sphinx: expose the extra unused bytes within the HopData struct

In this commit, we expose the extra 12 unused bytes within the HopData
struct. This allows callers to read/write to these extra bytes. With
this change, higher level applications will now be able to take
advantage of these extra bytes to implement things like AMP, spontaneous
payments, and so on.

Author: Roasbeef

sphinx.go

@@ -7,7 +7,6 @@ import (
 	"crypto/sha256"
 	"encoding/binary"
 	"io"
-	"io/ioutil"
 	"math/big"
 
 	"github.com/aead/chacha20"
@@ -38,7 +37,7 @@ const (
 
 	// hopDataSize is the fixed size of hop_data. BOLT 04 currently
 	// specifies this to be 1 byte realm, 8 byte channel_id, 8 byte amount
-	// to forward, 4 byte outgoing CLTV value, 16 bytes padding and 32
+	// to forward, 4 byte outgoing CLTV value, 12 bytes padding and 32
 	// bytes HMAC for a total of 65 bytes per hop.
 	hopDataSize = (1 + addressSize + 8 + 4 + padSize + hmacSize)
 
@@ -71,15 +70,9 @@ const (
 // the output of a SHA256 hash.
 type Hash256 [sha256.Size]byte
 
-var (
-	// paddingBytes are the padding bytes used to fill out the remainder of the
-	// unused portion of the per-hop payload.
-	paddingBytes [padSize]byte
-
-	// zeroHMAC is the special HMAC value that allows the final node to
-	// determine if it is the payment destination or not.
-	zeroHMAC [hmacSize]byte
-)
+// zeroHMAC is the special HMAC value that allows the final node to determine
+// if it is the payment destination or not.
+var zeroHMAC [hmacSize]byte
 
 // OnionPacket is the onion wrapped hop-to-hop routing information necessary to
 // propagate a message through the mix-net without intermediate nodes having
@@ -140,6 +133,13 @@ type HopData struct {
 	// should be included in the HTLC forwarded.
 	OutgoingCltv uint32
 
+	// ExtraBytes is the set of unused bytes within the onion payload. This
+	// extra set of bytes can be utilized by higher level applications to
+	// package additional data within the per-hop payload, or signal that a
+	// portion of the remaining set of hops are to be consumed as Extra
+	// Onion Blobs.
+	ExtraBytes [padSize]byte
+
 	// HMAC is an HMAC computed over the entire per-hop payload that also
 	// includes the higher-level (optional) associated data bytes.
 	HMAC [hmacSize]byte
@@ -164,7 +164,7 @@ func (hd *HopData) Encode(w io.Writer) error {
 		return err
 	}
 
-	if _, err := w.Write(paddingBytes[:]); err != nil {
+	if _, err := w.Write(hd.ExtraBytes[:]); err != nil {
 		return err
 	}
 
@@ -194,7 +194,7 @@ func (hd *HopData) Decode(r io.Reader) error {
 		return err
 	}
 
-	if _, err := io.CopyN(ioutil.Discard, r, padSize); err != nil {
+	if _, err := io.ReadFull(r, hd.ExtraBytes[:]); err != nil {
 		return err
 	}