TuviSRPLib

TuviSRPLib realizes specific SRP protocol that is used in ProtonMail. Also this library imitates behavior of server and client side. Realization is based on BounceCastle realization of SRP-6 protocol (https://github.com/bcgit/bc-csharp/tree/master/crypto/src/crypto/agreement/srp) with appropriate protocol changes

TuviSRPLib contains following main classes:

• ProtonSRPServer - imitates server side;
• ProtonSRPClient - imitates client side;
• ProtonSRPUtilities - contains main calculations;
• ExtendedHashDigest - realization of hash algorithm that used in Proton realization.

To use this library and imitate interaction between server and client follow next example:

	BigInteger N = new BigInteger(1, byteArray); // group order - any big prime number you want to use
    BigInteger g = new BigInteger("2"); // group generator - always equals 2 in Proton realization
    
    string password = "qwerty"; // any password
    string salt = "some bytes"; // exactly 10 symbols for Proton realization. Bcrypt uses salt with specific length.

    Encoding enc = Encoding.UTF8;
    byte[] passwordBytes = enc.GetBytes(password);
    byte[] saltBytes = enc.GetBytes(salt);

	// Sides creation
    ProtonSRPClient client = new ProtonSRPClient();
    ProtonSRPServer server = new ProtonSRPServer();
    IDigest digest = new ExtendedHashDigest();

	// Verifier creation
    var verifier = ProtonSRPUtilities.CalculateVerifier(digest, N, g, saltBytes, passwordBytes);

	// Sides initialization
    server.Init(N, g, verifier, digest, new SecureRandom());
    client.Init(N, g, digest, new SecureRandom());

	// Credential genration for both sides
    BigInteger pubA = client.GenerateClientCredentials(saltBytes, passwordBytes);
    BigInteger pubB = server.GenerateServerCredentials();

    server.CalculateSecret(pubA);
    client.CalculateSecret(pubB);

    BigInteger M1 = client.CalculateClientEvidenceMessage(); // M1 message creation
    
	if (server.VerifyClientEvidenceMessage(M1)) // M1 message verifying
	{
		BigInteger M2 = server.CalculateServerEvidenceMessage(); // M2 message creation

		if (client.VerifyServerEvidenceMessage(M2)) // M2 message verifying
		{
			BigInteger clientKey = client.CalculateSessionKey();
			BigInteger serverKey = server.CalculateSessionKey();
		}
	}