> > deciding on its effect.
> 
> This is a dangerous and wrong statement. For one, you are
> making a case based purely on previous cases in US courts,
> which is a very US-centric view, and dangerous for a discussion
> that potentially affects all jurisdictions. Secondly, even
> within US law, even if you're right in what you are saying, the
> absoluteness of your statement makes it wrong.

There are also purposes which have nothing to do with laws and
contracts.

One:

Signatures don't just prove that the content is authentic, in
practice they also work in the other direction - associating
content and online identity with the signing key.

A large amount of e-mails, consistently authored by the same
persona and signed by the same key is as strong a signal of
trustworthiness (of the key) as anything the web of trust or
keyservers can provide. In many ways, it's stronger and more
practical, because I probably care more about communicating with
the person that wrote all those messages, than I care about
government issued IDs or how diligent the author is at updating
keyservers or attending keysigning parties.

Um, in my opinion. I don't know if there is research which
quantifies these assertions. So take with as many grains of salt
as you feel appropriate. :-)

Two:

For automated signaling which affects the behaviour of the mail
client behind the scenes, it'd be nice if third parties can't
just inject/strip content. The OpenPGP header is an example of
such a thing, such things get proposed quite frequently. A
digital signature which covers [parts of] the header would be
immensely useful - I know PGP doesn't usually do this, but it
should and there are efforts to make it so. So that argues for
moving things in the opposite direction, signing more, not less.

This kind of thing is the main reason Mailpile signs by default.
I worry about the usability, but I just think signatures are too
useful to abandon them.

 - Bjarni

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