txscript: use DoubleHashRaw to write directly crypto.Hash for segwit …

…sighash In this commit, we optimize the sighash calc further by writing directly into the buffer used for serialization by the sha256.New() instance rather than to an intermediate buffer, which is then write to the hash buffer.
Roasbeef · Dec 19, 2023 · fa537e1 · fa537e1
1 parent dbb4141
commit fa537e1
Showing 1 changed file with 94 additions and 92 deletions.
diff --git a/txscript/sighash.go b/txscript/sighash.go
@@ -200,103 +200,105 @@ func calcWitnessSignatureHashRaw(subScript []byte, sigHashes *TxSigHashes,
 		return nil, fmt.Errorf("idx %d but %d txins", idx, len(tx.TxIn))
 	}
 
-	// We'll utilize this buffer throughout to incrementally calculate
-	// the signature hash for this transaction.
-	var sigHash bytes.Buffer
-
-	// First write out, then encode the transaction's version number.
-	var bVersion [4]byte
-	binary.LittleEndian.PutUint32(bVersion[:], uint32(tx.Version))
-	sigHash.Write(bVersion[:])
-
-	// Next write out the possibly pre-calculated hashes for the sequence
-	// numbers of all inputs, and the hashes of the previous outs for all
-	// outputs.
-	var zeroHash chainhash.Hash
-
-	// If anyone can pay isn't active, then we can use the cached
-	// hashPrevOuts, otherwise we just write zeroes for the prev outs.
-	if hashType&SigHashAnyOneCanPay == 0 {
-		sigHash.Write(sigHashes.HashPrevOutsV0[:])
-	} else {
-		sigHash.Write(zeroHash[:])
-	}
+	sigHashBytes := chainhash.DoubleHashRaw(func(w io.Writer) error {
+		// First write out, then encode the transaction's version
+		// number.
+		var bVersion [4]byte
+		binary.LittleEndian.PutUint32(bVersion[:], uint32(tx.Version))
+		w.Write(bVersion[:])
+
+		// Next write out the possibly pre-calculated hashes for the
+		// sequence numbers of all inputs, and the hashes of the
+		// previous outs for all outputs.
+		var zeroHash chainhash.Hash
+
+		// If anyone can pay isn't active, then we can use the cached
+		// hashPrevOuts, otherwise we just write zeroes for the prev
+		// outs.
+		if hashType&SigHashAnyOneCanPay == 0 {
+			w.Write(sigHashes.HashPrevOutsV0[:])
+		} else {
+			w.Write(zeroHash[:])
+		}
 
-	// If the sighash isn't anyone can pay, single, or none, the use the
-	// cached hash sequences, otherwise write all zeroes for the
-	// hashSequence.
-	if hashType&SigHashAnyOneCanPay == 0 &&
-		hashType&sigHashMask != SigHashSingle &&
-		hashType&sigHashMask != SigHashNone {
-		sigHash.Write(sigHashes.HashSequenceV0[:])
-	} else {
-		sigHash.Write(zeroHash[:])
-	}
+		// If the sighash isn't anyone can pay, single, or none, the
+		// use the cached hash sequences, otherwise write all zeroes
+		// for the hashSequence.
+		if hashType&SigHashAnyOneCanPay == 0 &&
+			hashType&sigHashMask != SigHashSingle &&
+			hashType&sigHashMask != SigHashNone {
 
-	txIn := tx.TxIn[idx]
-
-	// Next, write the outpoint being spent.
-	sigHash.Write(txIn.PreviousOutPoint.Hash[:])
-	var bIndex [4]byte
-	binary.LittleEndian.PutUint32(bIndex[:], txIn.PreviousOutPoint.Index)
-	sigHash.Write(bIndex[:])
-
-	if isWitnessPubKeyHashScript(subScript) {
-		// The script code for a p2wkh is a length prefix varint for
-		// the next 25 bytes, followed by a re-creation of the original
-		// p2pkh pk script.
-		sigHash.Write([]byte{0x19})
-		sigHash.Write([]byte{OP_DUP})
-		sigHash.Write([]byte{OP_HASH160})
-		sigHash.Write([]byte{OP_DATA_20})
-		sigHash.Write(extractWitnessPubKeyHash(subScript))
-		sigHash.Write([]byte{OP_EQUALVERIFY})
-		sigHash.Write([]byte{OP_CHECKSIG})
-	} else {
-		// For p2wsh outputs, and future outputs, the script code is
-		// the original script, with all code separators removed,
-		// serialized with a var int length prefix.
-		wire.WriteVarBytes(&sigHash, 0, subScript)
-	}
+			w.Write(sigHashes.HashSequenceV0[:])
+		} else {
+			w.Write(zeroHash[:])
+		}
 
