diff --git a/chain/chain_test.go b/chain/chain_test.go
index e60141d..f53bb98 100644
--- a/chain/chain_test.go
+++ b/chain/chain_test.go
@@ -129,3 +129,117 @@ func TestChain_TopBlock_Good(t *testing.T) {
 		t.Errorf("top height: got %d, want 2", topMeta.Height)
 	}
 }
+
+func TestChain_PutGetTransaction_Good(t *testing.T) {
+	c := newTestChain(t)
+
+	tx := &types.Transaction{
+		Version: 1,
+		Vin: []types.TxInput{
+			types.TxInputToKey{
+				Amount:     1000000000000,
+				KeyImage:   types.KeyImage{0x01},
+				EtcDetails: wire.EncodeVarint(0),
+			},
+		},
+		Vout: []types.TxOutput{
+			types.TxOutputBare{
+				Amount: 900000000000,
+				Target: types.TxOutToKey{Key: types.PublicKey{0x02}},
+			},
+		},
+		Extra:      wire.EncodeVarint(0),
+		Attachment: wire.EncodeVarint(0),
+	}
+	meta := &TxMeta{
+		KeeperBlock:         5,
+		GlobalOutputIndexes: []uint64{42},
+	}
+
+	txHash := types.Hash{0xde, 0xad}
+	if err := c.PutTransaction(txHash, tx, meta); err != nil {
+		t.Fatalf("PutTransaction: %v", err)
+	}
+
+	if !c.HasTransaction(txHash) {
+		t.Error("HasTransaction: got false, want true")
+	}
+
+	gotTx, gotMeta, err := c.GetTransaction(txHash)
+	if err != nil {
+		t.Fatalf("GetTransaction: %v", err)
+	}
+	if gotTx.Version != 1 {
+		t.Errorf("version: got %d, want 1", gotTx.Version)
+	}
+	if gotMeta.KeeperBlock != 5 {
+		t.Errorf("keeper_block: got %d, want 5", gotMeta.KeeperBlock)
+	}
+}
+
+func TestChain_KeyImage_Good(t *testing.T) {
+	c := newTestChain(t)
+
+	ki := types.KeyImage{0xaa, 0xbb}
+
+	spent, err := c.IsSpent(ki)
+	if err != nil {
+		t.Fatalf("IsSpent: %v", err)
+	}
+	if spent {
+		t.Error("IsSpent: got true before marking")
+	}
+
+	if err := c.MarkSpent(ki, 10); err != nil {
+		t.Fatalf("MarkSpent: %v", err)
+	}
+
+	spent, err = c.IsSpent(ki)
+	if err != nil {
+		t.Fatalf("IsSpent: %v", err)
+	}
+	if !spent {
+		t.Error("IsSpent: got false after marking")
+	}
+}
+
+func TestChain_OutputIndex_Good(t *testing.T) {
+	c := newTestChain(t)
+
+	txID := types.Hash{0x01}
+
+	gidx0, err := c.PutOutput(1000000000000, txID, 0)
+	if err != nil {
+		t.Fatalf("PutOutput(0): %v", err)
+	}
+	if gidx0 != 0 {
+		t.Errorf("gindex: got %d, want 0", gidx0)
+	}
+
+	gidx1, err := c.PutOutput(1000000000000, txID, 1)
+	if err != nil {
+		t.Fatalf("PutOutput(1): %v", err)
+	}
+	if gidx1 != 1 {
+		t.Errorf("gindex: got %d, want 1", gidx1)
+	}
+
+	count, err := c.OutputCount(1000000000000)
+	if err != nil {
+		t.Fatalf("OutputCount: %v", err)
+	}
+	if count != 2 {
+		t.Errorf("count: got %d, want 2", count)
+	}
+
+	gotTxID, gotOutNo, err := c.GetOutput(1000000000000, 0)
+	if err != nil {
+		t.Fatalf("GetOutput: %v", err)
+	}
+	if gotTxID != txID {
+		t.Errorf("tx_id mismatch")
+	}
+	if gotOutNo != 0 {
+		t.Errorf("out_no: got %d, want 0", gotOutNo)
+	}
+}
diff --git a/chain/index.go b/chain/index.go
new file mode 100644
index 0000000..848cc7f
--- /dev/null
+++ b/chain/index.go
@@ -0,0 +1,99 @@
+// Copyright (c) 2017-2026 Lethean (https://lt.hn)
+//
+// Licensed under the European Union Public Licence (EUPL) version 1.2.
+// SPDX-License-Identifier: EUPL-1.2
+
+package chain
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strconv"
+
+	store "forge.lthn.ai/core/go-store"
+	"forge.lthn.ai/core/go-blockchain/types"
+)
+
+// MarkSpent records a key image as spent at the given block height.
+func (c *Chain) MarkSpent(ki types.KeyImage, height uint64) error {
+	if err := c.store.Set(groupSpentKeys, ki.String(), strconv.FormatUint(height, 10)); err != nil {
+		return fmt.Errorf("chain: mark spent %s: %w", ki, err)
+	}
+	return nil
+}
+
+// IsSpent checks whether a key image has been spent.
+func (c *Chain) IsSpent(ki types.KeyImage) (bool, error) {
+	_, err := c.store.Get(groupSpentKeys, ki.String())
+	if errors.Is(err, store.ErrNotFound) {
+		return false, nil
+	}
+	if err != nil {
+		return false, fmt.Errorf("chain: check spent %s: %w", ki, err)
+	}
+	return true, nil
+}
+
+// outputGroup returns the go-store group for outputs of the given amount.
