return bitcoin transaction from sequence computation.

verifier.go

@@ -1,10 +1,9 @@
 package opentimestamps
 
 import (
+	"bytes"
 	"fmt"
 
-	"slices"
-
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 )
@@ -14,33 +13,35 @@ type Bitcoin interface {
 	GetBlockHeader(hash *chainhash.Hash) (*wire.BlockHeader, error)
 }
 
-func (seq Sequence) Verify(bitcoin Bitcoin, initial []byte) error {
+// Verify validates sequence of operations that starts with digest and ends on a Bitcoin attestation against
+// an actual Bitcoin block, as given by the provided Bitcoin interface.
+func (seq Sequence) Verify(bitcoin Bitcoin, digest []byte) (*wire.MsgTx, error) {
 	if len(seq) == 0 {
-		return fmt.Errorf("empty sequence")
+		return nil, fmt.Errorf("empty sequence")
 	}
 
 	att := seq[len(seq)-1]
 	if att.Attestation == nil || att.BitcoinBlockHeight == 0 {
-		return fmt.Errorf("sequence doesn't include a bitcoin attestation")
+		return nil, fmt.Errorf("sequence doesn't include a bitcoin attestation")
 	}
 
 	blockHash, err := bitcoin.GetBlockHash(int64(att.BitcoinBlockHeight))
 	if err != nil {
-		return fmt.Errorf("failed to get block %d hash: %w", att.BitcoinBlockHeight, err)
+		return nil, fmt.Errorf("failed to get block %d hash: %w", att.BitcoinBlockHeight, err)
 	}
 
 	blockHeader, err := bitcoin.GetBlockHeader(blockHash)
 	if err != nil {
-		return fmt.Errorf("failed to get block %s header: %w", blockHash, err)
+		return nil, fmt.Errorf("failed to get block %s header: %w", blockHash, err)
 	}
 
 	merkleRoot := blockHeader.MerkleRoot[:]
+	result, tx := seq.Compute(digest)
 
-	result := seq.Compute(initial)
-	if !slices.Equal(result, merkleRoot) {
-		return fmt.Errorf("sequence result '%x' doesn't match the bitcoin merkle root for block %d: %x",
+	if !bytes.Equal(result, merkleRoot) {
+		return nil, fmt.Errorf("sequence result '%x' doesn't match the bitcoin merkle root for block %d: %x",
 			result, att.BitcoinBlockHeight, merkleRoot)
 	}
 
-	return nil
+	return tx, nil
 }