.\" $OpenBSD: X509_STORE_CTX_set_verify_cb.3,v 1.9 2022/11/16 14:51:08 schwarze Exp $
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.Dd $Mdocdate: November 16 2022 $
.Dt X509_STORE_CTX_SET_VERIFY_CB 3
.Os
.Sh NAME
.Nm X509_STORE_CTX_verify_cb ,
.Nm X509_STORE_CTX_set_verify_cb ,
.Nm X509_STORE_CTX_get_verify_cb
.Nd set and retrieve verification callback
.Sh SYNOPSIS
.In openssl/x509_vfy.h
.Ft typedef int
.Fo (*X509_STORE_CTX_verify_cb)
.Fa "int ok"
.Fa "X509_STORE_CTX *ctx"
.Fc
.Ft void
.Fo X509_STORE_CTX_set_verify_cb
.Fa "X509_STORE_CTX *ctx"
.Fa "X509_STORE_CTX_verify_cb verify_cb"
.Fc
.Ft X509_STORE_CTX_verify_cb
.Fo X509_STORE_CTX_get_verify_cb
.Fa "X509_STORE_CTX *ctx"
.Fc
.Sh DESCRIPTION
.Fn X509_STORE_CTX_set_verify_cb
sets the verification callback of
.Fa ctx
to
.Fa verify_cb
overwriting any existing callback.
.Pp
The verification callback can be used to customise the operation of
certificate verification, either by overriding error conditions or
logging errors for debugging purposes.
.Pp
However, a verification callback is
.Em not
essential and the default operation is often sufficient.
.Pp
The
.Fa 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
.Dv X509_V_FLAG_NOTIFY_POLICY
is set, then
.Fa ok
is set to 2 to indicate the policy checking is complete.
.Pp
The
.Fa ctx
parameter to the callback is the
.Vt 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
.Xr X509_STORE_CTX_get_current_cert 3 .
Additional application data can be passed to the callback via the
.Sy ex_data
mechanism.
.Pp
The verification callback can be set and inherited from the parent
structure performing the operation.
In some cases (such as S/MIME verification) the
.Vt 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
.Vt X509_STORE .
.Sh RETURN VALUES
.Fn X509_STORE_CTX_get_verify_cb
returns a pointer to the current callback function
used by the specified
.Fa ctx .
If no callback was set using
.Fn X509_STORE_CTX_set_verify_cb ,
that is a pointer to a built-in static function
which does nothing except returning the
.Fa ok
argument passed to it.
.Sh EXAMPLES
Default callback operation:
.Bd -literal
int
verify_callback(int ok, X509_STORE_CTX *ctx)
{
	return ok;
}
.Ed
.Pp
Simple example, suppose a certificate in the chain is expired and we
wish to continue after this error:
.Bd -literal
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;
}
.Ed
.Pp
More complex example, we don't wish to continue after
.Sy any
certificate has expired just one specific case:
.Bd -literal
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;
}
.Ed
.Pp
Full featured logging callback.
In this case the
.Fa bio_err
is assumed to be a global logging
.Vt BIO ,
an alternative would to store a
.Vt BIO
in
.Fa ctx
using
.Sy ex_data .
.Bd -literal
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, "\en");
	} else
		BIO_puts(bio_err, "<no cert>\en");
	if (!ok)
		BIO_printf(bio_err, "verify error:num=%d:%s\en",
		    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, "\en");
		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, "\en");
		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, "\en");
		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\en",ok);
	return(ok);
}
.Ed
.Sh SEE ALSO
.Xr X509_STORE_CTX_get_error 3 ,
.Xr X509_STORE_CTX_get_ex_new_index 3 ,
.Xr X509_STORE_CTX_new 3 ,
.Xr X509_STORE_CTX_set_error 3 ,
.Xr X509_STORE_CTX_set_flags 3 ,
.Xr X509_STORE_CTX_set_verify 3 ,
.Xr X509_STORE_set_verify_cb 3 ,
.Xr X509_verify_cert 3 ,
.Xr X509_VERIFY_PARAM_set_flags 3
.Sh HISTORY
.Fn X509_STORE_CTX_set_verify_cb
first appeared in OpenSSL 0.9.6c and has been available since
.Ox 3.2 .
.Pp
.Fn X509_STORE_CTX_get_verify_cb
first appeared in OpenSSL 1.1.0 and has been available since
.Ox 7.1 .
.Pp
.Fn X509_STORE_CTX_verify_cb
first appeared in OpenSSL 1.1.0 and has been available since
.Ox 7.2 .
.Sh CAVEATS
In general a verification callback should
.Sy 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
.Sy any
certificate (including untrusted generated ones) will be accepted.