Needing TLS handshake to fail

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Needing TLS handshake to fail

Phillip Odam

I have a project that involves mutual / two way TLS and one of the requirements is that the TLS handshake must fail ie. be terminated before completion if the handshake is in anyway unsuccessful, eg. no client certificate provided or client certificate not trusted.

After having no success getting nginx (v1.16.1) & openssl (v1.0.2k-fips) to fail the handshake I ended up looking at the nginx source code, in particular src/event/ngx_event_openssl.c, and from what I read here I think a small but necessary code change is required.

Some possible approaches when choosing to remain using nginx as the server end of the mutual TLS connection

  • in static int ngx_ssl_verify_callback(int ok, X509_STORE_CTX *x509_store) make it configurable whether 1 is always returned or the value of ok
  • in ngx_int_t ngx_ssl_client_certificate(ngx_conf_t *cf, ngx_ssl_t *ssl, ngx_str_t *cert, ngx_int_t depth) make it configurable whether SSL_CTX_set_verify(ssl->ctx, SSL_VERIFY_PEER, ngx_ssl_verify_callback); is called or SSL_CTX_set_verify(ssl->ctx, SSL_VERIFY_PEER, NULL);

Is a code change required or is there a way for the handshake failure to be 'enabled' as opposed to ending up with a successfully established TLS connection. Admittedly within nginx there's all the detail that the TLS connection doesn't conform to the configured requirements of the TLS connection but this doesn't satisfy the requirements for the project.

I won't bother going in to the details of the project but will just say it's a third party certification body that requires the TLS handshake to be terminated before completion if the handshake is in anyway unsuccessful.



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