Openssl tls srp - ssl

Question

Id like to use srp in my current project. But im kinda at a loss as to how i would implement that with openssl. I got the client side running but i dont know how to write the server side. I also couldnt find any documentation orexample implementations of use. What i want is to store the login information inside a database and then retrieve that data when needed. Im using poco for most of the network part so writing the client was rather easy and i sucessfully tested it against other servers. So i would be gratefull for hints as to how to implement the server side.

Answer 1

There is an example of how to do this in file ssl/ssltest.c in OpenSSL archive.
At a high level you register an authentication callback. This callback is called automatically as necessary during SSL_Accept() to authenticate your users.
From ssltest.c example callback looks like the following:
static int MS_CALLBACK ssl_srp_server_param_cb(SSL *s, int *ad, void *arg)
{
SRP_SERVER_ARG * p = (SRP_SERVER_ARG *) arg;
if (strcmp(p->expected_user, SSL_get_srp_username(s)) != 0)
{
fprintf(stderr, "User %s doesn't exist\n", SSL_get_srp_username(s));
return SSL3_AL_FATAL;
}
if (SSL_set_srp_server_param_pw(s,p->expected_user,p->pass,NULL)<0)
{
*ad = SSL_AD_INTERNAL_ERROR;
return SSL3_AL_FATAL;
}
return SSL_ERROR_NONE;
}
To register you call
SSL_CTX_set_srp_username_callback(s_ctx, ssl_srp_server_param_cb);
The arg parameter of callback function is a void pointer passed to your callback so that you can reference any necessary application context in your application from the callback.
To set arg parameter call SSL_CTX_set_srp_cb_arg(s_ctx, mypointer);
It is really this easy. Be sure SRP ciphers are being included in your cipher list.
There is a way to get feedback on autentication failures that can be used to implement countermeasure against high rate online dictionary attack.
To do this call SSL_CTX_set_info_callback(s_ctx,mynotifycallback) to register your callback. Had success using condition below to filter unrelated notifications.
void mynotifycallback(const SSL *s, int reason, int ret)
{
if (reason & SSL_CB_ALERT && ret & SSL3_AD_BAD_RECORD_MAC && SSL_get_srp_username((SSL *)s))
// authentication failure
}