p2p/discover: move discv4 encoding to new 'v4wire' package (#21147)

This moves all v4 protocol definitions to a new package, p2p/discover/v4wire.
The new package will be used for low-level protocol tests.

p2p/discover/node.go

@@ -61,17 +61,6 @@ func (e encPubkey) id() enode.ID {
 	return enode.ID(crypto.Keccak256Hash(e[:]))
 }
 
-// recoverNodeKey computes the public key used to sign the
-// given hash from the signature.
-func recoverNodeKey(hash, sig []byte) (key encPubkey, err error) {
-	pubkey, err := crypto.Ecrecover(hash, sig)
-	if err != nil {
-		return key, err
-	}
-	copy(key[:], pubkey[1:])
-	return key, nil
-}
-
 func wrapNode(n *enode.Node) *node {
 	return &node{Node: *n}
 }

p2p/discover/v4_lookup_test.go

@@ -24,6 +24,7 @@ import (
 	"testing"
 
 	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/p2p/discover/v4wire"
 	"github.com/ethereum/go-ethereum/p2p/enode"
 	"github.com/ethereum/go-ethereum/p2p/enr"
 )
@@ -135,15 +136,15 @@ func TestUDPv4_LookupIteratorClose(t *testing.T) {
 
 func serveTestnet(test *udpTest, testnet *preminedTestnet) {
 	for done := false; !done; {
-		done = test.waitPacketOut(func(p packetV4, to *net.UDPAddr, hash []byte) {
+		done = test.waitPacketOut(func(p v4wire.Packet, to *net.UDPAddr, hash []byte) {
 			n, key := testnet.nodeByAddr(to)
 			switch p.(type) {
-			case *pingV4:
-				test.packetInFrom(nil, key, to, &pongV4{Expiration: futureExp, ReplyTok: hash})
-			case *findnodeV4:
+			case *v4wire.Ping:
+				test.packetInFrom(nil, key, to, &v4wire.Pong{Expiration: futureExp, ReplyTok: hash})
+			case *v4wire.Findnode:
 				dist := enode.LogDist(n.ID(), testnet.target.id())
 				nodes := testnet.nodesAtDistance(dist - 1)
-				test.packetInFrom(nil, key, to, &neighborsV4{Expiration: futureExp, Nodes: nodes})
+				test.packetInFrom(nil, key, to, &v4wire.Neighbors{Expiration: futureExp, Nodes: nodes})
 			}
 		})
 	}
@@ -270,8 +271,8 @@ func (tn *preminedTestnet) nodeByAddr(addr *net.UDPAddr) (*enode.Node, *ecdsa.Pr
 	return tn.node(dist, index), key
 }
 
-func (tn *preminedTestnet) nodesAtDistance(dist int) []rpcNode {
-	result := make([]rpcNode, len(tn.dists[dist]))
+func (tn *preminedTestnet) nodesAtDistance(dist int) []v4wire.Node {
+	result := make([]v4wire.Node, len(tn.dists[dist]))
 	for i := range result {
 		result[i] = nodeToRPC(wrapNode(tn.node(dist, i)))
 	}

