protected OutgoingMessage computeSharedSecret(final IncomingMessage in)
throws KeyAgreementException {
N = in.readMPI();
g = in.readMPI();
final BigInteger s = in.readMPI();
final BigInteger B = in.readMPI();
if (B.mod(N).equals(BigInteger.ZERO)) {
throw new KeyAgreementException("illegal value for B");
}
// generate an ephemeral keypair
final SRPKeyPairGenerator kpg = new SRPKeyPairGenerator();
final Map attributes = new HashMap();
if (rnd != null) {
attributes.put(SRPKeyPairGenerator.SOURCE_OF_RANDOMNESS, rnd);
}
attributes.put(SRPKeyPairGenerator.SHARED_MODULUS, N);
attributes.put(SRPKeyPairGenerator.GENERATOR, g);
kpg.setup(attributes);
userKeyPair = kpg.generate();
final BigInteger A = ((SRPPublicKey) userKeyPair.getPublic()).getY();
final BigInteger u = uValue(A, B); // u = H(A | B)
if (u.mod(N).equals(BigInteger.ZERO)) {
throw new KeyAgreementException("u is zero");
}
final BigInteger x;
try {
x = new BigInteger(1, srp.computeX(Util.trim(s), I, p));
} catch (Exception e) {
throw new KeyAgreementException("computeSharedSecret()", e);
}
// compute S = (B - 3g^x) ^ (a + ux)
final BigInteger a = ((SRPPrivateKey) userKeyPair.getPrivate()).getX();
final BigInteger S = B.subtract(THREE.multiply(g.modPow(x, N))).modPow(a.add(u.multiply(x)), N);
K = S;
final OutgoingMessage result = new OutgoingMessage();
result.writeMPI(A);
complete = true;
return result;
}
private OutgoingMessage sendIdentity(final IncomingMessage in) throws KeyAgreementException {
final OutgoingMessage result = new OutgoingMessage();
result.writeString(I);
return result;
}
