beck and davidben have plans to make ASN1_STRING opaque in OpenSSL 4 [1].
This breaks somewhere in the vicinity of 200 ports, so I don't think we'll
flip the switch anytime soon in libressl, but I want base to be ready for
that since that makes my life easier.

This is the first of two steps to convert isakmpd. ASN1_STRING_length()
and ASN1_STRING_get0_data() are dumb accessors. The latter returns a
const pointer, so add a minor adjustment for that, the rest is entirely
straightforward.

(In case you wonder: x509_cert_subjectaltname() isn't static because it
is "used" from regress/isakmpd/x509. That thing is completely broken and
hasn't been successfully compiled since reyk removed math_mp.h back in
2010, possibly much longer.)

[1]: https://github.com/openssl/openssl/issues/29117

Index: x509.c
===================================================================
RCS file: /cvs/src/sbin/isakmpd/x509.c,v
diff -u -p -r1.126 x509.c
--- x509.c	28 Apr 2024 16:43:42 -0000	1.126
+++ x509.c	24 Nov 2025 14:03:09 -0000
@@ -1098,11 +1098,11 @@ x509_cert_obtain(u_int8_t *id, size_t id
 
 /* Returns a pointer to the subjectAltName information of X509 certificate.  */
 int
-x509_cert_subjectaltname(X509 *scert, u_int8_t **altname, u_int32_t *len)
+x509_cert_subjectaltname(X509 *scert, const u_int8_t **altname, u_int32_t *len)
 {
 	X509_EXTENSION		*subjectaltname;
 	ASN1_OCTET_STRING	*sanasn1data;
-	u_int8_t		*sandata;
+	const u_int8_t		*sandata;
 	int			 extpos, santype, sanlen;
 
 	extpos = X509_get_ext_by_NID(scert, NID_subject_alt_name, -1);
@@ -1114,14 +1114,15 @@ x509_cert_subjectaltname(X509 *scert, u_
 	subjectaltname = X509_get_ext(scert, extpos);
 	sanasn1data = X509_EXTENSION_get_data(subjectaltname);
 
-	if (!subjectaltname || !sanasn1data || !sanasn1data->data ||
-	    sanasn1data->length < 4) {
+	if (!subjectaltname || !sanasn1data ||
+	    !ASN1_STRING_get0_data(sanasn1data) ||
+	     ASN1_STRING_length(sanasn1data) < 4) {
 		log_print("x509_cert_subjectaltname: invalid "
 		    "subjectaltname extension");
 		return 0;
 	}
 	/* SSL does not handle unknown ASN stuff well, do it by hand.  */
-	sandata = sanasn1data->data;
+	sandata = ASN1_STRING_get0_data(sanasn1data);
 	santype = sandata[2] & 0x3f;
 	sanlen = sandata[3];
 	sandata += 4;
@@ -1131,7 +1132,7 @@ x509_cert_subjectaltname(X509 *scert, u_
 	 * extra stuff in subjectAltName, so we will just take the first
 	 * salen bytes, and not worry about what follows.
 	 */
-	if (sanlen + 4 > sanasn1data->length) {
+	if (sanlen + 4 > ASN1_STRING_length(sanasn1data)) {
 		log_print("x509_cert_subjectaltname: subjectaltname invalid "
 		    "length");
 		return 0;
@@ -1148,7 +1149,7 @@ x509_cert_get_subjects(void *scert, int 
 	X509		*cert = scert;
 	X509_NAME	*subject;
 	int		type;
-	u_int8_t	*altname;
+	const u_int8_t	*altname;
 	u_int32_t	altlen;
 	u_int8_t	*buf = 0;
 	unsigned char	*ubuf;
Index: x509.h
===================================================================
RCS file: /cvs/src/sbin/isakmpd/x509.h,v
diff -u -p -r1.22 x509.h
--- x509.h	5 Aug 2007 09:43:09 -0000	1.22
+++ x509.h	24 Nov 2025 14:03:09 -0000
@@ -82,7 +82,7 @@ int		x509_ca_count(void);
 
 char           *x509_DN_string(u_int8_t *, size_t);
 int             x509_cert_insert(int, void *);
-int             x509_cert_subjectaltname(X509 * cert, u_char **, u_int *);
+int             x509_cert_subjectaltname(X509 * cert, const u_char **, u_int *);
 X509           *x509_from_asn(u_char *, u_int);
 int             x509_generate_kn(int, X509 *);
 int             x509_read_from_dir(X509_STORE *, char *, int, int *);