ope 0.1.1

This is an Order-preserving encryption (OPE) lib inspired by cryptdb's ope implementation.
Documentation 
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use hmac::{Hmac, Mac};
use sha2::{Digest, Sha256};

use crate::prng::BLOCK_SIZE;
use crate::ZZ;

type HmacSha256 = Hmac<Sha256>;

pub(crate) fn big_int_from_bytes(bytes: &[u8]) -> ZZ
{
	ZZ::from_bytes_be(num_bigint::Sign::Plus, bytes)
}

pub(crate) fn num_bits(mut n: u64) -> u64
{
	let mut bits = 0u64;

	while n > 0 {
		bits += 1;
		n >>= 1;
	}

	bits
}

pub(crate) fn hmac(v: &[u8], key: &[u8]) -> [u8; BLOCK_SIZE]
{
	let mut mac = HmacSha256::new_from_slice(key).unwrap();
	mac.update(v);
	let result = mac.finalize();

	let mut out = [0u8; BLOCK_SIZE];

	out.copy_from_slice(&result.into_bytes()[..BLOCK_SIZE]);

	out
}

pub(crate) fn sha(v: &[u8]) -> [u8; BLOCK_SIZE]
{
	let mut hasher = Sha256::new();
	hasher.update(v);
	let result = hasher.finalize();

	let mut out = [0u8; BLOCK_SIZE];

	out.copy_from_slice(&result[..BLOCK_SIZE]);

	out
}