p2p/discover/v4wire/v4wire.go

+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package v4wire implements the Discovery v4 Wire Protocol.
+package v4wire
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"errors"
+	"fmt"
+	"math/big"
+	"net"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common/math"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/enr"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+// RPC packet types
+const (
+	PingPacket = iota + 1 // zero is 'reserved'
+	PongPacket
+	FindnodePacket
+	NeighborsPacket
+	ENRRequestPacket
+	ENRResponsePacket
+)
+
+// RPC request structures
+type (
+	Ping struct {
+		Version    uint
+		From, To   Endpoint
+		Expiration uint64
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// Pong is the reply to ping.
+	Pong struct {
+		// This field should mirror the UDP envelope address
+		// of the ping packet, which provides a way to discover the
+		// the external address (after NAT).
+		To         Endpoint
+		ReplyTok   []byte // This contains the hash of the ping packet.
+		Expiration uint64 // Absolute timestamp at which the packet becomes invalid.
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// Findnode is a query for nodes close to the given target.
+	Findnode struct {
+		Target     Pubkey
+		Expiration uint64
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// Neighbors is the reply to findnode.
+	Neighbors struct {
+		Nodes      []Node
+		Expiration uint64
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// enrRequest queries for the remote node's record.
+	ENRRequest struct {
+		Expiration uint64
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// enrResponse is the reply to enrRequest.
+	ENRResponse struct {
+		ReplyTok []byte // Hash of the enrRequest packet.
+		Record   enr.Record
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+)
+
+// This number is the maximum number of neighbor nodes in a Neigbors packet.
+const MaxNeighbors = 12
+
+// This code computes the MaxNeighbors constant value.
+
+// func init() {
+// 	var maxNeighbors int
+// 	p := Neighbors{Expiration: ^uint64(0)}
+// 	maxSizeNode := Node{IP: make(net.IP, 16), UDP: ^uint16(0), TCP: ^uint16(0)}
+// 	for n := 0; ; n++ {
+// 		p.Nodes = append(p.Nodes, maxSizeNode)
+// 		size, _, err := rlp.EncodeToReader(p)
+// 		if err != nil {
+// 			// If this ever happens, it will be caught by the unit tests.
+// 			panic("cannot encode: " + err.Error())
+// 		}
+// 		if headSize+size+1 >= 1280 {
+// 			maxNeighbors = n
+// 			break
+// 		}
+// 	}
+// 	fmt.Println("maxNeighbors", maxNeighbors)
+// }
+
+// Pubkey represents an encoded 64-byte secp256k1 public key.
+type Pubkey [64]byte
+
+// ID returns the node ID corresponding to the public key.
+func (e Pubkey) ID() enode.ID {
+	return enode.ID(crypto.Keccak256Hash(e[:]))
+}
+
+// Node represents information about a node.
+type Node struct {
+	IP  net.IP // len 4 for IPv4 or 16 for IPv6
+	UDP uint16 // for discovery protocol
+	TCP uint16 // for RLPx protocol
+	ID  Pubkey
+}
+
+// Endpoint represents a network endpoint.
+type Endpoint struct {
+	IP  net.IP // len 4 for IPv4 or 16 for IPv6
+	UDP uint16 // for discovery protocol
+	TCP uint16 // for RLPx protocol
+}
+
+// NewEndpoint creates an endpoint.
+func NewEndpoint(addr *net.UDPAddr, tcpPort uint16) Endpoint {
+	ip := net.IP{}
+	if ip4 := addr.IP.To4(); ip4 != nil {
+		ip = ip4
+	} else if ip6 := addr.IP.To16(); ip6 != nil {
+		ip = ip6
+	}
+	return Endpoint{IP: ip, UDP: uint16(addr.Port), TCP: tcpPort}
+}
+
+type Packet interface {
+	// packet name and type for logging purposes.
+	Name() string
+	Kind() byte
+}
+
+func (req *Ping) Name() string   { return "PING/v4" }
+func (req *Ping) Kind() byte     { return PingPacket }
+func (req *Ping) ENRSeq() uint64 { return seqFromTail(req.Rest) }
+
+func (req *Pong) Name() string   { return "PONG/v4" }
