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_set_verify_cb_func(3) HISTORY
X509_STORE_CTX_get_get_issuer(), X509_STORE_CTX_get_check_issued(), X509_STORE_CTX_get_check_revocation(), X509_STORE_CTX_get_get_crl(), X509_STORE_CTX_get_check_crl(), X509_STORE_CTX_get_cert_crl(), X509_STORE_CTX_get_check_policy(), X509_STORE_CTX_get_lookup_certs(), X509_STORE_CTX_get_lookup_crls() and X509_STORE_CTX_get_cleanup() were added in OpenSSL 1.1.0.
COPYRIGHT
Copyright 2009-2016 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.