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// Copyright (c) 2013-2016 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package blockchain
import (
const (
// CoinbaseWitnessDataLen is the required length of the only element within
// the coinbase's witness data if the coinbase transaction contains a
// witness commitment.
CoinbaseWitnessDataLen = 32
// CoinbaseWitnessPkScriptLength is the length of the public key script
// containing an OP_RETURN, the WitnessMagicBytes, and the witness
// commitment itself. In order to be a valid candidate for the output
// containing the witness commitment
CoinbaseWitnessPkScriptLength = 38
var (
// WitnessMagicBytes is the prefix marker within the public key script
// of a coinbase output to indicate that this output holds the witness
// commitment for a block.
WitnessMagicBytes = []byte{
// nextPowerOfTwo returns the next highest power of two from a given number if
// it is not already a power of two. This is a helper function used during the
// calculation of a merkle tree.
func nextPowerOfTwo(n int) int {
// Return the number if it's already a power of 2.
if n&(n-1) == 0 {
return n
// Figure out and return the next power of two.
exponent := uint(math.Log2(float64(n))) + 1
return 1 << exponent // 2^exponent
// HashMerkleBranches takes two hashes, treated as the left and right tree
// nodes, and returns the hash of their concatenation. This is a helper
// function used to aid in the generation of a merkle tree.
func HashMerkleBranches(left *chainhash.Hash, right *chainhash.Hash) *chainhash.Hash {
// Concatenate the left and right nodes.
var hash [chainhash.HashSize * 2]byte
copy(hash[:chainhash.HashSize], left[:])
copy(hash[chainhash.HashSize:], right[:])
newHash := chainhash.DoubleHashH(hash[:])
return &newHash
// BuildMerkleTreeStore creates a merkle tree from a slice of transactions,
// stores it using a linear array, and returns a slice of the backing array. A
// linear array was chosen as opposed to an actual tree structure since it uses
// about half as much memory. The following describes a merkle tree and how it
// is stored in a linear array.
// A merkle tree is a tree in which every non-leaf node is the hash of its
// children nodes. A diagram depicting how this works for bitcoin transactions
// where h(x) is a double sha256 follows:
// root = h1234 = h(h12 + h34)
// / \
// h12 = h(h1 + h2) h34 = h(h3 + h4)
// / \ / \
// h1 = h(tx1) h2 = h(tx2) h3 = h(tx3) h4 = h(tx4)
// The above stored as a linear array is as follows:
// [h1 h2 h3 h4 h12 h34 root]
// As the above shows, the merkle root is always the last element in the array.
// The number of inputs is not always a power of two which results in a
// balanced tree structure as above. In that case, parent nodes with no
// children are also zero and parent nodes with only a single left node
// are calculated by concatenating the left node with itself before hashing.
// Since this function uses nodes that are pointers to the hashes, empty nodes
// will be nil.
// The additional bool parameter indicates if we are generating the merkle tree
// using witness transaction id's rather than regular transaction id's. This
// also presents an additional case wherein the wtxid of the coinbase transaction
// is the zeroHash.
func BuildMerkleTreeStore(transactions []*btcutil.Tx, witness bool) []*chainhash.Hash {
// Calculate how many entries are required to hold the binary merkle
// tree as a linear array and create an array of that size.
nextPoT := nextPowerOfTwo(len(transactions))
arraySize := nextPoT*2 - 1
merkles := make([]*chainhash.Hash, arraySize)
// Create the base transaction hashes and populate the array with them.
for i, tx := range transactions {
// If we're computing a witness merkle root, instead of the
// regular txid, we use the modified wtxid which includes a
// transaction's witness data within the digest. Additionally,
// the coinbase's wtxid is all zeroes.
switch {
case witness && i == 0:
var zeroHash chainhash.Hash
merkles[i] = &zeroHash
case witness:
wSha := tx.MsgTx().WitnessHash()
merkles[i] = &wSha
merkles[i] = tx.Hash()
// Start the array offset after the last transaction and adjusted to the
// next power of two.
offset := nextPoT
for i := 0; i < arraySize-1; i += 2 {
switch {
// When there is no left child node, the parent is nil too.
case merkles[i] == nil:
merkles[offset] = nil
// When there is no right child, the parent is generated by
// hashing the concatenation of the left child with itself.
case merkles[i+1] == nil:
newHash := HashMerkleBranches(merkles[i], merkles[i])
merkles[offset] = newHash
// The normal case sets the parent node to the double sha256
// of the concatentation of the left and right children.
