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// hkdf.h - written and placed in public domain by Jeffrey Walton. Copyright assigned to Crypto++ project
#ifndef CRYPTOPP_HASH_KEY_DERIVATION_FUNCTION_H
#define CRYPTOPP_HASH_KEY_DERIVATION_FUNCTION_H
#include "cryptlib.h"
#include "hmac.h"
#include "hrtimer.h"
#include "secblock.h"
#include <cstring>
NAMESPACE_BEGIN(CryptoPP)
//! abstract base class for key derivation function
class KeyDerivationFunction
{
public:
virtual size_t MaxDerivedKeyLength() const =0;
virtual bool UsesContext() const =0;
//! derive key from secret
virtual unsigned int DeriveKey(byte *derived, size_t derivedLen, const byte *secret, size_t secretLen, const byte *salt, size_t saltLen, const byte* context=NULL, size_t contextLen=0) const =0;
// If salt is missing, then use the NULL vector. The length depends on the Hash function.
static const byte s_NullVector[64];
};
const byte KeyDerivationFunction::s_NullVector[64] = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};
//! General, multipurpose KDF from RFC 5869. T should be a HashTransformation class
//! https://eprint.iacr.org/2010/264 and https://tools.ietf.org/html/rfc5869
template <class T>
class CRYPTOPP_DLL HKDF : public KeyDerivationFunction
{
public:
size_t MaxDerivedKeyLength() const {return static_cast<size_t>(T::DIGESTSIZE) * 255;}
bool UsesContext() const {return true;}
unsigned int DeriveKey(byte *derived, size_t derivedLen, const byte *secret, size_t secretLen, const byte *salt, size_t saltLen, const byte* context, size_t contextLen) const;
};
template <class T>
unsigned int HKDF<T>::DeriveKey(byte *derived, size_t derivedLen, const byte *secret, size_t secretLen, const byte *salt, size_t saltLen, const byte* context, size_t contextLen) const
{
static const size_t DIGEST_SIZE = static_cast<size_t>(T::DIGESTSIZE);
CRYPTOPP_COMPILE_ASSERT(DIGEST_SIZE <= COUNTOF(s_NullVector));
const unsigned int req = static_cast<unsigned int>(derivedLen);
assert(secret && secretLen);
assert(derived && derivedLen);
assert(derivedLen <= MaxDerivedKeyLength());
if(derivedLen > MaxDerivedKeyLength())
throw InvalidArgument("HKDF: derivedLen must be less than or equal to MaxDerivedKeyLength");
HMAC<T> hmac;
FixedSizeSecBlock<byte, DIGEST_SIZE> prk, buffer;
// Extract
const byte* key = (salt ? salt : s_NullVector);
const size_t klen = (salt ? saltLen : DIGEST_SIZE);
hmac.SetKey(key, klen);
hmac.CalculateDigest(prk, secret, secretLen);
// Expand
hmac.SetKey(prk.data(), prk.size());
byte block = 0;
while (derivedLen > 0)
{
if(block++) {hmac.Update(buffer, buffer.size());}
if(context && contextLen) {hmac.Update(context, contextLen);}
hmac.CalculateDigest(buffer, &block, 1);
size_t segmentLen = STDMIN(derivedLen, DIGEST_SIZE);
std::memcpy(derived, buffer, segmentLen);
derived += segmentLen;
derivedLen -= segmentLen;
}
return req;
}
NAMESPACE_END
#endif // CRYPTOPP_HASH_KEY_DERIVATION_FUNCTION_H
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