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justice_kit.go
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justice_kit.go
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package blob
import (
"bytes"
"crypto/rand"
"encoding/binary"
"errors"
"fmt"
"io"
"golang.org/x/crypto/chacha20poly1305"
"github.com/btcsuite/btcd/btcec"
"github.com/btcsuite/btcd/txscript"
"github.com/lightningnetwork/lnd/lnwallet"
"github.com/lightningnetwork/lnd/lnwire"
)
const (
// MinVersion is the minimum blob version supported by this package.
MinVersion = 0
// MaxVersion is the maximumm blob version supported by this package.
MaxVersion = 0
// NonceSize is the length of a chacha20poly1305 nonce, 24 bytes.
NonceSize = chacha20poly1305.NonceSizeX
// KeySize is the length of a chacha20poly1305 key, 32 bytes.
KeySize = chacha20poly1305.KeySize
// CiphertextExpansion is the number of bytes padded to a plaintext
// encrypted with chacha20poly1305, which comes from a 16-byte MAC.
CiphertextExpansion = 16
// V0PlaintextSize is the plaintext size of a version 0 encoded blob.
// 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 = 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.
// nonce: 24 bytes
// enciphered plaintext: n bytes
// MAC: 16 bytes
func Size(ver uint16) int {
return NonceSize + PlaintextSize(ver) + CiphertextExpansion
}
// PlaintextSize returns the size of the encoded-but-unencrypted blob in bytes.
func PlaintextSize(ver uint16) int {
switch ver {
case 0:
return V0PlaintextSize
default:
return 0
}
}
var (
// byteOrder specifies a big-endian encoding of all integer values.
byteOrder = binary.BigEndian
// ErrUnknownBlobVersion signals that we don't understand the requested
// blob encoding scheme.
ErrUnknownBlobVersion = errors.New("unknown blob version")
// ErrCiphertextTooSmall is a decryption error signaling that the
// ciphertext is smaller than the ciphertext expansion factor.
ErrCiphertextTooSmall = errors.New(
"ciphertext is too small for chacha20poly1305",
)
// ErrKeySize signals that the provided key is improperly sized.
ErrKeySize = fmt.Errorf(
"chacha20poly1305 key size must be %d bytes", KeySize,
)
// ErrNoCommitToRemoteOutput is returned when trying to retrieve the
// commit to-remote output from the blob, though none exists.
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.
type PubKey [33]byte
// JusticeKit is lé Blob of Justice. The JusticeKit contains information
// required to construct a justice transaction, that sweeps a remote party's
// revoked commitment transaction. It supports encryption and decryption using
// chacha20poly1305, allowing the client to encrypt the contents of the blob,
// and for a watchtower to later decrypt if action must be taken. The encoding
// format is versioned to allow future extensions.
type JusticeKit struct {
// SweepAddress is the witness program of the output where the client's
// fund will be deposited. This value is included in the blobs, as
// opposed to the session info, such that the sweep addresses can't be
// correlated across sessions and/or towers.
//
// NOTE: This is chosen to be the length of a maximally sized witness
// program.
SweepAddress []byte
// RevocationPubKey is the compressed pubkey that guards the revocation
// clause of the remote party's to-local output.
RevocationPubKey PubKey
// LocalDelayPubKey is the compressed pubkey in the to-local script of
// the remote party, which guards the path where the remote party
// claims their commitment output.
LocalDelayPubKey PubKey
// CSVDelay is the relative timelock in the remote party's to-local
// output, which the remote party must wait out before sweeping their
// commitment output.
CSVDelay uint32
// CommitToLocalSig is a signature under RevocationPubKey using
// SIGHASH_ALL.
CommitToLocalSig lnwire.Sig
// CommitToRemotePubKey is the public key in the to-remote output of the revoked
// commitment transaction.
//
// NOTE: This value is only used if it contains a valid compressed
// public key.
CommitToRemotePubKey PubKey
// CommitToRemoteSig is a signature under CommitToRemotePubKey using SIGHASH_ALL.
//
// NOTE: This value is only used if CommitToRemotePubKey contains a valid
// compressed public key.
