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X509_STORE_CTX_SET_VERIFY_CB(3) Library Functions Manual X509_STORE_CTX_SET_VERIFY_CB(3)

X509_STORE_CTX_set_verify_cb, X509_STORE_CTX_get_verify_cbset and retrieve verification callback

#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));

int
(*X509_STORE_CTX_get_verify_cb(X509_STORE_CTX *ctx))(int ok, X509_STORE_CTX *ctx);

() 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 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 an 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(3). Additional application data can be passed to the callback via the ex_data mechanism.

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.

X509_STORE_CTX_get_verify_cb() returns a pointer to the current callback function used by the specified ctx. If no callback was set using X509_STORE_CTX_set_verify_cb(), that is a pointer to a built-in static function which does nothing except returning the ok argument passed to it.

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);
}

X509_STORE_CTX_get_error(3), X509_STORE_CTX_get_ex_new_index(3), X509_STORE_CTX_new(3), X509_STORE_CTX_set_error(3), X509_STORE_CTX_set_flags(3), X509_STORE_set_verify_cb(3), X509_verify_cert(3), X509_VERIFY_PARAM_set_flags(3)

X509_STORE_CTX_set_verify_cb() first appeared in OpenSSL 0.9.6c and has been available since OpenBSD 3.2.

X509_STORE_CTX_get_verify_cb() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 7.1.

In general a verification callback should 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.

January 2, 2022 OpenBSD-current