sphinx: Fixed a buffer overflow in hmac generation

Our HMACs are truncated to 20 byte, but sodium still generates 32 byte
HMACs and we were handing in a buffer that was too small, so we
overflowing the buffer by 12 bytes. This manifested itself only in the
32 bit variant because of different alignment in the 64bit version.

Fixes #94.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

daemon/sphinx.c

@@ -184,11 +184,13 @@ static bool compute_hmac(
 static void compute_packet_hmac(struct onionpacket *packet, u8 *mukey, u8 *hmac)
 {
 	u8 mactemp[ROUTING_INFO_SIZE + TOTAL_HOP_PAYLOAD_SIZE + MESSAGE_SIZE];
+	u8 mac[32];
 
 	memcpy(mactemp, packet->routinginfo, ROUTING_INFO_SIZE);
 	memcpy(mactemp + ROUTING_INFO_SIZE, packet->hoppayloads, TOTAL_HOP_PAYLOAD_SIZE);
 	memcpy(mactemp + ROUTING_INFO_SIZE + TOTAL_HOP_PAYLOAD_SIZE, packet->payload, sizeof(packet->payload));
-	compute_hmac(hmac, mactemp, sizeof(mactemp), mukey, KEY_LEN);
+	compute_hmac(mac, mactemp, sizeof(mactemp), mukey, KEY_LEN);
+	memcpy(hmac, mac, 20);
 }
 
 static bool generate_key(void *k, const char *t, u8 tlen, const u8 *s)