diff --git a/chain/chain.go b/chain/chain.go
new file mode 100644
index 0000000..2cd49d6
--- /dev/null
+++ b/chain/chain.go
@@ -0,0 +1,47 @@
+// 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 stores and indexes the Lethean blockchain by syncing from
+// a C++ daemon via RPC.
+package chain
+
+import (
+	"fmt"
+
+	store "forge.lthn.ai/core/go-store"
+	"forge.lthn.ai/core/go-blockchain/types"
+)
+
+// Chain manages blockchain storage and indexing.
+type Chain struct {
+	store *store.Store
+}
+
+// New creates a Chain backed by the given store.
+func New(s *store.Store) *Chain {
+	return &Chain{store: s}
+}
+
+// Height returns the number of stored blocks (0 if empty).
+func (c *Chain) Height() (uint64, error) {
+	n, err := c.store.Count(groupBlocks)
+	if err != nil {
+		return 0, fmt.Errorf("chain: height: %w", err)
+	}
+	return uint64(n), nil
+}
+
+// TopBlock returns the highest stored block and its metadata.
+// Returns an error if the chain is empty.
+func (c *Chain) TopBlock() (*types.Block, *BlockMeta, error) {
+	h, err := c.Height()
+	if err != nil {
+		return nil, nil, err
+	}
+	if h == 0 {
+		return nil, nil, fmt.Errorf("chain: no blocks stored")
+	}
+	return c.GetBlockByHeight(h - 1)
+}
diff --git a/chain/chain_test.go b/chain/chain_test.go
new file mode 100644
index 0000000..e60141d
--- /dev/null
+++ b/chain/chain_test.go
@@ -0,0 +1,131 @@
+// 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 (
+	"testing"
+
+	store "forge.lthn.ai/core/go-store"
+	"forge.lthn.ai/core/go-blockchain/types"
+	"forge.lthn.ai/core/go-blockchain/wire"
+)
+
+func newTestChain(t *testing.T) *Chain {
+	t.Helper()
+	s, err := store.New(":memory:")
+	if err != nil {
+		t.Fatalf("store.New: %v", err)
+	}
+	t.Cleanup(func() { s.Close() })
+	return New(s)
+}
+
+// testCoinbaseTx returns a minimal v1 coinbase transaction that round-trips
+// cleanly through the wire encoder/decoder.
+func testCoinbaseTx(height uint64) types.Transaction {
+	return types.Transaction{
+		Version: 1,
+		Vin:     []types.TxInput{types.TxInputGenesis{Height: height}},
+		Vout: []types.TxOutput{types.TxOutputBare{
+			Amount: 1000000,
+			Target: types.TxOutToKey{Key: types.PublicKey{0x01}},
+		}},
+		Extra:      wire.EncodeVarint(0),
+		Attachment: wire.EncodeVarint(0),
+	}
+}
+
+func TestChain_Height_Empty(t *testing.T) {
+	c := newTestChain(t)
+	h, err := c.Height()
+	if err != nil {
+		t.Fatalf("Height: %v", err)
+	}
+	if h != 0 {
+		t.Errorf("height: got %d, want 0", h)
+	}
+}
+
+func TestChain_PutGetBlock_Good(t *testing.T) {
+	c := newTestChain(t)
+
+	blk := &types.Block{
+		BlockHeader: types.BlockHeader{
+			MajorVersion: 1,
+			Timestamp:    1770897600,
+		},
+		MinerTx: testCoinbaseTx(0),
+	}
+	meta := &BlockMeta{
+		Hash:       types.Hash{0xab, 0xcd},
+		Height:     0,
+		Timestamp:  1770897600,
+		Difficulty: 1,
+	}
+
+	if err := c.PutBlock(blk, meta); err != nil {
+		t.Fatalf("PutBlock: %v", err)
+	}
+
+	h, err := c.Height()
+	if err != nil {
+		t.Fatalf("Height: %v", err)
+	}
+	if h != 1 {
+		t.Errorf("height: got %d, want 1", h)
+	}
+
+	gotBlk, gotMeta, err := c.GetBlockByHeight(0)
+	if err != nil {
+		t.Fatalf("GetBlockByHeight: %v", err)
+	}
+	if gotBlk.MajorVersion != 1 {
+		t.Errorf("major_version: got %d, want 1", gotBlk.MajorVersion)
+	}
+	if gotMeta.Hash != meta.Hash {
+		t.Errorf("hash mismatch")
+	}
+
+	gotBlk2, gotMeta2, err := c.GetBlockByHash(meta.Hash)
+	if err != nil {
+		t.Fatalf("GetBlockByHash: %v", err)
+	}
+	if gotBlk2.Timestamp != blk.Timestamp {
+		t.Errorf("timestamp mismatch")
+	}
+	if gotMeta2.Height != 0 {
+		t.Errorf("height: got %d, want 0", gotMeta2.Height)
+	}
+}
+
+func TestChain_TopBlock_Good(t *testing.T) {
+	c := newTestChain(t)
+
+	for i := uint64(0); i < 3; i++ {
+		blk := &types.Block{
+			BlockHeader: types.BlockHeader{
+				MajorVersion: 1,
+				Timestamp:    1770897600 + i*120,
+			},
+			MinerTx: testCoinbaseTx(i),
+		}
+		meta := &BlockMeta{
+			Hash:   types.Hash{byte(i)},
+			Height: i,
+		}
+		if err := c.PutBlock(blk, meta); err != nil {
+			t.Fatalf("PutBlock(%d): %v", i, err)
+		}
+	}
+
+	_, topMeta, err := c.TopBlock()
+	if err != nil {
+		t.Fatalf("TopBlock: %v", err)
+	}
+	if topMeta.Height != 2 {
+		t.Errorf("top height: got %d, want 2", topMeta.Height)
+	}
+}
diff --git a/chain/store.go b/chain/store.go
new file mode 100644
index 0000000..2a08ed8
--- /dev/null
+++ b/chain/store.go
@@ -0,0 +1,115 @@
+// 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 (
+	"bytes"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strconv"
+
+	store "forge.lthn.ai/core/go-store"
+	"forge.lthn.ai/core/go-blockchain/types"
+	"forge.lthn.ai/core/go-blockchain/wire"
+)
+
+// Storage group constants matching the design schema.
