implement verifier.

verifier.go

@@ -1,11 +1,45 @@
 package opentimestamps
 
 import (
+	"fmt"
+
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
+	"golang.org/x/exp/slices"
 )
 
 type Bitcoin interface {
 	GetBlockHash(height int64) (*chainhash.Hash, error)
 	GetBlockHeader(hash *chainhash.Hash) (*wire.BlockHeader, error)
 }
+
+func (seq Sequence) Verify(bitcoin Bitcoin, initial []byte) error {
+	if len(seq) == 0 {
+		return 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")
+	}
+
+	blockHash, err := bitcoin.GetBlockHash(int64(att.BitcoinBlockHeight))
+	if err != nil {
+		return 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)
+	}
+
+	merkleRoot := blockHeader.MerkleRoot[:]
+
+	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",
+			result, att.BitcoinBlockHeight, merkleRoot)
+	}
+
+	return nil
+}