Re: Patch: cprng_fast performance - please review.

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

Closer inspection of HC-128 reveals that it uses secret-dependent
array indices[*], so for that reason alone I don't think we should
adopt it.  It also has a very large state, which is going to hurt the
cache on big systems and hurt the stack on little systems.

So, could you please split your changes into three parts, along the
following lines?  That will make it easier to discuss and measure the
parts separately.

(1) Isolate the algorithm from the cprng_fast bookkeeping (currently
the cprng_fast API is separate, but the bookkeeping is embedded in
arc4random.c).

(2) Convert the bookkeeping from one global state to per-CPU states.

(3) Switch algorithms.


[*] In hc128_init, key flows into w which flows into p/q which flow
into round_expression which does h(qp, pq[...]) which does qp[pq[...]
& 0xff].