CommitToRemoteSig lnwire.Sig
}
// CommitToLocalWitnessScript returns the serialized witness script for the
// commitment to-local output.
func (b *JusticeKit) CommitToLocalWitnessScript() ([]byte, error) {
revocationPubKey, err := btcec.ParsePubKey(
b.RevocationPubKey[:], btcec.S256(),
)
if err != nil {
return nil, err
}
localDelayedPubKey, err := btcec.ParsePubKey(
b.LocalDelayPubKey[:], btcec.S256(),
)
if err != nil {
return nil, err
}
return lnwallet.CommitScriptToSelf(
b.CSVDelay, localDelayedPubKey, revocationPubKey,
)
}
// CommitToLocalRevokeWitnessStack constructs a witness stack spending the
// revocation clause of the commitment to-local output.
// <revocation-sig> 1
func (b *JusticeKit) CommitToLocalRevokeWitnessStack() ([][]byte, error) {
toLocalSig, err := b.CommitToLocalSig.ToSignature()
if err != nil {
return nil, err
}
witnessStack := make([][]byte, 2)
witnessStack[0] = append(toLocalSig.Serialize(),
byte(txscript.SigHashAll))
witnessStack[1] = []byte{1}
return witnessStack, nil
}
// HasCommitToRemoteOutput returns true if the blob contains a to-remote p2wkh
// pubkey.
func (b *JusticeKit) HasCommitToRemoteOutput() bool {
return btcec.IsCompressedPubKey(b.CommitToRemotePubKey[:])
}
// CommitToRemoteWitnessScript returns the witness script for the commitment
// to-remote p2wkh output, which is the pubkey itself.
func (b *JusticeKit) CommitToRemoteWitnessScript() ([]byte, error) {
if !btcec.IsCompressedPubKey(b.CommitToRemotePubKey[:]) {
return nil, ErrNoCommitToRemoteOutput
}
return b.CommitToRemotePubKey[:], nil
}
// CommitToRemoteWitnessStack returns a witness stack spending the commitment
// to-remote output, which is a regular p2wkh.
// <to-remote-sig>
func (b *JusticeKit) CommitToRemoteWitnessStack() ([][]byte, error) {
toRemoteSig, err := b.CommitToRemoteSig.ToSignature()
if err != nil {
return nil, err
}
witnessStack := make([][]byte, 1)
witnessStack[0] = append(toRemoteSig.Serialize(),
byte(txscript.SigHashAll))
return witnessStack, nil
}
// Encrypt encodes the blob of justice using encoding version, and then
// creates a ciphertext using chacha20poly1305 under the chosen (nonce, key)
// pair.
//
// NOTE: It is the caller's responsibility to ensure that this method is only
// called once for a given (nonce, key) pair.
func (b *JusticeKit) Encrypt(key []byte, version uint16) ([]byte, error) {
// Fail if the nonce is not 32-bytes.
if len(key) != KeySize {
return nil, ErrKeySize
}
// Encode the plaintext using the provided version, to obtain the
// plaintext bytes.
var ptxtBuf bytes.Buffer
err := b.encode(&ptxtBuf, version)
if err != nil {
return nil, err
}
// Create a new chacha20poly1305 cipher, using a 32-byte key.
cipher, err := chacha20poly1305.NewX(key)
if err != nil {
return nil, err
}
// Allocate the ciphertext, which will contain the nonce, encrypted
// plaintext and MAC.
plaintext := ptxtBuf.Bytes()
ciphertext := make([]byte, Size(version))
// Generate a random 24-byte nonce in the ciphertext's prefix.
nonce := ciphertext[:NonceSize]
if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
return nil, err
}
// Finally, encrypt the plaintext using the given nonce, storing the
// result in the ciphertext buffer.
cipher.Seal(ciphertext[NonceSize:NonceSize], nonce, plaintext, nil)
return ciphertext, nil
}
// Decrypt unenciphers a blob of justice by decrypting the ciphertext using
// chacha20poly1305 with the chosen (nonce, key) pair. The internal plaintext is
// then deserialized using the given encoding version.
