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
{
        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

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