+=head1 DIAGNOSTICS
+
+When a verify operation fails the output messages can be somewhat cryptic. The
+general form of the error message is:
+
+ server.pem: /C=AU/ST=Queensland/O=CryptSoft Pty Ltd/CN=Test CA (1024 bit)
+ error 24 at 1 depth lookup:invalid CA certificate
+
+The first line contains the name of the certificate being verified followed by
+the subject name of the certificate. The second line contains the error number
+and the depth. The depth is number of the certificate being verified when a
+problem was detected starting with zero for the certificate being verified itself
+then 1 for the CA that signed the certificate and so on. Finally a text version
+of the error number is presented.
+
+An exhaustive list of the error codes and messages is shown below, this also
+includes the name of the error code as defined in the header file x509_vfy.h
+Some of the error codes are defined but never returned: these are described
+as "unused".
+
+=over 4
+
+=item B<0 X509_V_OK: ok>
+
+the operation was successful.
+
+=item B<2 X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT: unable to get issuer certificate>
+
+the issuer certificate could not be found: this occurs if the issuer certificate
+of an untrusted certificate cannot be found.
+
+=item B<3 X509_V_ERR_UNABLE_TO_GET_CRL unable to get certificate CRL>
+
+the CRL of a certificate could not be found. Unused.
+
+=item B<4 X509_V_ERR_UNABLE_TO_DECRYPT_CERT_SIGNATURE: unable to decrypt certificate's signature>
+
+the certificate signature could not be decrypted. This means that the actual signature value
+could not be determined rather than it not matching the expected value, this is only
+meaningful for RSA keys.
+
+=item B<5 X509_V_ERR_UNABLE_TO_DECRYPT_CRL_SIGNATURE: unable to decrypt CRL's's signature>
+
+the CRL signature could not be decrypted: this means that the actual signature value
+could not be determined rather than it not matching the expected value. Unused.
+
+=item B<6 X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY: unable to decode issuer public key>
+
+the public key in the certificate SubjectPublicKeyInfo could not be read.
+
+=item B<7 X509_V_ERR_CERT_SIGNATURE_FAILURE: certificate signature failure>
+
+the signature of the certificate is invalid.
+
+=item B<8 X509_V_ERR_CRL_SIGNATURE_FAILURE: CRL signature failure>
+
+the signature of the certificate is invalid. Unused.
+
+=item B<9 X509_V_ERR_CERT_NOT_YET_VALID: certificate is not yet valid>
+
+the certificate is not yet valid: the notBefore date is after the current time.
+
+=item B<10 X509_V_ERR_CRL_NOT_YET_VALID: CRL is not yet valid>
+
+the CRL is not yet valid. Unused.
+
+=item B<11 X509_V_ERR_CERT_HAS_EXPIRED: Certificate has expired>
+
+the certificate has expired: that is the notAfter date is before the current time.
+
+=item B<12 X509_V_ERR_CRL_HAS_EXPIRED: CRL has expired>
+
+the CRL has expired. Unused.
+
+=item B<13 X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD: format error in certificate's notBefore field>
+
+the certificate notBefore field contains an invalid time.