rpki-client: Enforce X509v3 SKIs to be the SHA-1 hash of the Subject Public Key

[Job Snijders]

Upon re-reading RFC 6487 section 4.8.2, SKIs are not at all arbitary
identifiers: they must be the SHA-1 hash of the 'Subject Public Key'.
The below changeset adds a SPK digest calculation and comparison to the
X509v3 extension containing the SKI.

OK?

Index: x509.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/x509.c,v
retrieving revision 1.65
diff -u -p -r1.65 x509.c
--- x509.c      16 Feb 2023 14:34:34 -0000      1.65
+++ x509.c      6 Mar 2023 20:06:24 -0000
@@ -186,15 +186,17 @@ out:
 
 /*
  * Parse X509v3 subject key identifier (SKI), RFC 6487 sec. 4.8.2.
- * Returns the SKI or NULL if it could not be parsed.
- * The SKI is formatted as a hex string.
+ * The SKI must be the SHA1 hash of the Subject Public Key.
+ * Returns the SKI formatted as hex string, or NULL if it couldn't be parsed.
  */
 int
 x509_get_ski(X509 *x, const char *fn, char **ski)
 {
-       const unsigned char     *d;
+       const unsigned char     *d, *spk;
        ASN1_OCTET_STRING       *os;
-       int                      dsz, crit, rc = 0;
+       X509_PUBKEY             *pkey;
+       unsigned char            spkd[SHA_DIGEST_LENGTH];
+       int                      spkz, dsz, crit, rc = 0;
 
        *ski = NULL;
        os = X509_get_ext_d2i(x, NID_subject_key_identifier, &crit, NULL);
@@ -216,9 +218,28 @@ x509_get_ski(X509 *x, const char *fn, ch
                goto out;
        }
 
+       if ((pkey = X509_get_X509_PUBKEY(x)) == NULL) {
+               warnx("%s: X509_get_X509_PUBKEY", fn);
+               goto out;
+       }
+       if (X509_PUBKEY_get0_param(NULL, &spk, &spkz, NULL, pkey) != 1) {
+               warnx("%s: X509_PUBKEY_get0_param", fn);
+               goto out;
+       }
+
+       if (!EVP_Digest(spk, spkz, spkd, NULL, EVP_sha1(), NULL)) {
+               warnx("%s: EVP_Digest failed", fn);
+               goto out;
+       }
+
+       if (memcmp(spkd, d, dsz) != 0) {
+               warnx("%s: SKI does not match SHA1 hash of SPK", fn);
+               goto out;
+       }
+
        *ski = hex_encode(d, dsz);
        rc = 1;
-out:
+ out:
        ASN1_OCTET_STRING_free(os);
        return rc;
 }