Consider using an internal random-number generator

In addition to the improvements suggested in #5, user [u/skeeto](https://reddit.com/u/skeeto) on Reddit also suggests using an internal PRNG. In skeeto's words:

> [`crypto/rand`] is slow (involves at least one syscall for each read), and can fail. It's fine for seeding a PRNG, but a poor option otherwise.

Start by reading issue #5 and reviewing the crypto/rand concern described here. Determine whether an internal PRNG can avoid per-read syscalls and failures without weakening the randomness required by encid; done means the approach and its security implications are settled, with relevant behavior verified.

Written by the indexing model from the issue text.