X509_STORE_CTX_set_verify_cb(3)

NAME

       X509_STORE_CTX_set_verify_cb - set verification callback

SYNOPSIS

        #include <openssl/x509_vfy.h>

        void X509_STORE_CTX_set_verify_cb(X509_STORE_CTX *ctx,
                                       int (*verify_cb)(int ok, X509_STORE_CTX *ctx));

DESCRIPTION

       X509_STORE_CTX_set_verify_cb() sets the verification callback of ctx to
       verify_cb overwriting any existing callback.

       The verification callback can be used to customise the operation of
       certificate verification, either by overriding error conditions or log-
       ging errors for debugging purposes.

       However a verification callback is not essential and the default opera-
       tion is often sufficient.

       The ok parameter to the callback indicates the value the callback
       should return to retain the default behaviour. If it is zero then and
       error condition is indicated. If it is 1 then no error occurred. If the
       flag X509_V_FLAG_NOTIFY_POLICY is set then ok is set to 2 to indicate
       the policy checking is complete.

       The ctx parameter to the callback is the X509_STORE_CTX structure that
       is performing the verification operation. A callback can examine this
       structure and receive additional information about the error, for exam-
       ple by calling X509_STORE_CTX_get_current_cert(). Additional applica-
       tion data can be passed to the callback via the ex_data mechanism.

WARNING

       In general a verification callback should NOT unconditionally return 1
       in all circumstances because this will allow verification to succeed no
       matter what the error. This effectively removes all security from the
       application because any certificate (including untrusted generated
       ones) will be accepted.

NOTES

       The verification callback can be set and inherited from the parent
       structure performing the operation. In some cases (such as S/MIME veri-
       fication) the X509_STORE_CTX structure is created and destroyed inter-
       nally and the only way to set a custom verification callback is by
       inheriting it from the associated X509_STORE.

RETURN VALUES

       X509_STORE_CTX_set_verify_cb() does not return a value.

EXAMPLES

       Default callback operation:

        int verify_callback(int ok, X509_STORE_CTX *ctx)
               {
               return ok;
               }

       Simple example, suppose a certificate in the chain is expired and we
       wish to continue after this error:

        int verify_callback(int ok, X509_STORE_CTX *ctx)
               {
               /* Tolerate certificate expiration */
               if (X509_STORE_CTX_get_error(ctx) == X509_V_ERR_CERT_HAS_EXPIRED)
                               return 1;
               /* Otherwise don't override */
               return ok;
               }

       More complex example, we don't wish to continue after any certificate
       has expired just one specific case:

        int verify_callback(int ok, X509_STORE_CTX *ctx)
               {
               int err = X509_STORE_CTX_get_error(ctx);
               X509 *err_cert = X509_STORE_CTX_get_current_cert(ctx);
               if (err == X509_V_ERR_CERT_HAS_EXPIRED)
                       {
                       if (check_is_acceptable_expired_cert(err_cert)
                               return 1;
                       }
               return ok;
               }

       Full featured logging callback. In this case the bio_err is assumed to
       be a global logging BIO, an alternative would to store a BIO in ctx
       using ex_data.

        int verify_callback(int ok, X509_STORE_CTX *ctx)
               {
               X509 *err_cert;
               int err,depth;

               err_cert = X509_STORE_CTX_get_current_cert(ctx);
               err =   X509_STORE_CTX_get_error(ctx);
               depth = X509_STORE_CTX_get_error_depth(ctx);

               BIO_printf(bio_err,"depth=%d ",depth);
               if (err_cert)
                       {
                       X509_NAME_print_ex(bio_err, X509_get_subject_name(err_cert),
                                               0, XN_FLAG_ONELINE);
                       BIO_puts(bio_err, "\n");
                       }
               else
                       BIO_puts(bio_err, "<no cert>\n");
               if (!ok)
                       BIO_printf(bio_err,"verify error:num=%d:%s\n",err,
                               X509_verify_cert_error_string(err));
               switch (err)
                       {
               case X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT:
                       BIO_puts(bio_err,"issuer= ");
                       X509_NAME_print_ex(bio_err, X509_get_issuer_name(err_cert),
                                               0, XN_FLAG_ONELINE);
                       BIO_puts(bio_err, "\n");
                       break;
               case X509_V_ERR_CERT_NOT_YET_VALID:
               case X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD:
                       BIO_printf(bio_err,"notBefore=");
                       ASN1_TIME_print(bio_err,X509_get_notBefore(err_cert));
                       BIO_printf(bio_err,"\n");
                       break;
               case X509_V_ERR_CERT_HAS_EXPIRED:
               case X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD:
                       BIO_printf(bio_err,"notAfter=");
                       ASN1_TIME_print(bio_err,X509_get_notAfter(err_cert));
                       BIO_printf(bio_err,"\n");
                       break;
               case X509_V_ERR_NO_EXPLICIT_POLICY:
                       policies_print(bio_err, ctx);
                       break;
                       }
               if (err == X509_V_OK && ok == 2)
                       /* print out policies */

               BIO_printf(bio_err,"verify return:%d\n",ok);
               return(ok);
               }

SEE ALSO

       X509_STORE_CTX_get_error(3) X509_STORE_set_verify_cb_func(3)
       X509_STORE_CTX_get_ex_new_index(3)

HISTORY

       X509_STORE_CTX_set_verify_cb() is available in all versions of SSLeay
       and OpenSSL.

1.0.2t                            2019-09-10   X509_STORE_CTX_set_verify_cb(3)