SrpParameters thread-safety issue

[Alexey Yakovlev]

Looks like SrpParameters aren't thread safe because HashAlgorithm.ComputeHash isn't thread safe.
The issue is easily reproduced like this:

[Test]
public async Task ParallelAuthenticationTest()
{
	var username = "demo";
	var password = "insecure";
	var parameters = new SrpParameters();
	var server = new SrpServer(parameters);

	// spawn multiple threads reusing the same SrpParameters instance
	var tasks = Enumerable.Range(0, 10).Select(i => Task.Run(() =>
	{
		var client = new SrpClient(parameters);

		// sign up
		var salt = client.GenerateSalt();
		var privateKey = client.DerivePrivateKey(salt, username, password);
		var verifier = client.DeriveVerifier(privateKey);

		// authenticate
		var clientEphemeral = client.GenerateEphemeral();
		var serverEphemeral = server.GenerateEphemeral(verifier);
		var clientSession = client.DeriveSession(clientEphemeral.Secret, serverEphemeral.Public, salt, username, privateKey);

		var serverSession = server.DeriveSession(serverEphemeral.Secret, clientEphemeral.Public, salt, username, verifier, clientSession.Proof);
		client.VerifySession(clientEphemeral.Public, clientSession, serverSession.Proof);

		// make sure both the client and the server have the same session key
		Assert.AreEqual(clientSession.Key, serverSession.Key);
	}));

	await Task.WhenAll(tasks);
}