how to get the Keyusage value from the X509 certificate?

Question

I want to retrieve the Key usage value from the X509 structured certificate , i tried the following code

 X509* lcert=NULL;
 lCert=PEM_read(filename); // fun         


        

Answer 1

From ssl\ssl_lib.c, line 2365, OpenSSL v 1.0.2d:

/* This call populates extension flags (ex_flags) */

X509_check_purpose(x, -1, 0);

So OpenSSL developers use this way.

If digging deeper, you may find call of x509v3_cache_extensions, that populate flags, guarded by locks.