the peer&#39;s certificate. At each level signatures and issuer attributes are checked. Whenever a verification error is found, the error number is stored in <b>x509_ctx</b> and <b>verify_callback</b> is called with <b>preverify_ok</b>=0. By applying X509_CTX_store_* functions <b>verify_callback</b> can locate the certificate in question and perform additional steps (see EXAMPLES). If no error is found for a certificate, <b>verify_callback</b> is called with <b>preverify_ok</b>=1 before advancing to the nex