Section: OpenSSL (1SSL)
openssl verify [CApath directory] [-CAfile file] [-purpose purpose] [-policy arg] [-ignore_critical] [-crl_check] [-crl_check_all] [-policy_check] [-explicit_policy] [-inhibit_any] [-inhibit_map] [-x509_strict] [-extended_crl] [-use_deltas] [-policy_print] [-untrusted file] [-help] [-issuer_checks] [-attime timestamp] [-verbose]  [certificates]
:-CAfile file A file of trusted certificates. The file should contain multiple certificates in PEM format concatenated together.
The verify program uses the same functions as the internal SSL and S/MIME verification, therefore this description applies to these verify operations too.
There is one crucial difference between the verify operations performed by the verify program: wherever possible an attempt is made to continue after an error whereas normally the verify operation would halt on the first error. This allows all the problems with a certificate chain to be determined.
The verify operation consists of a number of separate steps.
Firstly a certificate chain is built up starting from the supplied certificate and ending in the root CA. It is an error if the whole chain cannot be built up. The chain is built up by looking up the issuers certificate of the current certificate. If a certificate is found which is its own issuer it is assumed to be the root CA.
The process of 'looking up the issuers certificate' itself involves a number of steps. In versions of OpenSSL before 0.9.5a the first certificate whose subject name matched the issuer of the current certificate was assumed to be the issuers certificate. In OpenSSL 0.9.6 and later all certificates whose subject name matches the issuer name of the current certificate are subject to further tests. The relevant authority key identifier components of the current certificate (if present) must match the subject key identifier (if present) and issuer and serial number of the candidate issuer, in addition the keyUsage extension of the candidate issuer (if present) must permit certificate signing.
The lookup first looks in the list of untrusted certificates and if no match is found the remaining lookups are from the trusted certificates. The root CA is always looked up in the trusted certificate list: if the certificate to verify is a root certificate then an exact match must be found in the trusted list.
The second operation is to check every untrusted certificate's extensions for consistency with the supplied purpose. If the -purpose option is not included then no checks are done. The supplied or ``leaf'' certificate must have extensions compatible with the supplied purpose and all other certificates must also be valid CA certificates. The precise extensions required are described in more detail in the CERTIFICATE EXTENSIONS section of the x509 utility.
The third operation is to check the trust settings on the root CA. The root CA should be trusted for the supplied purpose. For compatibility with previous versions of SSLeay and OpenSSL a certificate with no trust settings is considered to be valid for all purposes.
The final operation is to check the validity of the certificate chain. The validity period is checked against the current system time and the notBefore and notAfter dates in the certificate. The certificate signatures are also checked at this point.
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''.
Although the issuer checks are a considerable improvement over the old technique they still suffer from limitations in the underlying X509_LOOKUP API. One consequence of this is that trusted certificates with matching subject name must either appear in a file (as specified by the -CAfile option) or a directory (as specified by -CApath. If they occur in both then only the certificates in the file will be recognised.
Previous versions of OpenSSL assume certificates with matching subject name are identical and mishandled them.
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