+const (
+	groupBlocks     = "blocks"
+	groupBlockIndex = "block_index"
+	groupTx         = "transactions"
+	groupSpentKeys  = "spent_keys"
+	groupOutputsPfx = "outputs:" // suffixed with amount
+)
+
+// heightKey returns a zero-padded 10-digit decimal key for the given height.
+func heightKey(h uint64) string {
+	return fmt.Sprintf("%010d", h)
+}
+
+// blockRecord is the JSON value stored in the blocks group.
+type blockRecord struct {
+	Meta BlockMeta `json:"meta"`
+	Blob string    `json:"blob"` // hex-encoded wire format
+}
+
+// PutBlock stores a block and updates the block_index.
+func (c *Chain) PutBlock(b *types.Block, meta *BlockMeta) error {
+	var buf bytes.Buffer
+	enc := wire.NewEncoder(&buf)
+	wire.EncodeBlock(enc, b)
+	if err := enc.Err(); err != nil {
+		return fmt.Errorf("chain: encode block %d: %w", meta.Height, err)
+	}
+
+	rec := blockRecord{
+		Meta: *meta,
+		Blob: hex.EncodeToString(buf.Bytes()),
+	}
+	val, err := json.Marshal(rec)
+	if err != nil {
+		return fmt.Errorf("chain: marshal block %d: %w", meta.Height, err)
+	}
+
+	if err := c.store.Set(groupBlocks, heightKey(meta.Height), string(val)); err != nil {
+		return fmt.Errorf("chain: store block %d: %w", meta.Height, err)
+	}
+
+	// Update hash -> height index.
+	hashHex := meta.Hash.String()
+	if err := c.store.Set(groupBlockIndex, hashHex, strconv.FormatUint(meta.Height, 10)); err != nil {
+		return fmt.Errorf("chain: index block %d: %w", meta.Height, err)
+	}
+
+	return nil
+}
+
+// GetBlockByHeight retrieves a block by its height.
+func (c *Chain) GetBlockByHeight(height uint64) (*types.Block, *BlockMeta, error) {
+	val, err := c.store.Get(groupBlocks, heightKey(height))
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return nil, nil, fmt.Errorf("chain: block %d not found", height)
+		}
+		return nil, nil, fmt.Errorf("chain: get block %d: %w", height, err)
+	}
+	return decodeBlockRecord(val)
+}
+
+// GetBlockByHash retrieves a block by its hash.