func Decrypt(key, ciphertext []byte, version uint16) (*JusticeKit, error) {
switch {
// Fail if the blob's overall length is less than required for the nonce
// and expansion factor.
case len(ciphertext) < NonceSize+CiphertextExpansion:
return nil, ErrCiphertextTooSmall
// Fail if the key is not 32-bytes.
case len(key) != KeySize:
return nil, ErrKeySize
}
// Create a new chacha20poly1305 cipher, using a 32-byte key.
cipher, err := chacha20poly1305.NewX(key)
if err != nil {
return nil, err
}
// Allocate the final buffer that will contain the blob's plaintext
// bytes, which is computed by subtracting the ciphertext expansion
// factor from the blob's length.
plaintext := make([]byte, len(ciphertext)-CiphertextExpansion)
// Decrypt the ciphertext, placing the resulting plaintext in our
// plaintext buffer.
nonce := ciphertext[:NonceSize]
_, err = cipher.Open(plaintext[:0], nonce, ciphertext[NonceSize:], nil)
if err != nil {
return nil, err
}
// If decryption succeeded, we will then decode the plaintext bytes
// using the specified blob version.
boj := &JusticeKit{}
err = boj.decode(bytes.NewReader(plaintext), version)
if err != nil {
return nil, err
}
return boj, nil
}
// encode serializes the JusticeKit according to the version, returning an
// error if the version is unknown.
func (b *JusticeKit) encode(w io.Writer, ver uint16) error {
switch ver {
case 0:
return b.encodeV0(w)
default:
return ErrUnknownBlobVersion
}
}
// decode deserializes the JusticeKit according to the version, returning an
// error if the version is unknown.
func (b *JusticeKit) decode(r io.Reader, ver uint16) error {
switch ver {
case 0:
return b.decodeV0(r)
default:
return ErrUnknownBlobVersion
}
}
// 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 274 bytes.
//
// blob version 0 plaintext encoding:
// 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
func (b *JusticeKit) encodeV0(w io.Writer) error {
// 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
}
// Write 33-byte revocation public key.
_, err = w.Write(b.RevocationPubKey[:])
if err != nil {
return err
}
// Write 33-byte local delay public key.
_, err = w.Write(b.LocalDelayPubKey[:])
if err != nil {
return err
}
// Write 4-byte CSV delay.
err = binary.Write(w, byteOrder, b.CSVDelay)
if err != nil {
return err
}
// Write 64-byte revocation signature for commit to-local output.
_, err = w.Write(b.CommitToLocalSig[:])
if err != nil {
return err
}
// Write 33-byte commit to-remote public key, which may be blank.
_, err = w.Write(b.CommitToRemotePubKey[:])
if err != nil {
return err
}
// Write 64-byte commit to-remote signature, which may be blank.
_, err = w.Write(b.CommitToRemoteSig[:])
return err
}
// decodeV0 reconstructs a JusticeKit from the io.Reader, using version 0
// 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 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
func (b *JusticeKit) decodeV0(r io.Reader) error {
// 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 {
return err
}
// Read 33-byte local delay public key.
_, err = io.ReadFull(r, b.LocalDelayPubKey[:])
if err != nil {
return err
}
// Read 4-byte CSV delay.
err = binary.Read(r, byteOrder, &b.CSVDelay)
if err != nil {
return err
}
// Read 64-byte revocation signature for commit to-local output.
_, err = io.ReadFull(r, b.CommitToLocalSig[:])
if err != nil {
return err
}
var (
commitToRemotePubkey PubKey
commitToRemoteSig lnwire.Sig
)
// Read 33-byte commit to-remote public key, which may be discarded.
_, err = io.ReadFull(r, commitToRemotePubkey[:])
if err != nil {
return err
}
// Read 64-byte commit to-remote signature, which may be discarded.
_, err = io.ReadFull(r, commitToRemoteSig[:])
if err != nil {
return err
}
// Only populate the commit to-remote fields in the decoded blob if a
// valid compressed public key was read from the reader.
if btcec.IsCompressedPubKey(commitToRemotePubkey[:]) {
b.CommitToRemotePubKey = commitToRemotePubkey
b.CommitToRemoteSig = commitToRemoteSig
}
return nil
}