Not supported critical extensions

Question

I've read the great post written by @Thomas Pornin Which properties of a X.509 certificate should be critical and which not?

and it raised my question.

STORY:

Let's assume that I have a client C and a server S. Now, client try to connect to S by SSL. Therefore, S presents its certificate CERT. Now, C tries to verify a S. For that moment we can ommit signature verification and hostname verification details and focus on extensions. Obviously, C uses OpenSSL to verify certificate.

But, for some critical extensions OpenSSL says: I don't support that.

PROBLEMS:

1. What I should do? I suppose that I should writting a code that verifies extensions not supported by OpenSSL, yes?

2. Especially, how to verify an extension? I can implement it on my own but I have to know what does it mean to verify/validate an extension. Please help.

Answer 1

...but I have to know what does it mean to verify/validate an extension...

There is no generic rule how an extension should be verified. The rules depend instead on the specific extension and are usually documented in the standard where the specific extension is defined.

Thus, if the certificate contains an extension which is marked as critical and which is not specifically addressed by either the TLS library or by your application according to the rules specific to this extension type than you should not accept the certificate.