-	// Next, add the input amount, and sequence number of the input being
-	// signed.
-	var bAmount [8]byte
-	binary.LittleEndian.PutUint64(bAmount[:], uint64(amt))
-	sigHash.Write(bAmount[:])
-	var bSequence [4]byte
-	binary.LittleEndian.PutUint32(bSequence[:], txIn.Sequence)
-	sigHash.Write(bSequence[:])
-
-	// If the current signature mode isn't single, or none, then we can
-	// re-use the pre-generated hashoutputs sighash fragment. Otherwise,
-	// we'll serialize and add only the target output index to the signature
-	// pre-image.
-	if hashType&sigHashMask != SigHashSingle &&
-		hashType&sigHashMask != SigHashNone {
-		sigHash.Write(sigHashes.HashOutputsV0[:])
-	} else if hashType&sigHashMask == SigHashSingle && idx < len(tx.TxOut) {
-		var b bytes.Buffer
-		wire.WriteTxOut(&b, 0, 0, tx.TxOut[idx])
-		sigHash.Write(chainhash.DoubleHashB(b.Bytes()))
-	} else {
-		sigHash.Write(zeroHash[:])
-	}
+		txIn := tx.TxIn[idx]
+
+		// Next, write the outpoint being spent.
+		w.Write(txIn.PreviousOutPoint.Hash[:])
+		var bIndex [4]byte
+		binary.LittleEndian.PutUint32(
+			bIndex[:], txIn.PreviousOutPoint.Index,
+		)
+		w.Write(bIndex[:])
+
+		if isWitnessPubKeyHashScript(subScript) {
+			// The script code for a p2wkh is a length prefix
+			// varint for the next 25 bytes, followed by a
+			// re-creation of the original p2pkh pk script.
+			w.Write([]byte{0x19})
+			w.Write([]byte{OP_DUP})
+			w.Write([]byte{OP_HASH160})
+			w.Write([]byte{OP_DATA_20})
+			w.Write(extractWitnessPubKeyHash(subScript))
+			w.Write([]byte{OP_EQUALVERIFY})
+			w.Write([]byte{OP_CHECKSIG})
+		} else {
+			// For p2wsh outputs, and future outputs, the script
+			// code is the original script, with all code
+			// separators removed, serialized with a var int length
+			// prefix.
+			wire.WriteVarBytes(w, 0, subScript)
+		}
 
-	// Finally, write out the transaction's locktime, and the sig hash
-	// type.
-	var bLockTime [4]byte
-	binary.LittleEndian.PutUint32(bLockTime[:], tx.LockTime)
-	sigHash.Write(bLockTime[:])
-	var bHashType [4]byte
-	binary.LittleEndian.PutUint32(bHashType[:], uint32(hashType))
-	sigHash.Write(bHashType[:])
+		// Next, add the input amount, and sequence number of the input
+		// being signed.
+		var bAmount [8]byte
+		binary.LittleEndian.PutUint64(bAmount[:], uint64(amt))
+		w.Write(bAmount[:])
+		var bSequence [4]byte
+		binary.LittleEndian.PutUint32(bSequence[:], txIn.Sequence)
+		w.Write(bSequence[:])
+
+		// If the current signature mode isn't single, or none, then we
+		// can re-use the pre-generated hashoutputs sighash fragment.
+		// Otherwise, we'll serialize and add only the target output
+		// index to the signature pre-image.
+		if hashType&sigHashMask != SigHashSingle &&
+			hashType&sigHashMask != SigHashNone {
+
+			w.Write(sigHashes.HashOutputsV0[:])
+		} else if hashType&sigHashMask == SigHashSingle &&
+			idx < len(tx.TxOut) {
+
+			var b bytes.Buffer
+			wire.WriteTxOut(&b, 0, 0, tx.TxOut[idx])
+			w.Write(chainhash.DoubleHashB(b.Bytes()))
+		} else {
+			w.Write(zeroHash[:])
+		}
 
-	sigHashBytes := chainhash.DoubleHashRaw(func(w io.Writer) error {
-		// TODO(rosabeef): put entire calc func into this? then no
-		// intermediate buffer
-		_, err := sigHash.WriteTo(w)
-		return err
+		// Finally, write out the transaction's locktime, and the sig
+		// hash type.
+		var bLockTime [4]byte
+		binary.LittleEndian.PutUint32(bLockTime[:], tx.LockTime)
+		w.Write(bLockTime[:])
+		var bHashType [4]byte
+		binary.LittleEndian.PutUint32(bHashType[:], uint32(hashType))
+		w.Write(bHashType[:])
+
+		return nil
 	})
 
 	return sigHashBytes[:], nil