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 logging errors for debugging
       purposes.

       However a verification callback is not essential and the default operation 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 example by calling X509_STORE_CTX_get_current_cert().
       Additional application 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 verification) the X509_STORE_CTX structure is
       created and destroyed internally 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.