getpid() can be really expensive sometimes, and it can fail to
detect some kind of fork+prng mistakes, so we need to avoid it if
it's safe to do so.
This patch might slow down fast_prng a lot on any old operating
system that lacks a way to prevent ram from being inherited, AND
requires a syscall for any getpid() calls. But it should make sure
that we either crash or continue safely on incorrect fork+prng usage
elsewhere in the future.
This module is currently implemented to use the same technique as
libottery (later used by the bsds' arc4random replacement), using
AES-CTR-256 as its underlying stream cipher. It's backtracking-
resistant immediately after each call, and prediction-resistant
after a while.
Here's how it works:
We generate psuedorandom bytes using AES-CTR-256. We generate BUFLEN bytes
at a time. When we do this, we keep the first SEED_LEN bytes as the key
and the IV for our next invocation of AES_CTR, and yield the remaining
BUFLEN - SEED_LEN bytes to the user as they invoke the PRNG. As we yield
bytes to the user, we clear them from the buffer.
Every RESEED_AFTER times we refill the buffer, we mix in an additional
SEED_LEN bytes from our strong PRNG into the seed.
If the user ever asks for a huge number of bytes at once, we pull SEED_LEN
bytes from the PRNG and use them with our stream cipher to fill the user's
request.
