Justifications:
- crypto_(auth|hash|generichash|onetimeauth|shorthash)*:
it's legal to hash or HMAC a 0-length message
- crypto_box*: it's legal to encrypt a 0-length message
- crypto_sign*: it's legal to sign a 0-length message
- utils:
comparing two 0-length byte arrays is legal
memzero on a 0-length byte array is a no-op
converting an empty hex string to binary results in an empty binary string
converting an empty binary string to hex results in an empty hex string
converting an empty b64 string to binary results in an empty binary string
converting an empty binary string to b64 results in an empty b64 string
sodium_add / sodium_sub on zero-length arrays is a no-op
For the functions declared in utils.h, I moved the logic into private functions that
have the __attribute__ ((nonnull)) check, but they are only called when the
corresponding length argument is non-0. I didn't do this for the hash/box/sign
functions since it would have been a lot more work and quite a large refactor.
Return -1 on incomplete sequences and on complete sequences
with trailing, non-ignored characters if no pointers to store the
last parsed byte has been provided
Unlike hex encoding, due to optional padding, computing the correct size is
not straightforward. Ensuring that the string ends with `\0` is fine, but
if the size is not exact, some unrelated data might be send around by the
application. So, zero it to be safe.
A constant-time version of memcmp(), useful to compare nonces and counters
in little-endian format, that plays well with sodium_increment().
Unlike sodium_memcmp() which can compare anything for equality,
sodium_compare() is designed to compare things that are comparable, byte by
byte. Therefore, the prototype is slightly different: its arguments are
supposed to be `const unsigned char *`.
The names sodium_memcmp() and sodium_compare() are slightly confusing.
But we're not going to rename sodium_memcmp(), and I cannot think of a
better name for sodium_compare() than sodium_compare().
