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.