[messaging] Question regarding Whatsapp/Signal Safety Numbers

[Trevor Perrin]

On Wed, Sep 27, 2017 at 6:01 PM, Vincent Breitmoser
<look at my.amazin.horse> wrote:
>
> Simply hashing all of the public keys and user ids together into one
> Alice+Bob-specific safety number has none of these problems, yielding
> the same 100 bits preimage attack scenario, in only half the digits.

Hi Vincent,

If you hash everything together you have to worry about
collision-resistance, so you still need a similar-sized value (e.g.
200 bits).

So that doesn't reduce the size, but that does lose the ability to
extract out individual "fingerprints" from the safety number halves.

Trevor