brontide: modify the max payload length to be 65535 bytes total

This commit modifies the current implementation of the p2p crypto
protocol to further constrain the max allowed payload size. With this
change we now use 16-bits (2-bytes) for the maximum payload length.
This change puts us closer to strict adherence of the Noise spec, and
simplifies the memory management w.r.t implementing the current version
of our scheme.

Note that this doesn’t restrict the size of messages that are able to
be sent over the wire within the LN p2p protocol. Larger message can
safely be encapsulated within the crypt messages via fragmentation that
will detected take place if a larger message is detected.

brontide/noise.go

@@ -4,7 +4,9 @@ import (
 	"crypto/cipher"
 	"crypto/sha256"
 	"encoding/binary"
+	"errors"
 	"io"
+	"math"
 
 	"golang.org/x/crypto/hkdf"
 
@@ -19,6 +21,18 @@ const (
 	// exact same string for this value, along with prologue of the Bitcoin
 	// network, then the initial handshake will fail.
 	protocolName = "Noise_XK_secp256k1_ChaChaPoly_SHA256"
+
+	// macSize is the length in bytes of the tags generated by poly1305.
+	macSize = 16
+
+	// lengthHeaderSize is the number of bytes used to prefix encode the
+	// length of a message payload.
+	lengthHeaderSize = 2
+)
+
+var (
+	ErrMaxMessageLengthExceeded = errors.New("the generated payload exceeds " +
+		"the max allowed message length of (2^16)-1")
 )
 
 // TODO(roasbeef): free buffer pool?
@@ -533,12 +547,18 @@ func (b *BrontideMachine) split() {
 // must be used as the AD to the AEAD construction when being decrypted by the
 // other side.
 func (b *BrontideMachine) WriteMessage(w io.Writer, p []byte) error {
-	// The full length of the packet includes the 16 byte MAC.
-	fullLength := uint64(len(p) + 16)
+	// The total length of each message payload including the MAC size
+	// payload exceed the largest number encodable within a 16-bit unsigned
+	// integer.
+	if len(p)+macSize > math.MaxUint16 {
+		return ErrMaxMessageLengthExceeded
+	}
 
-	// TODO(roasbeef): The Summit decided on 24 bits?
-	var pktLen [8]byte
-	binary.BigEndian.PutUint64(pktLen[:], fullLength)
+	// The full length of the packet includes the 16 byte MAC.
+	fullLength := uint16(len(p) + macSize)
+
+	var pktLen [2]byte
+	binary.BigEndian.PutUint16(pktLen[:], fullLength)
 
 	// First, write out the encrypted+MAC'd length prefix for the packet.
 	cipherLen := b.sendCipher.Encrypt(nil, nil, pktLen[:])
@@ -560,7 +580,7 @@ func (b *BrontideMachine) WriteMessage(w io.Writer, p []byte) error {
 // ReadMessage attemps to read the next message from the passed io.Reader. In
 // the case of an authentication error, a non-nil error is returned.
 func (b *BrontideMachine) ReadMessage(r io.Reader) ([]byte, error) {
-	var cipherLen [8 + 16]byte
+	var cipherLen [lengthHeaderSize + macSize]byte
 	if _, err := io.ReadFull(r, cipherLen[:]); err != nil {
 		return nil, err
 	}
@@ -573,7 +593,7 @@ func (b *BrontideMachine) ReadMessage(r io.Reader) ([]byte, error) {
 
 	// Next, using the length read from the packet header, read the
 	// encrypted packet itself.
-	pktLen := binary.BigEndian.Uint64(pktLenBytes)
+	pktLen := binary.BigEndian.Uint16(pktLenBytes)
 	ciperText := make([]byte, pktLen)
 	if _, err := io.ReadFull(r, ciperText[:]); err != nil {
 		return nil, err

brontide/noise_test.go

@@ -2,6 +2,7 @@ package brontide
 
 import (
 	"bytes"
+	"math"
 	"net"
 	"testing"
 
@@ -97,6 +98,44 @@ func TestConnectionCorrectness(t *testing.T) {
 	}
 }
 
+func TestMaxPayloadLength(t *testing.T) {
+	b := BrontideMachine{}
+	b.split()
+
+	// Create a payload that's juust over the maximum alloted payload
+	// length.
+	payloadToReject := make([]byte, math.MaxUint16+1)
+
+	var buf bytes.Buffer
+
+	// A write of the payload generated above to the state machine should
+	// be rejected as it's over the max payload length.
+	err := b.WriteMessage(&buf, payloadToReject)
+	if err != ErrMaxMessageLengthExceeded {
+		t.Fatalf("payload is over the max allowed length, the write " +
+			"should have been rejected")
+	}
+
+	// Generate another payload which with the MAC acounted for, should be
+	// accepted as a valid payload.
+	payloadToAccept := make([]byte, math.MaxUint16-macSize)
+	if err := b.WriteMessage(&buf, payloadToAccept); err != nil {
+		t.Fatalf("write for payload was rejected, should have been " +
+			"accepted")
+	}
+
+	// Generate a final payload which is juuust over the max payload length
+	// when the MAC is accounted for.
+	payloadToReject = make([]byte, math.MaxUint16-macSize+1)
+
+	// This payload should be rejected.
+	err = b.WriteMessage(&buf, payloadToReject)
+	if err != ErrMaxMessageLengthExceeded {
+		t.Fatalf("payload is over the max allowed length, the write " +
+			"should have been rejected")
+	}
+}
+
 func TestNoiseIdentityHiding(t *testing.T) {
 	// TODO(roasbeef): fin
 }