+func (req *Pong) Kind() byte     { return PongPacket }
+func (req *Pong) ENRSeq() uint64 { return seqFromTail(req.Rest) }
+
+func (req *Findnode) Name() string { return "FINDNODE/v4" }
+func (req *Findnode) Kind() byte   { return FindnodePacket }
+
+func (req *Neighbors) Name() string { return "NEIGHBORS/v4" }
+func (req *Neighbors) Kind() byte   { return NeighborsPacket }
+
+func (req *ENRRequest) Name() string { return "ENRREQUEST/v4" }
+func (req *ENRRequest) Kind() byte   { return ENRRequestPacket }
+
+func (req *ENRResponse) Name() string { return "ENRRESPONSE/v4" }
+func (req *ENRResponse) Kind() byte   { return ENRResponsePacket }
+
+// Expired checks whether the given UNIX time stamp is in the past.
+func Expired(ts uint64) bool {
+	return time.Unix(int64(ts), 0).Before(time.Now())
+}
+
+func seqFromTail(tail []rlp.RawValue) uint64 {
+	if len(tail) == 0 {
+		return 0
+	}
+	var seq uint64
+	rlp.DecodeBytes(tail[0], &seq)
+	return seq
+}
+
+// Encoder/decoder.
+
+const (
+	macSize  = 32
+	sigSize  = crypto.SignatureLength
+	headSize = macSize + sigSize // space of packet frame data
+)
+
+var (
+	ErrPacketTooSmall = errors.New("too small")
+	ErrBadHash        = errors.New("bad hash")
+	ErrBadPoint       = errors.New("invalid curve point")
+)
+
+var headSpace = make([]byte, headSize)
+
+// Decode reads a discovery v4 packet.
+func Decode(input []byte) (Packet, Pubkey, []byte, error) {
+	if len(input) < headSize+1 {
+		return nil, Pubkey{}, nil, ErrPacketTooSmall
+	}
+	hash, sig, sigdata := input[:macSize], input[macSize:headSize], input[headSize:]
+	shouldhash := crypto.Keccak256(input[macSize:])
+	if !bytes.Equal(hash, shouldhash) {
+		return nil, Pubkey{}, nil, ErrBadHash
+	}
+	fromKey, err := recoverNodeKey(crypto.Keccak256(input[headSize:]), sig)
+	if err != nil {
+		return nil, fromKey, hash, err
+	}
+
+	var req Packet
+	switch ptype := sigdata[0]; ptype {
+	case PingPacket:
+		req = new(Ping)
+	case PongPacket:
+		req = new(Pong)
+	case FindnodePacket:
+		req = new(Findnode)
+	case NeighborsPacket:
+		req = new(Neighbors)
+	case ENRRequestPacket:
+		req = new(ENRRequest)
+	case ENRResponsePacket:
+		req = new(ENRResponse)
+	default:
+		return nil, fromKey, hash, fmt.Errorf("unknown type: %d", ptype)
+	}
+	s := rlp.NewStream(bytes.NewReader(sigdata[1:]), 0)
+	err = s.Decode(req)
+	return req, fromKey, hash, err
+}
+
+// Encode encodes a discovery packet.
+func Encode(priv *ecdsa.PrivateKey, req Packet) (packet, hash []byte, err error) {
+	b := new(bytes.Buffer)
+	b.Write(headSpace)
+	b.WriteByte(req.Kind())
+	if err := rlp.Encode(b, req); err != nil {
+		return nil, nil, err
+	}
+	packet = b.Bytes()
+	sig, err := crypto.Sign(crypto.Keccak256(packet[headSize:]), priv)
+	if err != nil {
+		return nil, nil, err
+	}
+	copy(packet[macSize:], sig)
+	// Add the hash to the front. Note: this doesn't protect the packet in any way.
+	hash = crypto.Keccak256(packet[macSize:])
+	copy(packet, hash)
+	return packet, hash, nil
+}
+
+// recoverNodeKey computes the public key used to sign the given hash from the signature.
+func recoverNodeKey(hash, sig []byte) (key Pubkey, err error) {
+	pubkey, err := crypto.Ecrecover(hash, sig)
+	if err != nil {
+		return key, err
+	}
+	copy(key[:], pubkey[1:])
+	return key, nil
+}
+
+// EncodePubkey encodes a secp256k1 public key.
+func EncodePubkey(key *ecdsa.PublicKey) Pubkey {
+	var e Pubkey
+	math.ReadBits(key.X, e[:len(e)/2])
+	math.ReadBits(key.Y, e[len(e)/2:])
+	return e
+}
+
+// DecodePubkey reads an encoded secp256k1 public key.
+func DecodePubkey(curve elliptic.Curve, e Pubkey) (*ecdsa.PublicKey, error) {
+	p := &ecdsa.PublicKey{Curve: curve, X: new(big.Int), Y: new(big.Int)}
+	half := len(e) / 2
+	p.X.SetBytes(e[:half])
+	p.Y.SetBytes(e[half:])
+	if !p.Curve.IsOnCurve(p.X, p.Y) {
+		return nil, ErrBadPoint
+	}
+	return p, nil
+}

