Apart from the extensions, EE certs are still insufficiently parsed.
The Certificate and the TBSCertificate are independent of the cert
purpose, so we need one function to deal with those.

cert_parse_internal() does part of this: walk the TBSCertificate's
SEQUENCE per RFC 5280 and validate one entry after the other and check
the restrictions imposed by RFC 6487. Well, almost. Since we
deal with subject and issuer in one go, the order isn't 100% the same.

cert_parse_internal() brings part of a simplification promised earlier.
It allocates struct cert itself and populates it, and takes ownership
of the cert passed in.

The new cert_check_sigalg() and cert_check_validity_period() are
straightforward enough. Two missing bits are indicated in an XXX:
the SPKI and checking for completeness of extensions.

diff --git a/usr.sbin/rpki-client/cert.c b/usr.sbin/rpki-client/cert.c
index 10101b21aa6..9029b9290fe 100644
--- a/usr.sbin/rpki-client/cert.c
+++ b/usr.sbin/rpki-client/cert.c
@@ -1106,18 +1106,41 @@ certificate_policies(const char *fn, struct cert *cert, X509_EXTENSION *ext)
 }
 
 static int
-cert_check_subject_and_issuer(const char *fn, const X509 *x)
+cert_check_sigalg(const char *fn, const struct cert *cert)
+{
+	int nid;
+
+	if ((nid = X509_get_signature_nid(cert->x509)) == NID_undef) {
+		warnx("%s: unknown signature type", fn);
+		return 0;
+	}
+
+	if (nid == NID_sha256WithRSAEncryption)
+		return 1;
+	if (experimental && nid == NID_ecdsa_with_SHA256) {
+		if (verbose)
+			warnx("%s: P-256 support is experimental", fn);
+		return 1;
+	}
+
+	warnx("%s: RFC 7935: wrong signature algorithm %s, want %s",
+	    fn, nid2str(nid), LN_sha256WithRSAEncryption);
+	return 0;
+}
+
+static int
+cert_check_subject_and_issuer(const char *fn, const struct cert *cert)
 {
 	const X509_NAME *name;
 
-	if ((name = X509_get_subject_name(x)) == NULL) {
+	if ((name = X509_get_subject_name(cert->x509)) == NULL) {
 		warnx("%s: X509_get_subject_name", fn);
 		return 0;
 	}
 	if (!x509_valid_name(fn, "subject", name))
 		return 0;
 
-	if ((name = X509_get_issuer_name(x)) == NULL) {
+	if ((name = X509_get_issuer_name(cert->x509)) == NULL) {
 		warnx("%s: X509_get_issuer_name", fn);
 		return 0;
 	}
@@ -1127,6 +1150,28 @@ cert_check_subject_and_issuer(const char *fn, const X509 *x)
 	return 1;
 }
 
+static int
+cert_check_validity_period(const char *fn, struct cert *cert)
+{
+	time_t		 now = get_current_time();
+
+	if (!x509_get_notbefore(cert->x509, fn, &cert->notbefore))
+		return 0;
+	if (!x509_get_notafter(cert->x509, fn, &cert->notafter))
+		return 0;
+
+	if (cert->notbefore > now) {
+		warnx("%s: certificate not yet valid", fn);
+		return 0;
+	}
+	if (cert->notafter < now) {
+		warnx("%s: certificate has expired", fn);
+		return 0;
+	}
+
+	return 1;
+}
+
 /*
  * Check the cert's purpose: the cA bit in basic constraints distinguishes
  * between TA/CA and EE/BGPsec router and the key usage bits must match.
@@ -1400,6 +1445,8 @@ cert_parse_extensions(const char *fn, struct cert *cert)
 		}
 	}
 
+	/* XXX - validate required fields. */
+
 	return 1;
 
  dup:
@@ -1409,38 +1456,82 @@ cert_parse_extensions(const char *fn, struct cert *cert)
 	return 0;
 }
 
-/*
- * Lightweight version of cert_parse_pre() for EE certs.
- * Parses the two RFC 3779 extensions, and performs some sanity checks.
- * Returns cert on success and NULL on failure.
- */
-struct cert *
-cert_parse_ee_cert(const char *fn, int talid, X509 *x)
+static struct cert *
+cert_parse_internal(const char *fn, X509 *x)
 {
 	struct cert		*cert;
+	const ASN1_INTEGER	*serial;
+	const ASN1_BIT_STRING	*issuer_uid = NULL, *subject_uid = NULL;
 
-	if ((cert = calloc(1, sizeof(struct cert))) == NULL)
+	if ((cert = calloc(1, sizeof(*cert))) == NULL)
 		err(1, NULL);
+	cert->x509 = x;
 
-	if (!X509_up_ref(x)) {
-		warnx("%s: X509_up_ref failed", fn);
+	if (!x509_cache_extensions(x, fn))
 		goto out;
-	}
-
-	cert->x509 = x;
-	cert->talid = talid;
 
 	if (X509_get_version(x) != 2) {
 		warnx("%s: RFC 6487 4.1: X.509 version must be v3", fn);
 		goto out;
 	}
 
