public static boolean verifySignature(byte[] message, DSASignature ds, DSAPublicKey dpk)
throws IOException {
/* Inspired by Bouncycastle's DSASigner class */
SHA1 md = new SHA1();
md.update(message);
byte[] sha_message = new byte[md.getDigestLength()];
md.digest(sha_message);
BigInteger m = new BigInteger(1, sha_message);
BigInteger r = ds.getR();
BigInteger s = ds.getS();
BigInteger g = dpk.getG();
BigInteger p = dpk.getP();
BigInteger q = dpk.getQ();
BigInteger y = dpk.getY();
BigInteger zero = BigInteger.ZERO;
if (log.isEnabled()) {
log.log(60, "ssh-dss signature: m: " + m.toString(16));
log.log(60, "ssh-dss signature: r: " + r.toString(16));
log.log(60, "ssh-dss signature: s: " + s.toString(16));
log.log(60, "ssh-dss signature: g: " + g.toString(16));
log.log(60, "ssh-dss signature: p: " + p.toString(16));
log.log(60, "ssh-dss signature: q: " + q.toString(16));
log.log(60, "ssh-dss signature: y: " + y.toString(16));
}
if (zero.compareTo(r) >= 0 || q.compareTo(r) <= 0) {
log.log(20, "ssh-dss signature: zero.compareTo(r) >= 0 || q.compareTo(r) <= 0");
return false;
}
if (zero.compareTo(s) >= 0 || q.compareTo(s) <= 0) {
log.log(20, "ssh-dss signature: zero.compareTo(s) >= 0 || q.compareTo(s) <= 0");
return false;
}
BigInteger w = s.modInverse(q);
BigInteger u1 = m.multiply(w).mod(q);
BigInteger u2 = r.multiply(w).mod(q);
u1 = g.modPow(u1, p);
u2 = y.modPow(u2, p);
BigInteger v = u1.multiply(u2).mod(p).mod(q);
return v.equals(r);
}
public static DSASignature generateSignature(byte[] message, DSAPrivateKey pk, SecureRandom rnd) {
SHA1 md = new SHA1();
md.update(message);
byte[] sha_message = new byte[md.getDigestLength()];
md.digest(sha_message);
BigInteger m = new BigInteger(1, sha_message);
BigInteger k;
int qBitLength = pk.getQ().bitLength();
do {
k = new BigInteger(qBitLength, rnd);
} while (k.compareTo(pk.getQ()) >= 0);
BigInteger r = pk.getG().modPow(k, pk.getP()).mod(pk.getQ());
k = k.modInverse(pk.getQ()).multiply(m.add((pk).getX().multiply(r)));
BigInteger s = k.mod(pk.getQ());
return new DSASignature(r, s);
}