[ccache] Questions about two hot functions in ccache

[Martin Pool]

On 20 October 2010 17:44, Justin Lebar <justin.lebar at gmail.com> wrote:
> My cryptographically-inclined friend suggested we use a universal hash
> function or something a bit stronger, such as VHASH.
>
> These functions take a "key", which we could choose at random and fix
> in the code.
>
> VHASH outputs 64-bit digests with collision probability 2^61, so in
> expectation you'd need to hash 2^30 files before you saw a collision.
> If that wasn't good enough, we could compute two VHASH digests with
> different keys and concatenate them.

Is VHASH expected to be faster than MD4?  I don't think adding more
strength will help with anything.  The odds of an accidental MD4
collision are low, and I don't know of any attack by which being able
to predict or produce ccache collisions accomplishes anything for the
attacker.  (If they can write to the cache you have bigger problems.)

-- 
Martin