[messaging] Useability of public-key fingerprints

Robert Ransom rransom.8774 at gmail.com
Wed Jan 29 22:19:54 PST 2014

On 1/29/14, Moxie Marlinspike <moxie at thoughtcrime.org> wrote:

> Subsequent questions tend to include things like "wait, what's the
> difference between a key and a fingerprint?" There's no great reason a
> person who wants to send messages should need to know that.

The difference is that you can encrypt messages to a key offline, but
you need to be connected to the Internet (and to a working directory
server of some sort) in order to encrypt messages to a fingerprint.

> My intuition is that we just shouldn't be showing the user a fingerprint
> at all if even remotely possible (TOFU).  If it's necessary to display a
> real fingerprint at some point, the user isn't going to have any idea
> what's going on, so it probably doesn't matter whether it's a set of
> gibberish words, a hex string, or b32 character string.

That's another of Ross Anderson's usability lessons: if you want the
user to check a fingerprint, make the user type it in and have the
software compare it.  (And in that case, it may as well be a key or
password of some sort, especially with ECC.)

Robert Ransom

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