Header-only constexpr SHA256 C++ 14 literal

THis is a C++14 constexpr implementation of SHA256, which allows (among others) the usage of a "_sha256" literal to get compile-time SHA256 hashes:

#include "sha256_literal.h"

static constexpr sha256::HashType H = "mypassword"_sha256;
// HashType is a std::array<uint8_t, 32>

More "low-level" API are also provided:

static constexpr auto H0 = sha256::compute_str("hello");
static constexpr auto H1 = sha256::compute({'A','B'});

This can be used for instance to compare user-provided passwords to a hard-coded one in a binary (although this is not recommended to do so directly, passwords should always be salted when hashed!):

#include "sha256.h"
#include "sha256_literal.h"

bool isGoodPwd(const char* Pwd) {
  static constexpr auto PasswordHash = "myverysecretpassword"_sha256;
  return sha256::compute((const uint8_t*) Pwd, strlen(Pwd)) == PasswordHash;
}

See the tests.cpp file for tests, and the example.cpp for a usage example.

A runtime implementation of SHA256 is also provided in sha256.cpp. This does not respect the NIST API standards, and is only there for convience.

This has been tested with clang 4.0 and GCC 6.3 under linux.

Licensing

This code is licensed under GPLv3, except for the QBDI project (https://github.com/QBDI/QBDI/) that uses it for benchmarking purpose where we tolerate the usage of this code under the Apache2 license.