@Override
public Signature sign(final byte[] data) {
if (!this.getKeyPair().hasPrivateKey()) {
throw new CryptoException("cannot sign without private key");
}
// Hash the private key to improve randomness.
final byte[] hash =
Hashes.sha3_512(ArrayUtils.toByteArray(this.getKeyPair().getPrivateKey().getRaw(), 32));
// r = H(hash_b,...,hash_2b-1, data) where b=256.
final Ed25519EncodedFieldElement r =
new Ed25519EncodedFieldElement(
Hashes.sha3_512(
Arrays.copyOfRange(
hash, 32, 64), // only include the last 32 bytes of the private key hash
data));
// Reduce size of r since we are calculating mod group order anyway
final Ed25519EncodedFieldElement rModQ = r.modQ();
// R = rModQ * base point.
final Ed25519GroupElement R = Ed25519Group.BASE_POINT.scalarMultiply(rModQ);
final Ed25519EncodedGroupElement encodedR = R.encode();
// S = (r + H(encodedR, encodedA, data) * a) mod group order where
// encodedR and encodedA are the little endian encodings of the group element R and the public
// key A and
// a is the lower 32 bytes of hash after clamping.
final Ed25519EncodedFieldElement h =
new Ed25519EncodedFieldElement(
Hashes.sha3_512(encodedR.getRaw(), this.getKeyPair().getPublicKey().getRaw(), data));
final Ed25519EncodedFieldElement hModQ = h.modQ();
final Ed25519EncodedFieldElement encodedS =
hModQ.multiplyAndAddModQ(
Ed25519Utils.prepareForScalarMultiply(this.getKeyPair().getPrivateKey()), rModQ);
// Signature is (encodedR, encodedS)
final Signature signature = new Signature(encodedR.getRaw(), encodedS.getRaw());
if (!this.isCanonicalSignature(signature)) {
throw new CryptoException("Generated signature is not canonical");
}
return signature;
}
@Override
public boolean verify(final byte[] data, final Signature signature) {
if (!this.isCanonicalSignature(signature)) {
return false;
}
if (1
== ArrayUtils.isEqualConstantTime(
this.getKeyPair().getPublicKey().getRaw(), new byte[32])) {
return false;
}
// h = H(encodedR, encodedA, data).
final byte[] rawEncodedR = signature.getBinaryR();
final byte[] rawEncodedA = this.getKeyPair().getPublicKey().getRaw();
final Ed25519EncodedFieldElement h =
new Ed25519EncodedFieldElement(Hashes.sha3_512(rawEncodedR, rawEncodedA, data));
// hReduced = h mod group order
final Ed25519EncodedFieldElement hModQ = h.modQ();
Ed25519GroupElement A = this.getKeyPair().getPublicKey().getAsGroupElement();
if (null == A) {
// Must compute A.
A = new Ed25519EncodedGroupElement(rawEncodedA).decode();
A.precomputeForDoubleScalarMultiplication();
}
// R = encodedS * B - H(encodedR, encodedA, data) * A
final Ed25519GroupElement calculatedR =
Ed25519Group.BASE_POINT.doubleScalarMultiplyVariableTime(
A, hModQ, new Ed25519EncodedFieldElement(signature.getBinaryS()));
// Compare calculated R to given R.
final byte[] encodedCalculatedR = calculatedR.encode().getRaw();
final int result = ArrayUtils.isEqualConstantTime(encodedCalculatedR, rawEncodedR);
return 1 == result;
}