p2p/discover/v4wire/v4wire_test.go

+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package v4wire
+
+import (
+	"encoding/hex"
+	"net"
+	"reflect"
+	"testing"
+
+	"github.com/davecgh/go-spew/spew"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+// EIP-8 test vectors.
+var testPackets = []struct {
+	input      string
+	wantPacket interface{}
+}{
+	{
+		input: "71dbda3a79554728d4f94411e42ee1f8b0d561c10e1e5f5893367948c6a7d70bb87b235fa28a77070271b6c164a2dce8c7e13a5739b53b5e96f2e5acb0e458a02902f5965d55ecbeb2ebb6cabb8b2b232896a36b737666c55265ad0a68412f250001ea04cb847f000001820cfa8215a8d790000000000000000000000000000000018208ae820d058443b9a355",
+		wantPacket: &Ping{
+			Version:    4,
+			From:       Endpoint{net.ParseIP("127.0.0.1").To4(), 3322, 5544},
+			To:         Endpoint{net.ParseIP("::1"), 2222, 3333},
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{},
+		},
+	},
+	{
+		input: "e9614ccfd9fc3e74360018522d30e1419a143407ffcce748de3e22116b7e8dc92ff74788c0b6663aaa3d67d641936511c8f8d6ad8698b820a7cf9e1be7155e9a241f556658c55428ec0563514365799a4be2be5a685a80971ddcfa80cb422cdd0101ec04cb847f000001820cfa8215a8d790000000000000000000000000000000018208ae820d058443b9a3550102",
+		wantPacket: &Ping{
+			Version:    4,
+			From:       Endpoint{net.ParseIP("127.0.0.1").To4(), 3322, 5544},
+			To:         Endpoint{net.ParseIP("::1"), 2222, 3333},
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{{0x01}, {0x02}},
+		},
+	},
+	{
+		input: "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",
+		wantPacket: &Ping{
+			Version:    555,
+			From:       Endpoint{net.ParseIP("2001:db8:3c4d:15::abcd:ef12"), 3322, 5544},
+			To:         Endpoint{net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"), 2222, 33338},
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{{0xC5, 0x01, 0x02, 0x03, 0x04, 0x05}},
+		},
+	},
+	{
+		input: "09b2428d83348d27cdf7064ad9024f526cebc19e4958f0fdad87c15eb598dd61d08423e0bf66b2069869e1724125f820d851c136684082774f870e614d95a2855d000f05d1648b2d5945470bc187c2d2216fbe870f43ed0909009882e176a46b0102f846d79020010db885a308d313198a2e037073488208ae82823aa0fbc914b16819237dcd8801d7e53f69e9719adecb3cc0e790c57e91ca4461c9548443b9a355c6010203c2040506a0c969a58f6f9095004c0177a6b47f451530cab38966a25cca5cb58f055542124e",
+		wantPacket: &Pong{
+			To:         Endpoint{net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"), 2222, 33338},
+			ReplyTok:   common.Hex2Bytes("fbc914b16819237dcd8801d7e53f69e9719adecb3cc0e790c57e91ca4461c954"),
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{{0xC6, 0x01, 0x02, 0x03, 0xC2, 0x04, 0x05}, {0x06}},
+		},
+	},
+	{
