bug#58472: [PATCH] Make `message-unique-id' less prone to collisions

[Paul Eggert <eggert@cs.ucla.edu>]

On 2022-10-12 19:46, Stefan Kangas wrote:

> The main goal is to avoid collisions, but using the time also gives an
> idea of when the message was sent, which is kind of nice.

That info is in the Date: line, along with zillions of other Received: 
lines. There should be no need to repeat it in the Message-ID line.

> Time also
> guarantees a somewhat unique value even if the user has happened to set
> the random seed.

If that's a concern, we should be using more-random data, e.g., with

    (base64-encode-string
     (secure-hash 'md5 'iv-auto 128 nil t))

if we want 128 bits of randomness (this yields a string like 
"B8a3usyu5QSE/rTLu0nIHg==").

As an aside, it's weird that there's no easy way to ask Emacs for an 
N-bit random integer, where the randomness is taken from system entropy. 
Shouldn't we extend Emacs to support that? E.g., (make-string 128 
'iv-auto) could give you an N-byte entropy-derived random string, or 
(random -N) could give you an N-bit entropy-derived random nonnegative 
integer, or something like that. Then we could write something like this:

   (base64-encode-string (make-string 16 'iv-auto))

to get a Message-ID component with 16 bytes (128 bits) of entropy.