OpenSSL certificate revocation check in client program using OCSP stapling

Question

I have an embedded C client program that securely connects to a server using OpenSSL. The server provides its certificate during the handshake and the client has to check the revocation status of this certificate. Currently I do this by using OCSP.

All of this works, but now I need to re-implement the client's revocation check using OCSP stapling (assuming the server will start providing this).

Currently I get the server certificate using X509 *cert = SSL_get_peer_certificate(ssl) to check the subjectAltName against my server's domain and get the authorityInfoAccess (for OCSP URI).

Assuming I have an SSL * ssl; and I successfully set everything up and connected via SSL_connect(ssl);, what do I do at this point to get at the OCSP stapling information and verify the certificate I just received? I can't find any sample code for how to actually implement this using the OpenSSL library.

Answer 1

There are a couple steps:

1. Have the client send the status_request extension via SSL_set_tlsext_status_type(ssl, TLSEXT_STATUSTYPE_ocsp).

2. Register a callback (and argument) to examine the OCSP response via SSL_CTX_set_tlsext_status_cb(ctx, ocsp_resp_cb) and SSL_CTX_set_tlsext_status_arg(ctx, arg)

3. Write the callback function. The one used by s_client demonstrates how to get at the response information:

   static int ocsp_resp_cb(SSL *s, void *arg)
   {
   const unsigned char *p;
   int len;
   OCSP_RESPONSE *rsp;
   len = SSL_get_tlsext_status_ocsp_resp(s, &p);
   BIO_puts(arg, "OCSP response: ");
   if (!p)
       {
       BIO_puts(arg, "no response sent\n");
       return 1;
       }
   rsp = d2i_OCSP_RESPONSE(NULL, &p, len);
   if (!rsp)
       {
       BIO_puts(arg, "response parse error\n");
       BIO_dump_indent(arg, (char *)p, len, 4);
   return 0;
   }
   BIO_puts(arg, "\n======================================\n");
   OCSP_RESPONSE_print(arg, rsp, 0);
   BIO_puts(arg, "======================================\n");
   OCSP_RESPONSE_free(rsp);
   return 1;
   }