newHash := HashMerkleBranches(merkles[i], merkles[i+1])
merkles[offset] = newHash
return merkles
// ExtractWitnessCommitment attempts to locate, and return the witness
// commitment for a block. The witness commitment is of the form:
// SHA256(witness root || witness nonce). The function additionally returns a
// boolean indicating if the witness root was located within any of the txOut's
// in the passed transaction. The witness commitment is stored as the data push
// for an OP_RETURN with special magic bytes to aide in location.
func ExtractWitnessCommitment(tx *btcutil.Tx) ([]byte, bool) {
// The witness commitment *must* be located within one of the coinbase
// transaction's outputs.
if !IsCoinBase(tx) {
return nil, false
msgTx := tx.MsgTx()
for i := len(msgTx.TxOut) - 1; i >= 0; i-- {
// The public key script that contains the witness commitment
// must shared a prefix with the WitnessMagicBytes, and be at
// least 38 bytes.
pkScript := msgTx.TxOut[i].PkScript
if len(pkScript) >= CoinbaseWitnessPkScriptLength &&
bytes.HasPrefix(pkScript, WitnessMagicBytes) {
// The witness commitment itself is a 32-byte hash
// directly after the WitnessMagicBytes. The remaining
// bytes beyond the 38th byte currently have no consensus
// meaning.
start := len(WitnessMagicBytes)
end := CoinbaseWitnessPkScriptLength
return msgTx.TxOut[i].PkScript[start:end], true
return nil, false
// ValidateWitnessCommitment validates the witness commitment (if any) found
// within the coinbase transaction of the passed block.
func ValidateWitnessCommitment(blk *btcutil.Block) error {
// If the block doesn't have any transactions at all, then we won't be
// able to extract a commitment from the non-existent coinbase
// transaction. So we exit early here.
if len(blk.Transactions()) == 0 {
str := "cannot validate witness commitment of block without " +
return ruleError(ErrNoTransactions, str)
coinbaseTx := blk.Transactions()[0]
if len(coinbaseTx.MsgTx().TxIn) == 0 {
return ruleError(ErrNoTxInputs, "transaction has no inputs")
witnessCommitment, witnessFound := ExtractWitnessCommitment(coinbaseTx)
// If we can't find a witness commitment in any of the coinbase's
// outputs, then the block MUST NOT contain any transactions with
// witness data.
if !witnessFound {
for _, tx := range blk.Transactions() {
msgTx := tx.MsgTx()
if msgTx.HasWitness() {
str := fmt.Sprintf("block contains transaction with witness" +
" data, yet no witness commitment present")
return ruleError(ErrUnexpectedWitness, str)
return nil
// At this point the block contains a witness commitment, so the
// coinbase transaction MUST have exactly one witness element within
// its witness data and that element must be exactly
// CoinbaseWitnessDataLen bytes.
coinbaseWitness := coinbaseTx.MsgTx().TxIn[0].Witness
if len(coinbaseWitness) != 1 {
str := fmt.Sprintf("the coinbase transaction has %d items in "+
"its witness stack when only one is allowed",
return ruleError(ErrInvalidWitnessCommitment, str)
witnessNonce := coinbaseWitness[0]
if len(witnessNonce) != CoinbaseWitnessDataLen {
str := fmt.Sprintf("the coinbase transaction witness nonce "+
"has %d bytes when it must be %d bytes",
len(witnessNonce), CoinbaseWitnessDataLen)
return ruleError(ErrInvalidWitnessCommitment, str)
// Finally, with the preliminary checks out of the way, we can check if
// the extracted witnessCommitment is equal to:
// SHA256(witnessMerkleRoot || witnessNonce). Where witnessNonce is the
// coinbase transaction's only witness item.
witnessMerkleTree := BuildMerkleTreeStore(blk.Transactions(), true)
witnessMerkleRoot := witnessMerkleTree[len(witnessMerkleTree)-1]
var witnessPreimage [chainhash.HashSize * 2]byte
copy(witnessPreimage[:], witnessMerkleRoot[:])
copy(witnessPreimage[chainhash.HashSize:], witnessNonce)
computedCommitment := chainhash.DoubleHashB(witnessPreimage[:])
if !bytes.Equal(computedCommitment, witnessCommitment) {
str := fmt.Sprintf("witness commitment does not match: "+
"computed %v, coinbase includes %v", computedCommitment,
return ruleError(ErrWitnessCommitmentMismatch, str)
return nil