84 lines
2.7 KiB
C++
84 lines
2.7 KiB
C++
// //////////////////////////////////////////////////////////
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// hmac.h
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// Copyright (c) 2015 Stephan Brumme. All rights reserved.
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// see http://create.stephan-brumme.com/disclaimer.html
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//
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#pragma once
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// based on http://tools.ietf.org/html/rfc2104
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// see also http://en.wikipedia.org/wiki/Hash-based_message_authentication_code
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/** Usage:
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std::string msg = "The quick brown fox jumps over the lazy dog";
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std::string key = "key";
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std::string md5hmac = hmac< MD5 >(msg, key);
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std::string sha1hmac = hmac< SHA1 >(msg, key);
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std::string sha2hmac = hmac<SHA256>(msg, key);
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Note:
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To keep my code simple, HMAC computation currently needs the whole message at once.
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This is in contrast to the hashes MD5, SHA1, etc. where an add() method is available
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for incremental computation.
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You can use any hash for HMAC as long as it provides:
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- constant HashMethod::BlockSize (typically 64)
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- constant HashMethod::HashBytes (length of hash in bytes, e.g. 20 for SHA1)
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- HashMethod::add(buffer, bufferSize)
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- HashMethod::getHash(unsigned char buffer[HashMethod::BlockSize])
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*/
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#include <string>
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#include <cstring> // memcpy
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/// compute HMAC hash of data and key using MD5, SHA1 or SHA256
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template <typename HashMethod>
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std::string hmac(const void* data, size_t numDataBytes, const void* key, size_t numKeyBytes)
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{
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// initialize key with zeros
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unsigned char usedKey[HashMethod::BlockSize] = {0};
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// adjust length of key: must contain exactly blockSize bytes
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if (numKeyBytes <= HashMethod::BlockSize)
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{
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// copy key
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memcpy(usedKey, key, numKeyBytes);
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}
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else
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{
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// shorten key: usedKey = hashed(key)
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HashMethod keyHasher;
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keyHasher.add(key, numKeyBytes);
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keyHasher.getHash(usedKey);
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}
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// create initial XOR padding
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for (size_t i = 0; i < HashMethod::BlockSize; i++)
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usedKey[i] ^= 0x36;
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// inside = hash((usedKey ^ 0x36) + data)
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unsigned char inside[HashMethod::HashBytes];
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HashMethod insideHasher;
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insideHasher.add(usedKey, HashMethod::BlockSize);
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insideHasher.add(data, numDataBytes);
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insideHasher.getHash(inside);
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// undo usedKey's previous 0x36 XORing and apply a XOR by 0x5C
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for (size_t i = 0; i < HashMethod::BlockSize; i++)
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usedKey[i] ^= 0x5C ^ 0x36;
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// hash((usedKey ^ 0x5C) + hash((usedKey ^ 0x36) + data))
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HashMethod finalHasher;
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finalHasher.add(usedKey, HashMethod::BlockSize);
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finalHasher.add(inside, HashMethod::HashBytes);
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return finalHasher.getHash();
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}
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/// convenience function for std::string
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template <typename HashMethod>
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std::string hmac(const std::string& data, const std::string& key)
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{
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return hmac<HashMethod>(data.c_str(), data.size(), key.c_str(), key.size());
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}
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