+		input: "c7c44041b9f7c7e41934417ebac9a8e1a4c6298f74553f2fcfdcae6ed6fe53163eb3d2b52e39fe91831b8a927bf4fc222c3902202027e5e9eb812195f95d20061ef5cd31d502e47ecb61183f74a504fe04c51e73df81f25c4d506b26db4517490103f84eb840ca634cae0d49acb401d8a4c6b6fe8c55b70d115bf400769cc1400f3258cd31387574077f301b421bc84df7266c44e9e6d569fc56be00812904767bf5ccd1fc7f8443b9a35582999983999999280dc62cc8255c73471e0a61da0c89acdc0e035e260add7fc0c04ad9ebf3919644c91cb247affc82b69bd2ca235c71eab8e49737c937a2c396",
+		wantPacket: &Findnode{
+			Target:     hexPubkey("ca634cae0d49acb401d8a4c6b6fe8c55b70d115bf400769cc1400f3258cd31387574077f301b421bc84df7266c44e9e6d569fc56be00812904767bf5ccd1fc7f"),
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{{0x82, 0x99, 0x99}, {0x83, 0x99, 0x99, 0x99}},
+		},
+	},
+	{
+		input: "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",
+		wantPacket: &Neighbors{
+			Nodes: []Node{
+				{
+					ID:  hexPubkey("3155e1427f85f10a5c9a7755877748041af1bcd8d474ec065eb33df57a97babf54bfd2103575fa829115d224c523596b401065a97f74010610fce76382c0bf32"),
+					IP:  net.ParseIP("99.33.22.55").To4(),
+					UDP: 4444,
+					TCP: 4445,
+				},
+				{
+					ID:  hexPubkey("312c55512422cf9b8a4097e9a6ad79402e87a15ae909a4bfefa22398f03d20951933beea1e4dfa6f968212385e829f04c2d314fc2d4e255e0d3bc08792b069db"),
+					IP:  net.ParseIP("1.2.3.4").To4(),
+					UDP: 1,
+					TCP: 1,
+				},
+				{
+					ID:  hexPubkey("38643200b172dcfef857492156971f0e6aa2c538d8b74010f8e140811d53b98c765dd2d96126051913f44582e8c199ad7c6d6819e9a56483f637feaac9448aac"),
+					IP:  net.ParseIP("2001:db8:3c4d:15::abcd:ef12"),
+					UDP: 3333,
+					TCP: 3333,
+				},
+				{
+					ID:  hexPubkey("8dcab8618c3253b558d459da53bd8fa68935a719aff8b811197101a4b2b47dd2d47295286fc00cc081bb542d760717d1bdd6bec2c37cd72eca367d6dd3b9df73"),
+					IP:  net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"),
+					UDP: 999,
+					TCP: 1000,
+				},
+			},
+			Expiration: 1136239445,
+			Rest:       []rlp.RawValue{{0x01}, {0x02}, {0x03}},
+		},
+	},
+}
+
+// This test checks that the decoder accepts packets according to EIP-8.
+func TestForwardCompatibility(t *testing.T) {
+	testkey, _ := crypto.HexToECDSA("b71c71a67e1177ad4e901695e1b4b9ee17ae16c6668d313eac2f96dbcda3f291")
+	wantNodeKey := EncodePubkey(&testkey.PublicKey)
+
+	for _, test := range testPackets {
+		input, err := hex.DecodeString(test.input)
+		if err != nil {
+			t.Fatalf("invalid hex: %s", test.input)
+		}
+		packet, nodekey, _, err := Decode(input)
+		if err != nil {
+			t.Errorf("did not accept packet %s\n%v", test.input, err)
+			continue
+		}
+		if !reflect.DeepEqual(packet, test.wantPacket) {
+			t.Errorf("got %s\nwant %s", spew.Sdump(packet), spew.Sdump(test.wantPacket))
+		}
+		if nodekey != wantNodeKey {
+			t.Errorf("got id %v\nwant id %v", nodekey, wantNodeKey)
+		}
+	}
+}
+
+func hexPubkey(h string) (ret Pubkey) {
+	b, err := hex.DecodeString(h)
+	if err != nil {
+		panic(err)
+	}
+	if len(b) != len(ret) {
+		panic("invalid length")
+	}
+	copy(ret[:], b)
+	return ret
+}