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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 "hrtimer.h"
#include "secblock.h"
#include "hmac.h"
NAMESPACE_BEGIN(CryptoPP)
//! abstract base class for key derivation function
class KeyDerivationFunction
{
public:
//! maximum number of bytes which can be produced under a secuirty context
virtual size_t MaxDerivedKeyLength() const =0;
virtual bool Usesinfo() const =0;
//! derive a 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* info=NULL, size_t infoLen=0) const =0;
virtual ~KeyDerivationFunction() {}
};
//! 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 HKDF : public KeyDerivationFunction
{
public:
CRYPTOPP_CONSTANT(DIGESTSIZE = T::DIGESTSIZE);
CRYPTOPP_CONSTANT(SALTSIZE = T::DIGESTSIZE);
static const char* StaticAlgorithmName () {
static const std::string name(std::string("HKDF(") + std::string(T::StaticAlgorithmName()) + std::string(")"));
return name.c_str();
}
size_t MaxDerivedKeyLength() const {return static_cast<size_t>(T::DIGESTSIZE) * 255;}
bool Usesinfo() 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* info, size_t infoLen) const;
protected:
// If salt is missing (NULL), then use the NULL vector. Missing is different than EMPTY (0 length). The length
// of s_NullVector used depends on the Hash function. SHA-256 will use 32 bytes of s_NullVector.
typedef byte NullVectorType[SALTSIZE];
static const NullVectorType& GetNullVector() {
static const NullVectorType s_NullVector = {0};
return s_NullVector;
}
};
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* info, size_t infoLen) const
{
static const size_t DIGEST_SIZE = static_cast<size_t>(T::DIGESTSIZE);
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 : GetNullVector());
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 (info && infoLen) {hmac.Update(info, infoLen);}
hmac.CalculateDigest(buffer, &block, 1);
#if CRYPTOPP_MSC_VERSION
const size_t segmentLen = STDMIN(derivedLen, DIGEST_SIZE);
memcpy_s(derived, segmentLen, buffer, segmentLen);
#else
const size_t segmentLen = STDMIN(derivedLen, DIGEST_SIZE);
std::memcpy(derived, buffer, segmentLen);
#endif
derived += segmentLen;
derivedLen -= segmentLen;
}
return req;
}
NAMESPACE_END
#endif // CRYPTOPP_HASH_KEY_DERIVATION_FUNCTION_H
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