No server certificate and "no shared cipher" when clients connect

Question

I am writing a SSL server and client for communication. I have the following code for server

SSL_CTX* InitServerCTX(void)
{       
   SSL_METHOD *method;
   SSL_CTX *ctx;
   SSL_library_init();
   OpenSSL_add_all_algorithms();        /* load & register all cryptos, etc. */
   SSL_load_error_strings();

  ERR_load_crypto_strings();
  OpenSSL_add_all_ciphers();

  ctx = SSL_CTX_new(SSLv23_server_method());            /* Create new context */
  if ( ctx == NULL )
  {
       ERR_print_errors_fp(stderr);
       abort();
  }

  SSL_CTX_set_cipher_list(ctx, "HIGH:MEDIUM:!eNULL:!aNULL:!RC4");

  return ctx;  }

After this the code for accept is

 int client = accept(server, (sockaddr*)&addr, &len);       /* accept  connection as usual */
 printf("Connection: %s:%d\n",
 inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));
 ssl = SSL_new(ctx);                            /* get new SSL state with context */
 SSL_set_fd(ssl, client);
 int ret = SSL_accept(ssl);

And here is the client code

SSL_CTX* InitCTX(void)
{   
    SSL_METHOD *method;
    SSL_CTX *ctx;
    SSL_library_init();
    OpenSSL_add_all_algorithms();       /* Load cryptos, et.al. */
    SSL_load_error_strings();           /* Bring in and register error messages */
    ctx = SSL_CTX_new(SSLv23_client_method());          /* Create new context */
    if ( ctx == NULL )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }

    SSL_CTX_set_cipher_list(ctx, "HIGH:MEDIUM:!eNULL:!aNULL:!RC4");
    return ctx;
}

For connecting it is

ssl = SSL_new(ctx);                     /* create new SSL connection state */
SSL_set_fd(ssl, server);                /* attach the socket descriptor */
int ret = SSL_connect(ssl) ;

I am not using any certificates or keys.

When i try to connect using this approach i am getting no shared ciphers error on the server side. I think this is some configuration issue with respect to the ciphers. Can someone please point me the right direction.

Thanks

Typically, in ***this*** use case (no server auth), you use Anonymous Diffie-Hellman (ADH) or EC Anonymous Diffie-Hellman (ECADH). You should remove the ***`!aNULL`*** from the cipher suite list at *both* the client and server. Some user agents, like browsers, will not accept it. A related topic is [opportunistic encryption](http://en.wikipedia.org/wiki/opportunistic_encryption). It is used in use cases like MTAs and mail servers, and it ***may*** be applicable in your use case. — jww, Sep 19 '16 at 16:32

score 4 · Answer 1 · edited Jun 20 '20 at 09:12

4

 SSL_CTX_set_cipher_list(ctx, "HIGH:MEDIUM:!eNULL:!aNULL:!RC4");
I am not using any certificates or keys.

Since you neither use certificates nor SRP the only possible ciphers are thus where no authentication of the server is done. But you did explicitly exclude these ciphers with !aNULL in both client and server. This means that none of the ciphers offered by the client or accepted by the server is able to work with no authentication which results in "no shared ciphers". From the documentation of ciphers:

aNULL
the cipher suites offering no authentication. This is currently the anonymous DH algorithms. These cipher suites are vulnerable to a "man in the middle" attack and so their use is normally discouraged.

edited Jun 20 '20 at 09:12

Community

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answered Sep 19 '16 at 09:14

Steffen Ullrich

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To expand on this, openssl doesn't error about a lot of bad configurations. You'll get the 'no shared ciphers' when your certificates/cipher list/cipher-specific-configurations don't match up. If you're just doing some quick testing follow https://stackoverflow.com/questions/10175812/how-to-create-a-self-signed-certificate-with-openssl to make some certs, then pass them to `SSL_CTX_use_certificate_chain_file`, `SSL_CTX_use_PrivateKey_file` and `SSL_CTX_load_verify_locations` – C. Broadbent Sep 19 '16 at 09:18
so if i allow all ciphers and dont specify the above line..will it allow me to connect? – kunal Sep 19 '16 at 09:21
@kunal: if you allow all ciphers or just aNULL in both client and server it will probably work or at least not result in this error (did not take a closer look at the rest of the code). But beware that this is highly insecure, i.e. man in the middle attack is easy unless you add some kind of proper server authentication. – Steffen Ullrich Sep 19 '16 at 09:24
You need to specifically enable them by removing the `!` in front of the `aNULL` cipher group, since these ciphers are disabled by default. You *should not do this*. Instead set up some certificates for internal testing, or use the preshared key ciphers. – C. Broadbent Sep 19 '16 at 09:28
@SteffenUllrich i just want to test session negotiation to highest protocol supported and not use any version specific methods..i will update the code to secure ciphers once i am able to establish this – kunal Sep 19 '16 at 09:28
i tried removing ! now i am setting the ciphers using SSL_CTX_set_cipher_list(ctx, "aNULL") but i am still getting the same error. – kunal Sep 19 '16 at 09:33
@kunal: I have no idea what your current code is now (see also [How to create a Minimal, **Complete**, and Verifiable example](http://stackoverflow.com/help/mcve). I recommend that don't try to create both server and client but start slowly by first creating one side and testing it against known working peers, i.e. `openssl s_server` and `openssl s_client`. – Steffen Ullrich Sep 19 '16 at 11:46

No server certificate and "no shared cipher" when clients connect

1 Answers1