This module is a copy of shamodule.c, with the SHA-1
compression function replaced with the SHA-256
compression function (copied from the LibTomCrypt
public-domain crypto library).

SHA-256 is similar to SHA-1: it's a US Federal Standard
hash algorithm (FIPS 180-2).  The difference is that it
produces a 256 bit hash value, instead of a 160 bit
hash value.  SHA-256 thus has 128 bits of resistance
against birthday attacks, which makes it secure in
certain protocols where SHA-1 is questionable (e.g.
digital signatures; or RNGs or Key-Derivation Functions
where you want to produce keys for 256-bit ciphers).

There's other flavors of SHA, but they're not as
useful: SHA-384 and SHA-512 are defined on 64-bit
values, so are slow on 32-bit architectures.  SHA-224
is just silly (it saves 32 bits over SHA-256; that's
its sole rationale).