Method Verify

extra/yassl/taocrypt/src/dsa.cpp:216–243

bool DSA_Verifier::Verify(const byte* sha_digest, const byte* sig)
{
    const Integer& p = key_.GetModulus();
    const Integer& q = key_.GetSubGroupOrder();
    const Integer& g = key_.GetSubGroupGenerator();
    const Integer& y = key_.GetPublicPart();

    int sz = q.ByteCount();

    r_.Decode(sig, sz);
    s_.Decode(sig + sz, sz);

    if (r_ >= q || r_ < 1 || s_ >= q || s_ < 1)
        return false;

    Integer H(sha_digest, SHA::DIGEST_SIZE);  // sha Hash(m)

    Integer w = s_.InverseMod(q);
    Integer u1 = (H  * w) % q;
    Integer u2 = (r_ * w) % q;

    // verify r == ((g^u1 * y^u2) mod p) mod q
    ModularArithmetic ma(p);
    Integer v = ma.CascadeExponentiate(g, u1, y, u2);
    v %= q;

    return r_ == v;
}