-	if (!cert_check_subject_and_issuer(fn, x))
+	if ((serial = X509_get0_serialNumber(x)) == NULL) {
+		warnx("%s: RFC 6487 4.2: missing serialNumber", fn);
+		goto out;
+	}
+	if (!x509_valid_seqnum(fn, "RFC 6487 4.2: serialNumber", serial))
+		goto out;
+
+	if (!cert_check_sigalg(fn, cert))
+		goto out;
+
+	if (!cert_check_subject_and_issuer(fn, cert))
+		goto out;
+
+	if (!cert_check_validity_period(fn, cert))
+		goto out;
+
+	/* XXX - add SPKI here. */
+
+	/*
+	 * Disallowed for CA certs in RFC 5280, 4.1.2.8. Uniqueness of subjects
+	 * per RFC 6487, 4.5 makes them meaningless.
+	 */
+	X509_get0_uids(x, &issuer_uid, &subject_uid);
+	if (issuer_uid != NULL || subject_uid != NULL) {
+		warnx("%s: issuer or subject unique identifiers not allowed",
+		    fn);
 		goto out;
+	}
 
 	if (!cert_parse_extensions(fn, cert))
 		goto out;
 
+	return cert;
+
+ out:
+	cert_free(cert);
+	return NULL;
+}
+
+/*
+ * Parse an EE cert extracted from a CMS signed object. Store all cert and
+ * extension data we need later in the returned struct cert.
+ * Check the certificate's purpose and validate the TA constraints.
+ * Returns cert on success and NULL on failure.
+ */
+struct cert *
+cert_parse_ee_cert(const char *fn, int talid, X509 *x)
+{
+	struct cert		*cert = NULL;
+
+	if (!X509_up_ref(x))
+		goto out;
+
+	if ((cert = cert_parse_internal(fn, x)) == NULL)
+		goto out;
+	cert->talid = talid;
+
 	if (cert->purpose != CERT_PURPOSE_EE) {
 		warnx("%s: expected EE cert, got %s", fn,
 		    purpose2str(cert->purpose));
@@ -1465,21 +1556,16 @@ cert_parse_ee_cert(const char *fn, int talid, X509 *x)
 struct cert *
 cert_parse_pre(const char *fn, const unsigned char *der, size_t len)
 {
-	struct cert		*cert;
+	struct cert		*cert = NULL;
 	const unsigned char	*oder;
 	size_t			 j;
 	X509			*x = NULL;
-	const ASN1_BIT_STRING	*issuer_uid = NULL, *subject_uid = NULL;
 	EVP_PKEY		*pkey;
-	int			 nid;
 
 	/* just fail for empty buffers, the warning was printed elsewhere */
 	if (der == NULL)
 		return NULL;
 
-	if ((cert = calloc(1, sizeof(struct cert))) == NULL)
-		err(1, NULL);
-
 	oder = der;
 	if ((x = d2i_X509(NULL, &der, len)) == NULL) {
 		warnx("%s: d2i_X509", fn);
@@ -1494,49 +1580,7 @@ cert_parse_pre(const char *fn, const unsigned char *der, size_t len)
 		warnx("%s: X509_up_ref failed", fn);
 		goto out;
 	}
-	cert->x509 = x;
-
-	if (!x509_cache_extensions(x, fn))
-		goto out;
-
-	if (X509_get_version(x) != 2) {
-		warnx("%s: RFC 6487 4.1: X.509 version must be v3", fn);
-		goto out;
-	}
-
-	if ((nid = X509_get_signature_nid(x)) == NID_undef) {
-		warnx("%s: unknown signature type", fn);
-		goto out;
-	}
-	if (experimental && nid == NID_ecdsa_with_SHA256) {
-		if (verbose)
-			warnx("%s: P-256 support is experimental", fn);
-	} else if (nid != NID_sha256WithRSAEncryption) {
-		warnx("%s: RFC 7935: wrong signature algorithm %s, want %s",
-		    fn, nid2str(nid), LN_sha256WithRSAEncryption);
-		goto out;
-	}
-
-	/*
-	 * Disallowed for CA certs in RFC 5280, 4.1.2.8. Uniqueness of subjects
-	 * per RFC 6487, 4.5 makes them meaningless.
-	 */
-	X509_get0_uids(x, &issuer_uid, &subject_uid);
-	if (issuer_uid != NULL || subject_uid != NULL) {
-		warnx("%s: issuer or subject unique identifiers not allowed",
-		    fn);
-		goto out;
-	}
-
-	if (!cert_check_subject_and_issuer(fn, x))
-		goto out;
-
-	if (!cert_parse_extensions(fn, cert))
-		goto out;
-
-	if (!x509_get_notbefore(x, fn, &cert->notbefore))
-		goto out;
-	if (!x509_get_notafter(x, fn, &cert->notafter))
+	if ((cert = cert_parse_internal(fn, x)) == NULL)
 		goto out;
 
 	/* Validation on required fields. */