watchtower/blob/justice_kit: add variable length sweep addr

This commit fixes an oversight in the previous
design of the watchtower blob, by introducing
a length byte for sweep addresses. The previous
format supposed that addresses would be padded
to 42 bytes, but had no indication of the
address's actual length.

To rememdy this, we introduce a single byte
indicating the actual size of the address,
such that the padding can be removed upon
decoding.

watchtower/blob/justice_kit.go

@@ -33,14 +33,19 @@ const (
 	CiphertextExpansion = 16
 
 	// V0PlaintextSize is the plaintext size of a version 0 encoded blob.
-	//    sweep address:                  42 bytes
+	//    sweep address length:            1 byte
+	//    padded sweep address:           42 bytes
 	//    revocation pubkey:              33 bytes
 	//    local delay pubkey:             33 bytes
 	//    csv delay:                       4 bytes
 	//    commit to-local revocation sig: 64 bytes
 	//    commit to-remote pubkey:        33 bytes, maybe blank
 	//    commit to-remote sig:           64 bytes, maybe blank
-	V0PlaintextSize = 273
+	V0PlaintextSize = 274
+
+	// MaxSweepAddrSize defines the maximum sweep address size that can be
+	// encoded in a blob.
+	MaxSweepAddrSize = 42
 )
 
 // Size returns the size of the encoded-and-encrypted blob in bytes.
@@ -89,6 +94,14 @@ var (
 	ErrNoCommitToRemoteOutput = errors.New(
 		"cannot obtain commit to-remote p2wkh output script from blob",
 	)
+
+	// ErrSweepAddressToLong is returned when trying to encode or decode a
+	// sweep address with length greater than the maximum length of 42
+	// bytes, which supports p2wkh and p2sh addresses.
+	ErrSweepAddressToLong = fmt.Errorf(
+		"sweep address must be less than or equal to %d bytes long",
+		MaxSweepAddrSize,
+	)
 )
 
 // PubKey is a 33-byte, serialized compressed public key.
@@ -108,7 +121,7 @@ type JusticeKit struct {
 	//
 	// NOTE: This is chosen to be the length of a maximally sized witness
 	// program.
-	SweepAddress [42]byte
+	SweepAddress []byte
 
 	// RevocationPubKey is the compressed pubkey that guards the revocation
 	// clause of the remote party's to-local output.
@@ -318,10 +331,11 @@ func (b *JusticeKit) decode(r io.Reader, ver uint16) error {
 // encodeV0 encodes the JusticeKit using the version 0 encoding scheme to the
 // provided io.Writer. The encoding supports sweeping of the commit to-local
 // output, and  optionally the  commit to-remote output. The encoding produces a
-// constant-size plaintext size of 273 bytes.
+// constant-size plaintext size of 274 bytes.
 //
 // blob version 0 plaintext encoding:
-//    sweep address:                  42 bytes
+//    sweep address length:            1 byte
+//    padded sweep address:           42 bytes
 //    revocation pubkey:              33 bytes
 //    local delay pubkey:             33 bytes
 //    csv delay:                       4 bytes
@@ -329,8 +343,23 @@ func (b *JusticeKit) decode(r io.Reader, ver uint16) error {
 //    commit to-remote pubkey:        33 bytes, maybe blank
 //    commit to-remote sig:           64 bytes, maybe blank
 func (b *JusticeKit) encodeV0(w io.Writer) error {
-	// Write 42-byte sweep address where client funds will be deposited.
-	_, err := w.Write(b.SweepAddress[:])
+	// Assert the sweep address length is sane.
+	if len(b.SweepAddress) > MaxSweepAddrSize {
+		return ErrSweepAddressToLong
+	}
+
+	// Write the actual length of the sweep address as a single byte.
+	err := binary.Write(w, byteOrder, uint8(len(b.SweepAddress)))
+	if err != nil {
+		return err
+	}
+
+	// Pad the sweep address to our maximum length of 42 bytes.
+	var sweepAddressBuf [MaxSweepAddrSize]byte
+	copy(sweepAddressBuf[:], b.SweepAddress)
+
+	// Write padded 42-byte sweep address.
+	_, err = w.Write(sweepAddressBuf[:])
 	if err != nil {
 		return err
 	}
@@ -371,12 +400,13 @@ func (b *JusticeKit) encodeV0(w io.Writer) error {
 }
 
 // decodeV0 reconstructs a JusticeKit from the io.Reader, using version 0
-// encoding scheme. This will parse a constant size input stream of 273 bytes to
+// encoding scheme. This will parse a constant size input stream of 274 bytes to
 // recover information for the commit to-local output, and possibly the commit
 // to-remote output.
 //
 // blob version 0 plaintext encoding:
-//    sweep address:                  42 bytes
+//    sweep address length:            1 byte
+//    padded sweep address:           42 bytes
 //    revocation pubkey:              33 bytes
 //    local delay pubkey:             33 bytes
 //    csv delay:                       4 bytes
@@ -384,12 +414,29 @@ func (b *JusticeKit) encodeV0(w io.Writer) error {
 //    commit to-remote pubkey:        33 bytes, maybe blank
 //    commit to-remote sig:           64 bytes, maybe blank
 func (b *JusticeKit) decodeV0(r io.Reader) error {
-	// Read 42-byte sweep address where client funds will be deposited.
-	_, err := io.ReadFull(r, b.SweepAddress[:])
+	// Read the sweep address length as a single byte.
+	var sweepAddrLen uint8
+	err := binary.Read(r, byteOrder, &sweepAddrLen)
 	if err != nil {
 		return err
 	}
 
+	// Assert the sweep address length is sane.
+	if sweepAddrLen > MaxSweepAddrSize {
+		return ErrSweepAddressToLong
+	}
+
+	// Read padded 42-byte sweep address.
+	var sweepAddressBuf [MaxSweepAddrSize]byte
+	_, err = io.ReadFull(r, sweepAddressBuf[:])
+	if err != nil {
+		return err
+	}
+
+	// Parse sweep address from padded buffer.
+	b.SweepAddress = make([]byte, sweepAddrLen)
+	copy(b.SweepAddress, sweepAddressBuf[:])
+
 	// Read 33-byte revocation public key.
 	_, err = io.ReadFull(r, b.RevocationPubKey[:])
 	if err != nil {