/**
* Create first message in SMP exchange. Input is Alice's secret value which this protocol aims to
* compare to Bob's. The return value is a serialized MPI array whose elements correspond to the
* following: [0] = g2a, Alice's half of DH exchange to determine g2 [1] = c2, [2] = d2, Alice's
* ZK proof of knowledge of g2a exponent [3] = g3a, Alice's half of DH exchange to determine g3
* [4] = c3, [5] = d3, Alice's ZK proof of knowledge of g3a exponent
*
* @throws OTRException
*/
public static byte[] step1(SMState astate, byte[] secret, Provider prov) throws OTRException {
/* Initialize the sm state or update the secret */
// Util.checkBytes("secret", secret);
MPI secret_mpi = new MPI(secret);
astate.secret = secret_mpi;
astate.receivedQuestion = 0;
astate.x2 = randomExponent(prov);
astate.x3 = randomExponent(prov);
MPI[] msg1 = new MPI[6];
msg1[0] = prov.powm(astate.g1, astate.x2, new MPI(MODULUS_S));
MPI[] res = proofKnowLog(astate.g1, astate.x2, 1, prov);
msg1[1] = res[0];
msg1[2] = res[1];
msg1[3] = prov.powm(astate.g1, astate.x3, new MPI(MODULUS_S));
res = proofKnowLog(astate.g1, astate.x3, 2, prov);
msg1[4] = res[0];
msg1[5] = res[1];
byte[] ret = serializeMPIArray(msg1);
astate.smProgState = PROG_OK;
return ret;
}
/**
* Receive the first message in SMP exchange, which was generated by step1. Input is saved until
* the user inputs their secret information. No output.
*
* @throws OTRException
*/
public static void step2a(SMState bstate, byte[] input, int received_question, Provider prov)
throws OTRException {
/* Initialize the sm state if needed */
bstate.receivedQuestion = received_question;
bstate.smProgState = PROG_CHEATED;
/* Read from input to find the mpis */
MPI[] msg1 = unserializeMPIArray(input);
if (checkGroupElem(msg1[0], prov)
|| checkExpon(msg1[2], prov)
|| checkGroupElem(msg1[3], prov)
|| checkExpon(msg1[5], prov)) {
throw new OTRException("Invalid parameter");
}
/* Store Alice's g3a value for later in the protocol */
bstate.g3o = msg1[3];
/* Verify Alice's proofs */
if (checkKnowLog(msg1[1], msg1[2], bstate.g1, msg1[0], 1, prov) != 0
|| checkKnowLog(msg1[4], msg1[5], bstate.g1, msg1[3], 2, prov) != 0) {
throw new OTRException("Proof checking failed");
}
/* Create Bob's half of the generators g2 and g3 */
bstate.x2 = randomExponent(prov);
bstate.x3 = randomExponent(prov);
/* Combine the two halves from Bob and Alice and determine g2 and g3 */
bstate.g2 = prov.powm(msg1[0], bstate.x2, new MPI(MODULUS_S));
// Util.checkBytes("g2b", bstate.g2.getValue());
bstate.g3 = prov.powm(msg1[3], bstate.x3, new MPI(MODULUS_S));
// Util.checkBytes("g3b", bstate.g3.getValue());
bstate.smProgState = PROG_OK;
}