Needing TLS handshake to fail

[Maxim Dounin]

Hello!

On Wed, Sep 04, 2019 at 08:35:05AM -0400, Phillip Odam wrote:

> Hi,
> 
> I tried asking the following on the general mailing list but I'm 
> guessing this is tending more towards development.
> 
> 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 
> https://www.openssl.org/docs/man1.0.2/man3/SSL_CTX_set_verify.html 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. We're currently looking at alternate software but would 
> really love to be able to pull this back in to nginx when/if the day 
> comes that nginx supports this.
> 
> Assuming development is required, is this something already on the 
> backlog? My only comment for enabling the TLS handshake failure is it'd 
> be really nice if it were configurable at the level of the virtual host.

No, there is nothing like this in the backlog.  As you can see 
from the code, nginx approach is to proceed with handshake 
regardless of client certificate verification results, and then 
reject requests on HTTP level (and/or just make verification 
results available for logging).  This approach is believed to be 
more user friendly - and, in particular, it allows one to 
configure an error page explaining the problem.

Note well that patches to change this are likely to be rejected 
unless there is some justification better than "there is a third 
party that requires this".

-- 
Maxim Dounin
http://mdounin.ru/