internal static BigInteger ComputeX(ClientInfo clientInfo, SrpInfo srpInfo)
{
var method = srpInfo.SrpMethod;
var iterations = srpInfo.Iterations;
if (iterations == 0)
{
throw new UnsupportedFeatureException("0 iterations is not supported");
}
// TODO: Add constants for 1024, 6144 and 8192
if (method != "SRPg-4096")
{
throw new UnsupportedFeatureException($"Method '{method}' is not supported");
}
var k1 = Util.Hkdf(method: method,
ikm: srpInfo.Salt,
salt: clientInfo.Username.ToBytes());
var k2 = Util.Pbes2(method: srpInfo.KeyMethod,
password: clientInfo.Password,
salt: k1,
iterations: iterations);
var x = clientInfo.AccountKey.CombineWith(k2);
return(x.ToBigInt());
}
//
// Internal
//
internal static byte[] Perform(BigInteger secretA,
ClientInfo clientInfo,
SrpInfo srpInfo,
string sessionId,
RestClient rest)
{
var sharedA = ComputeSharedA(secretA);
var sharedB = ExchangeAForB(sharedA, sessionId, rest);
ValidateB(sharedB);
return(ComputeKey(secretA, sharedA, sharedB, clientInfo, srpInfo, sessionId));
}
internal static byte[] ComputeKey(BigInteger secretA,
BigInteger sharedA,
BigInteger sharedB,
ClientInfo clientInfo,
SrpInfo srpInfo,
string sessionId)
{
// Some arbitrary crypto computation, variable names don't have a lot of meaning
var ab = sharedA.ToHex() + sharedB.ToHex();
var hashAb = Crypto.Sha256(ab).ToBigInt();
var s = sessionId.ToBytes().ToBigInt();
var x = ComputeX(clientInfo, srpInfo);
var y = sharedB - SirpG.ModExp(x, SirpN) * s;
var z = y.ModExp(secretA + hashAb * x, SirpN);
return(Crypto.Sha256(z.ToHex()));
}
// Returns the session encryption key
public static AesKey Perform(ClientInfo clientInfo, SrpInfo srpInfo, string sessionId, RestClient rest)
{
var key = Perform(GenerateSecretA(), clientInfo, srpInfo, sessionId, rest);
return(new AesKey(sessionId, key));
}
