emsa2.cpp

// emsa2.cpp - written and placed in the public domain by Wei Dai

#include "pch.h"
#include "emsa2.h"

#ifndef CRYPTOPP_IMPORTS

NAMESPACE_BEGIN(CryptoPP)

void EMSA2Pad::ComputeMessageRepresentative(RandomNumberGenerator& /*rng*/, 
	const byte* recoverableMessage, size_t recoverableMessageLength,
	HashTransformation &hash, HashIdentifier hashIdentifier, bool messageEmpty,
	byte *representative, size_t representativeBitLength) const
{
	CRYPTOPP_UNUSED(recoverableMessage), CRYPTOPP_UNUSED(recoverableMessageLength), CRYPTOPP_UNUSED(representativeBitLength);
	assert(representativeBitLength >= MinRepresentativeBitLength(hashIdentifier.second, hash.DigestSize()));

	if (representativeBitLength % 8 != 7)
		throw PK_SignatureScheme::InvalidKeyLength("EMSA2: EMSA2 requires a key length that is a multiple of 8");

	size_t digestSize = hash.DigestSize();
	size_t representativeByteLength = BitsToBytes(representativeBitLength);

	representative[0] = messageEmpty ? 0x4b : 0x6b;
	memset(representative+1, 0xbb, representativeByteLength-digestSize-4);	// pad with 0xbb
	byte *afterP2 = representative+representativeByteLength-digestSize-3;
	afterP2[0] = 0xba;
	hash.Final(afterP2+1);
	representative[representativeByteLength-2] = *hashIdentifier.first;
	representative[representativeByteLength-1] = 0xcc;
}

NAMESPACE_END

#endif