/**
* Create final message in SMP exchange. Input is a message generated by otrl_sm_step3. Output is
* a serialized mpi array whose elements correspond to the following: [0] = rb, calculated as
* (Qa/Qb)^x3 where x3 is the exponent used in g3b [1] = cr, [2] = d7, Bob's ZK proof that rb is
* formed correctly This method also checks if Alice and Bob's secrets were the same. If so, it
* returns NO_ERROR. If the secrets differ, an INV_VALUE error is returned instead.
*
* @throws OTRException
*/
public static byte[] step4(SMState bstate, byte[] input, Provider prov) throws OTRException {
/* Read from input to find the mpis */
MPI[] msg3 = unserializeMPIArray(input);
bstate.smProgState = PROG_CHEATED;
MPI[] msg4 = new MPI[3];
if (checkGroupElem(msg3[0], prov)
|| checkGroupElem(msg3[1], prov)
|| checkGroupElem(msg3[5], prov)
|| checkExpon(msg3[3], prov)
|| checkExpon(msg3[4], prov)
|| checkExpon(msg3[7], prov)) {
throw new OTRException("Invalid Parameter");
}
/* Verify Alice's coordinate equality proof */
if (checkEqualCoords(msg3[2], msg3[3], msg3[4], msg3[0], msg3[1], bstate, 6, prov) != 0)
throw new OTRException("Invalid Parameter");
/* Find Pa/Pb and Qa/Qb */
MPI inv = prov.invm(bstate.p, new MPI(MODULUS_S));
bstate.pab = prov.mulm(msg3[0], inv, new MPI(MODULUS_S));
inv = prov.invm(bstate.q, new MPI(MODULUS_S));
bstate.qab = prov.mulm(msg3[1], inv, new MPI(MODULUS_S));
/* Verify Alice's log equality proof */
if (checkEqualLogs(msg3[6], msg3[7], msg3[5], bstate, 7, prov) != 0) {
throw new OTRException("Proof checking failed");
}
/* Calculate Rb and proof */
msg4[0] = prov.powm(bstate.qab, bstate.x3, new MPI(MODULUS_S));
MPI[] res = proofEqualLogs(bstate, 8, prov);
msg4[1] = res[0];
msg4[2] = res[1];
byte[] output = serializeMPIArray(msg4);
/* Calculate Rab and verify that secrets match */
MPI rab = prov.powm(msg3[5], bstate.x3, new MPI(MODULUS_S));
// Util.checkBytes("rab", rab.getValue());
// Util.checkBytes("pab", bstate.pab.getValue());
int comp = prov.compareMPI(rab, bstate.pab);
bstate.smProgState = (comp != 0) ? PROG_FAILED : PROG_SUCCEEDED;
return output;
}
/**
* Create third message in SMP exchange. Input is a message generated by otrl_sm_step2b. Output is
* a serialized mpi array whose elements correspond to the following: [0] = pa, [1] = qa, Alice's
* halves of the (Pa/Pb) and (Qa/Qb) values [2] = cp, [3] = d5, [4] = d6, Alice's ZK proof that
* pa, qa formed correctly [5] = ra, calculated as (Qa/Qb)^x3 where x3 is the exponent used in g3a
* [6] = cr, [7] = d7, Alice's ZK proof that ra is formed correctly
*
* @throws OTRException
*/
public static byte[] step3(SMState astate, byte[] input, Provider prov) throws OTRException {
/* Read from input to find the mpis */
astate.smProgState = PROG_CHEATED;
MPI[] msg2 = unserializeMPIArray(input);
if (checkGroupElem(msg2[0], prov)
|| checkGroupElem(msg2[3], prov)
|| checkGroupElem(msg2[6], prov)
|| checkGroupElem(msg2[7], prov)
|| checkExpon(msg2[2], prov)
|| checkExpon(msg2[5], prov)
|| checkExpon(msg2[9], prov)
|| checkExpon(msg2[10], prov)) {
throw new OTRException("Invalid Parameter");
}
MPI[] msg3 = new MPI[8];
/* Store Bob's g3a value for later in the protocol */
astate.g3o = msg2[3];
/* Verify Bob's knowledge of discreet log proofs */
if (checkKnowLog(msg2[1], msg2[2], astate.g1, msg2[0], 3, prov) != 0
|| checkKnowLog(msg2[4], msg2[5], astate.g1, msg2[3], 4, prov) != 0) {
throw new OTRException("Proof checking failed");
}
/* Combine the two halves from Bob and Alice and determine g2 and g3 */
astate.g2 = prov.powm(msg2[0], astate.x2, new MPI(MODULUS_S));
// Util.checkBytes("g2a", astate.g2.getValue());
astate.g3 = prov.powm(msg2[3], astate.x3, new MPI(MODULUS_S));
// Util.checkBytes("g3a", astate.g3.getValue());
/* Verify Bob's coordinate equality proof */
if (checkEqualCoords(msg2[8], msg2[9], msg2[10], msg2[6], msg2[7], astate, 5, prov) != 0)
throw new OTRException("Invalid Parameter");
/* Calculate P and Q values for Alice */
MPI r = randomExponent(prov);
// MPI r = new MPI(SM.GENERATOR_S);
astate.p = prov.powm(astate.g3, r, new MPI(MODULUS_S));
// Util.checkBytes("Pa", astate.p.getValue());
msg3[0] = astate.p;
MPI qa1 = prov.powm(astate.g1, r, new MPI(MODULUS_S));
// Util.checkBytes("Qa1", qa1.getValue());
MPI qa2 = prov.powm(astate.g2, astate.secret, new MPI(MODULUS_S));
// Util.checkBytes("Qa2", qa2.getValue());
// Util.checkBytes("g2", astate.g2.getValue());
// Util.checkBytes("secret", astate.secret.getValue());
astate.q = prov.mulm(qa1, qa2, new MPI(MODULUS_S));
msg3[1] = astate.q;
// Util.checkBytes("Qa", astate.q.getValue());
MPI[] res = proofEqualCoords(astate, r, 6, prov);
msg3[2] = res[0];
msg3[3] = res[1];
msg3[4] = res[2];
/* Calculate Ra and proof */
MPI inv = prov.invm(msg2[6], new MPI(MODULUS_S));
astate.pab = prov.mulm(astate.p, inv, new MPI(MODULUS_S));
inv = prov.invm(msg2[7], new MPI(MODULUS_S));
astate.qab = prov.mulm(astate.q, inv, new MPI(MODULUS_S));
msg3[5] = prov.powm(astate.qab, astate.x3, new MPI(MODULUS_S));
res = proofEqualLogs(astate, 7, prov);
msg3[6] = res[0];
msg3[7] = res[1];
byte[] output = serializeMPIArray(msg3);
astate.smProgState = PROG_OK;
return output;
}