TSIG: fix possible weakness in signature checking

Use binary comparison instead of string comparison for the received MAC.

The transaction signature was checked only partially, if the MAC
contained a zero byte. If this was the very first byte of the signature,
the checking didn't happen at all. In addition, possible ASCII
characters in the MAC were compared case insensitively.

src/libknot/tsig-op.c

@@ -748,8 +748,7 @@ static int knot_tsig_check_digest(const knot_rrset_t *tsig_rr,
 	dbg_tsig_verb("TSIG: given digest:\n");
 	dbg_tsig_hex_verb((char *)tsig_mac, mac_length);
 
-	if (strncasecmp((char *)(tsig_mac), (char *)digest_tmp,
-	                mac_length) != 0) {
+	if (memcmp(tsig_mac, digest_tmp, mac_length) != 0) {
 		return KNOT_TSIG_EBADSIG;
 	}