+func (c *Chain) GetBlockByHash(hash types.Hash) (*types.Block, *BlockMeta, error) {
+	heightStr, err := c.store.Get(groupBlockIndex, hash.String())
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return nil, nil, fmt.Errorf("chain: block %s not found", hash)
+		}
+		return nil, nil, fmt.Errorf("chain: get block index %s: %w", hash, err)
+	}
+	height, err := strconv.ParseUint(heightStr, 10, 64)
+	if err != nil {
+		return nil, nil, fmt.Errorf("chain: parse height %q: %w", heightStr, err)
+	}
+	return c.GetBlockByHeight(height)
+}
+
+func decodeBlockRecord(val string) (*types.Block, *BlockMeta, error) {
+	var rec blockRecord
+	if err := json.Unmarshal([]byte(val), &rec); err != nil {
+		return nil, nil, fmt.Errorf("chain: unmarshal block: %w", err)
+	}
+	blob, err := hex.DecodeString(rec.Blob)
+	if err != nil {
+		return nil, nil, fmt.Errorf("chain: decode block hex: %w", err)
+	}
+	dec := wire.NewDecoder(bytes.NewReader(blob))
+	blk := wire.DecodeBlock(dec)
+	if err := dec.Err(); err != nil {
+		return nil, nil, fmt.Errorf("chain: decode block wire: %w", err)
+	}
+	return &blk, &rec.Meta, nil
+}
diff --git a/go.mod b/go.mod
index 550763e..5909416 100644
--- a/go.mod
+++ b/go.mod
@@ -8,7 +8,19 @@ require (
 	golang.org/x/crypto v0.48.0
 )
 
-require golang.org/x/sys v0.41.0 // indirect
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 // indirect
+	golang.org/x/sys v0.41.0 // indirect
+	modernc.org/libc v1.67.6 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
+	modernc.org/sqlite v1.46.1 // indirect
+)
 
 replace forge.lthn.ai/core/go-p2p => /home/claude/Code/core/go-p2p
 
diff --git a/go.sum b/go.sum
index 59fa593..62528dd 100644
--- a/go.sum
+++ b/go.sum
@@ -1,12 +1,63 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
+github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
+github.com/google/pprof v0.0.0-20250317173921-a4b03ec1a45e h1:ijClszYn+mADRFY17kjQEVQ1XRhq2/JR1M3sGqeJoxs=
+github.com/google/pprof v0.0.0-20250317173921-a4b03ec1a45e/go.mod h1:boTsfXsheKC2y+lKOCMpSfarhxDeIzfZG1jqGcPl3cA=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/hashicorp/golang-lru/v2 v2.0.7 h1:a+bsQ5rvGLjzHuww6tVxozPZFVghXaHOwFs4luLUK2k=
+github.com/hashicorp/golang-lru/v2 v2.0.7/go.mod h1:QeFd9opnmA6QUJc5vARoKUSoFhyfM2/ZepoAG6RGpeM=
+github.com/mattn/go-isatty v0.0.20 h1:xfD0iDuEKnDkl03q4limB+vH+GxLEtL/jb4xVJSWWEY=
+github.com/mattn/go-isatty v0.0.20/go.mod h1:W+V8PltTTMOvKvAeJH7IuucS94S2C6jfK/D7dTCTo3Y=
+github.com/ncruces/go-strftime v1.0.0 h1:HMFp8mLCTPp341M/ZnA4qaf7ZlsbTc+miZjCLOFAw7w=
+github.com/ncruces/go-strftime v1.0.0/go.mod h1:Fwc5htZGVVkseilnfgOVb9mKy6w1naJmn9CehxcKcls=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec h1:W09IVJc94icq4NjY3clb7Lk8O1qJ8BdBEF8z0ibU0rE=
+github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec/go.mod h1:qqbHyh8v60DhA7CoWK5oRCqLrMHRGoxYCSS9EjAz6Eo=
 github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
 github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
 golang.org/x/crypto v0.48.0 h1:/VRzVqiRSggnhY7gNRxPauEQ5Drw9haKdM0jqfcCFts=
 golang.org/x/crypto v0.48.0/go.mod h1:r0kV5h3qnFPlQnBSrULhlsRfryS2pmewsg+XfMgkVos=
+golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546 h1:mgKeJMpvi0yx/sU5GsxQ7p6s2wtOnGAHZWCHUM4KGzY=
+golang.org/x/exp v0.0.0-20251023183803-a4bb9ffd2546/go.mod h1:j/pmGrbnkbPtQfxEe5D0VQhZC6qKbfKifgD0oM7sR70=
+golang.org/x/mod v0.29.0 h1:HV8lRxZC4l2cr3Zq1LvtOsi/ThTgWnUk/y64QSs8GwA=
+golang.org/x/mod v0.29.0/go.mod h1:NyhrlYXJ2H4eJiRy/WDBO6HMqZQ6q9nk4JzS3NuCK+w=
+golang.org/x/sync v0.17.0 h1:l60nONMj9l5drqw6jlhIELNv9I0A4OFgRsG9k2oT9Ug=
+golang.org/x/sync v0.17.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
+golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.41.0 h1:Ivj+2Cp/ylzLiEU89QhWblYnOE9zerudt9Ftecq2C6k=
 golang.org/x/sys v0.41.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/tools v0.38.0 h1:Hx2Xv8hISq8Lm16jvBZ2VQf+RLmbd7wVUsALibYI/IQ=
+golang.org/x/tools v0.38.0/go.mod h1:yEsQ/d/YK8cjh0L6rZlY8tgtlKiBNTL14pGDJPJpYQs=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+modernc.org/cc/v4 v4.27.1 h1:9W30zRlYrefrDV2JE2O8VDtJ1yPGownxciz5rrbQZis=
+modernc.org/cc/v4 v4.27.1/go.mod h1:uVtb5OGqUKpoLWhqwNQo/8LwvoiEBLvZXIQ/SmO6mL0=
+modernc.org/ccgo/v4 v4.30.1 h1:4r4U1J6Fhj98NKfSjnPUN7Ze2c6MnAdL0hWw6+LrJpc=
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+modernc.org/fileutil v1.3.40 h1:ZGMswMNc9JOCrcrakF1HrvmergNLAmxOPjizirpfqBA=
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+modernc.org/gc/v2 v2.6.5 h1:nyqdV8q46KvTpZlsw66kWqwXRHdjIlJOhG6kxiV/9xI=
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