[dnssec] review, macros instead of magic constants in keytag computation

src/dnssec/lib/key/keytag.c

@@ -20,6 +20,7 @@
 
 #include "binary.h"
 #include "error.h"
+#include "key/dnskey.h"
 #include "shared.h"
 
 /*!
@@ -72,11 +73,11 @@ int dnssec_keytag(const dnssec_binary_t *rdata, uint16_t *keytag)
 		return DNSSEC_EINVAL;
 	}
 
-	if (!rdata->data || rdata->size < 4) {
+	if (!rdata->data || rdata->size < DNSKEY_RDATA_OFFSET_PUBKEY) {
 		return DNSSEC_MALFORMED_DATA;
 	}
 
-	uint8_t algorithm = rdata->data[3];
+	uint8_t algorithm = rdata->data[DNSKEY_RDATA_OFFSET_ALGORITHM];
 	if (algorithm == 1) {
 		*keytag = keytag_compat(rdata);
 	} else {