X509_CRL_get_signature_nid(crl) does OBJ_obj2nid(crl->sig_alg->algorithm)
and similarly for X509_get_signature_nid(). That's a bit simpler than
what we currently have. The proper error check for OBJ_obj2nid() is to
see if nid is NID_undef, so do that, although it wouldn't really be
necessary here. I can leave that out to preserve behavior if that's
preferred.

PS: The sig_alg members of X509_CRL_seq_tt and X509_seq_tt aren't optional
(there's no ASN1_TFLG_OPTIONAL) and neither is the algorithm in
X509_ALGOR_seq_tt, so none of these is NULL after a successful call to
d2i_X509_CRL and d2i_X509 and the get_signature_nid API is safe to call.

Index: cert.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/cert.c,v
diff -u -p -r1.145 cert.c
--- cert.c	10 Jun 2024 10:50:13 -0000	1.145
+++ cert.c	10 Jun 2024 21:07:22 -0000
@@ -797,9 +797,7 @@ cert_parse_pre(const char *fn, const uns
 	int			 i, extsz;
 	X509			*x = NULL;
 	X509_EXTENSION		*ext = NULL;
-	const X509_ALGOR	*palg;
 	const ASN1_BIT_STRING	*piuid = NULL, *psuid = NULL;
-	const ASN1_OBJECT	*cobj;
 	ASN1_OBJECT		*obj;
 	EVP_PKEY		*pkey;
 	int			 nid, ip, as, sia, cp, crldp, aia, aki, ski,
@@ -832,13 +830,10 @@ cert_parse_pre(const char *fn, const uns
 		goto out;
 	}
 
-	X509_get0_signature(NULL, &palg, x);
-	if (palg == NULL) {
-		warnx("%s: X509_get0_signature", fn);
+	if ((nid = X509_get_signature_nid(x)) == NID_undef) {
+		warnx("%s: unknown signature type", fn);
 		goto out;
 	}
-	X509_ALGOR_get0(&cobj, NULL, NULL, palg);
-	nid = OBJ_obj2nid(cobj);
 	if (experimental && nid == NID_ecdsa_with_SHA256) {
 		if (verbose)
 			warnx("%s: P-256 support is experimental", fn);
Index: crl.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/crl.c,v
diff -u -p -r1.37 crl.c
--- crl.c	5 Jun 2024 13:36:28 -0000	1.37
+++ crl.c	10 Jun 2024 21:09:55 -0000
@@ -165,9 +165,7 @@ crl_parse(const char *fn, const unsigned
 {
 	const unsigned char	*oder;
 	struct crl		*crl;
-	const X509_ALGOR	*palg;
 	const X509_NAME		*name;
-	const ASN1_OBJECT	*cobj;
 	const ASN1_TIME		*at;
 	int			 count, nid, rc = 0;
 
@@ -200,13 +198,10 @@ crl_parse(const char *fn, const unsigned
 	if (!x509_valid_name(fn, "issuer", name))
 		goto out;
 
-	X509_CRL_get0_signature(crl->x509_crl, NULL, &palg);
-	if (palg == NULL) {
-		warnx("%s: X509_CRL_get0_signature", fn);
+	if ((nid = X509_CRL_get_signature_nid(crl->x509_crl)) == NID_undef) {
+		warnx("%s: unknown signature type", fn);
 		goto out;
 	}
-	X509_ALGOR_get0(&cobj, NULL, NULL, palg);
-	nid = OBJ_obj2nid(cobj);
 	if (experimental && nid == NID_ecdsa_with_SHA256) {
 		if (verbose)
 			warnx("%s: P-256 support is experimental", fn);