hmac.h

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// //////////////////////////////////////////////////////////
// hmac.h
// Copyright (c) 2015 Stephan Brumme. All rights reserved.
// see http://create.stephan-brumme.com/disclaimer.html
//
 
#pragma once
 
// based on http://tools.ietf.org/html/rfc2104
// see also http://en.wikipedia.org/wiki/Hash-based_message_authentication_code

 
#include <string>
#include <cstring> // memcpy

template <typename HashMethod>
std::string hmac(const void* data, size_t numDataBytes, const void* key, size_t numKeyBytes)
{
  // initialize key with zeros
  unsigned char usedKey[HashMethod::BlockSize] = {0};
 
  // adjust length of key: must contain exactly blockSize bytes
  if (numKeyBytes <= HashMethod::BlockSize)
  {
    // copy key
    memcpy(usedKey, key, numKeyBytes);
  }
  else
  {
    // shorten key: usedKey = hashed(key)
    HashMethod keyHasher;
    keyHasher.add(key, numKeyBytes);
    keyHasher.getHash(usedKey);
  }
 
  // create initial XOR padding
  for (size_t i = 0; i < HashMethod::BlockSize; i++)
    usedKey[i] ^= 0x36;
 
  // inside = hash((usedKey ^ 0x36) + data)
  unsigned char inside[HashMethod::HashBytes];
  HashMethod insideHasher;
  insideHasher.add(usedKey, HashMethod::BlockSize);
  insideHasher.add(data,    numDataBytes);
  insideHasher.getHash(inside);
 
  // undo usedKey's previous 0x36 XORing and apply a XOR by 0x5C
  for (size_t i = 0; i < HashMethod::BlockSize; i++)
    usedKey[i] ^= 0x5C ^ 0x36;
 
  // hash((usedKey ^ 0x5C) + hash((usedKey ^ 0x36) + data))
  HashMethod finalHasher;
  finalHasher.add(usedKey, HashMethod::BlockSize);
  finalHasher.add(inside,  HashMethod::HashBytes);
 
  return finalHasher.getHash();
}
 

template <typename HashMethod>
std::string hmac(const std::string& data, const std::string& key)
{
  return hmac<HashMethod>(data.c_str(), data.size(), key.c_str(), key.size());
}