+func outputGroup(amount uint64) string {
+	return groupOutputsPfx + strconv.FormatUint(amount, 10)
+}
+
+// PutOutput appends an output to the global index for the given amount.
+// Returns the assigned global index.
+func (c *Chain) PutOutput(amount uint64, txID types.Hash, outNo uint32) (uint64, error) {
+	grp := outputGroup(amount)
+	count, err := c.store.Count(grp)
+	if err != nil {
+		return 0, fmt.Errorf("chain: output count: %w", err)
+	}
+	gindex := uint64(count)
+
+	entry := outputEntry{
+		TxID:  txID.String(),
+		OutNo: outNo,
+	}
+	val, err := json.Marshal(entry)
+	if err != nil {
+		return 0, fmt.Errorf("chain: marshal output: %w", err)
+	}
+
+	key := strconv.FormatUint(gindex, 10)
+	if err := c.store.Set(grp, key, string(val)); err != nil {
+		return 0, fmt.Errorf("chain: store output: %w", err)
+	}
+	return gindex, nil
+}
+
+// GetOutput retrieves an output by amount and global index.
+func (c *Chain) GetOutput(amount uint64, gindex uint64) (types.Hash, uint32, error) {
+	grp := outputGroup(amount)
+	key := strconv.FormatUint(gindex, 10)
+	val, err := c.store.Get(grp, key)
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return types.Hash{}, 0, fmt.Errorf("chain: output %d:%d not found", amount, gindex)
+		}
+		return types.Hash{}, 0, fmt.Errorf("chain: get output: %w", err)
+	}
+
+	var entry outputEntry
+	if err := json.Unmarshal([]byte(val), &entry); err != nil {
+		return types.Hash{}, 0, fmt.Errorf("chain: unmarshal output: %w", err)
+	}
+	hash, err := types.HashFromHex(entry.TxID)
+	if err != nil {
+		return types.Hash{}, 0, fmt.Errorf("chain: parse output tx_id: %w", err)
+	}
+	return hash, entry.OutNo, nil
+}
+
+// OutputCount returns the number of outputs indexed for the given amount.
+func (c *Chain) OutputCount(amount uint64) (uint64, error) {
+	n, err := c.store.Count(outputGroup(amount))
+	if err != nil {
+		return 0, fmt.Errorf("chain: output count: %w", err)
+	}
+	return uint64(n), nil
+}
diff --git a/chain/store.go b/chain/store.go
index 2a08ed8..fc43219 100644
--- a/chain/store.go
+++ b/chain/store.go
@@ -97,6 +97,68 @@ func (c *Chain) GetBlockByHash(hash types.Hash) (*types.Block, *BlockMeta, error
 	return c.GetBlockByHeight(height)
 }
 
+// txRecord is the JSON value stored in the transactions group.
+type txRecord struct {
+	Meta TxMeta `json:"meta"`
+	Blob string `json:"blob"` // hex-encoded wire format
+}
+
+// PutTransaction stores a transaction with metadata.
+func (c *Chain) PutTransaction(hash types.Hash, tx *types.Transaction, meta *TxMeta) error {
+	var buf bytes.Buffer
+	enc := wire.NewEncoder(&buf)
+	wire.EncodeTransaction(enc, tx)
+	if err := enc.Err(); err != nil {
+		return fmt.Errorf("chain: encode tx %s: %w", hash, err)
+	}
+
+	rec := txRecord{
+		Meta: *meta,
+		Blob: hex.EncodeToString(buf.Bytes()),
+	}
+	val, err := json.Marshal(rec)
+	if err != nil {
+		return fmt.Errorf("chain: marshal tx %s: %w", hash, err)
+	}
+
+	if err := c.store.Set(groupTx, hash.String(), string(val)); err != nil {
+		return fmt.Errorf("chain: store tx %s: %w", hash, err)
+	}
+	return nil
+}
+
+// GetTransaction retrieves a transaction by hash.
+func (c *Chain) GetTransaction(hash types.Hash) (*types.Transaction, *TxMeta, error) {
+	val, err := c.store.Get(groupTx, hash.String())
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return nil, nil, fmt.Errorf("chain: tx %s not found", hash)
+		}
+		return nil, nil, fmt.Errorf("chain: get tx %s: %w", hash, err)
+	}
+
+	var rec txRecord
+	if err := json.Unmarshal([]byte(val), &rec); err != nil {
+		return nil, nil, fmt.Errorf("chain: unmarshal tx: %w", err)
+	}
+	blob, err := hex.DecodeString(rec.Blob)
+	if err != nil {
+		return nil, nil, fmt.Errorf("chain: decode tx hex: %w", err)
+	}
+	dec := wire.NewDecoder(bytes.NewReader(blob))
+	tx := wire.DecodeTransaction(dec)
+	if err := dec.Err(); err != nil {
+		return nil, nil, fmt.Errorf("chain: decode tx wire: %w", err)
+	}
+	return &tx, &rec.Meta, nil
+}
+
+// HasTransaction checks whether a transaction exists in the store.
+func (c *Chain) HasTransaction(hash types.Hash) bool {
+	_, err := c.store.Get(groupTx, hash.String())
+	return err == nil
+}
+
 func decodeBlockRecord(val string) (*types.Block, *BlockMeta, error) {
 	var rec blockRecord
 	if err := json.Unmarshal([]byte(val), &rec); err != nil {