path: root/p2p/handshake.go

package p2p

import (
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/rand"
    "errors"
    "fmt"
    "hash"
    "io"
    "net"

    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/crypto/ecies"
    "github.com/ethereum/go-ethereum/crypto/secp256k1"
    "github.com/ethereum/go-ethereum/crypto/sha3"
    "github.com/ethereum/go-ethereum/p2p/discover"
    "github.com/ethereum/go-ethereum/rlp"
)

const (
    sskLen = 16 // ecies.MaxSharedKeyLength(pubKey) / 2
    sigLen = 65 // elliptic S256
    pubLen = 64 // 512 bit pubkey in uncompressed representation without format byte
    shaLen = 32 // hash length (for nonce etc)

    authMsgLen  = sigLen + shaLen + pubLen + shaLen + 1
    authRespLen = pubLen + shaLen + 1

    eciesBytes     = 65 + 16 + 32
    encAuthMsgLen  = authMsgLen + eciesBytes  // size of the final ECIES payload sent as initiator's handshake
    encAuthRespLen = authRespLen + eciesBytes // size of the final ECIES payload sent as receiver's handshake
)

// conn represents a remote connection after encryption handshake
// and protocol handshake have completed.
//
// The MsgReadWriter is usually layered as follows:
//
//     netWrapper       (I/O timeouts, thread-safe ReadMsg, WriteMsg)
//     rlpxFrameRW      (message encoding, encryption, authentication)
//     bufio.ReadWriter (buffering)
//     net.Conn         (network I/O)
//
type conn struct {
    MsgReadWriter
    *protoHandshake
}

// secrets represents the connection secrets
// which are negotiated during the encryption handshake.
type secrets struct {
    RemoteID              discover.NodeID
    AES, MAC              []byte
    EgressMAC, IngressMAC hash.Hash
    Token                 []byte
}

// protoHandshake is the RLP structure of the protocol handshake.
type protoHandshake struct {
    Version    uint64
    Name       string
    Caps       []Cap
    ListenPort uint64
    ID         discover.NodeID
}

// setupConn starts a protocol session on the given connection.
// It runs the encryption handshake and the protocol handshake.
// If dial is non-nil, the connection the local node is the initiator.
// If atcap is true, the connection will be disconnected with DiscTooManyPeers
// after the key exchange.
func setupConn(fd net.Conn, prv *ecdsa.PrivateKey, our *protoHandshake, dial *discover.Node, atcap bool) (*conn, error) {
    if dial == nil {
        return setupInboundConn(fd, prv, our, atcap)
    } else {
        return setupOutboundConn(fd, prv, our, dial, atcap)
    }
}

func setupInboundConn(fd net.Conn, prv *ecdsa.PrivateKey, our *protoHandshake, atcap bool) (*conn, error) {
    secrets, err := receiverEncHandshake(fd, prv, nil)
    if err != nil {
        return nil, fmt.Errorf("encryption handshake failed: %v", err)
    }
    rw := newRlpxFrameRW(fd, secrets)
    if atcap {
        SendItems(rw, discMsg, DiscTooManyPeers)
        return nil, errors.New("we have too many peers")
    }
    // Run the protocol handshake using authenticated messages.
    rhs, err := readProtocolHandshake(rw, secrets.RemoteID, our)
    if err != nil {
        return nil, err
    }
    if err := Send(rw, handshakeMsg, our); err != nil {
        return nil, fmt.Errorf("protocol handshake write error: %v", err)
    }
    return &conn{rw, rhs}, nil
}

func setupOutboundConn(fd net.Conn, prv *ecdsa.PrivateKey, our *protoHandshake, dial *discover.Node, atcap bool) (*conn, error) {
    secrets, err := initiatorEncHandshake(fd, prv, dial.ID, nil)
    if err != nil {
        return nil, fmt.Errorf("encryption handshake failed: %v", err)
    }
    rw := newRlpxFrameRW(fd, secrets)
    if atcap {
        SendItems(rw, discMsg, DiscTooManyPeers)
        return nil, errors.New("we have too many peers")
    }
    // Run the protocol handshake using authenticated messages.
    //
    // Note that even though writing the handshake is first, we prefer
    // returning the handshake read error. If the remote side
    // disconnects us early with a valid reason, we should return it
    // as the error so it can be tracked elsewhere.
    werr := make(chan error)
    go func() { werr <- Send(rw, handshakeMsg, our) }()
    rhs, err := readProtocolHandshake(rw, secrets.RemoteID, our)
    if err != nil {
        return nil, err
    }
    if err := <-werr; err != nil {
        return nil, fmt.Errorf("protocol handshake write error: %v", err)
    }
    if rhs.ID != dial.ID {
        return nil, errors.New("dialed node id mismatch")
    }
    return &conn{rw, rhs}, nil
}

// encHandshake contains the state of the encryption handshake.
type encHandshake struct {
    initiator bool
    remoteID  discover.NodeID

    remotePub            *ecies.PublicKey  // remote-pubk
    initNonce, respNonce []byte            // nonce
    randomPrivKey        *ecies.PrivateKey // ecdhe-random
    remoteRandomPub      *ecies.PublicKey  // ecdhe-random-pubk
}

// secrets is called after the handshake is completed.
// It extracts the connection secrets from the handshake values.
func (h *encHandshake) secrets(auth, authResp []byte) (secrets, error) {
    ecdheSecret, err := h.randomPrivKey.GenerateShared(h.remoteRandomPub, sskLen, sskLen)
    if err != nil {
        return secrets{}, err
    }

    // derive base secrets from ephemeral key agreement
    sharedSecret := crypto.Sha3(ecdheSecret, crypto.Sha3(h.respNonce, h.initNonce))
    aesSecret := crypto.Sha3(ecdheSecret, sharedSecret)
    s := secrets{
        RemoteID: h.remoteID,
        AES:      aesSecret,
        MAC:      crypto.Sha3(ecdheSecret, aesSecret),
        Token:    crypto.Sha3(sharedSecret),
    }

    // setup sha3 instances for the MACs
    mac1 := sha3.NewKeccak256()
    mac1.Write(xor(s.MAC, h.respNonce))
    mac1.Write(auth)
    mac2 := sha3.NewKeccak256()
    mac2.Write(xor(s.MAC, h.initNonce))
    mac2.Write(authResp)
    if h.initiator {
        s.EgressMAC, s.IngressMAC = mac1, mac2
    } else {
        s.EgressMAC, s.IngressMAC = mac2, mac1
    }

    return s, nil
}

func (h *encHandshake) ecdhShared(prv *ecdsa.PrivateKey) ([]byte, error) {
    return ecies.ImportECDSA(prv).GenerateShared(h.remotePub, sskLen, sskLen)
}

// initiatorEncHandshake negotiates a session token on conn.
// it should be called on the dialing side of the connection.
//
// prv is the local client's private key.
// token is the token from a previous session with this node.
func initiatorEncHandshake(conn io.ReadWriter, prv *ecdsa.PrivateKey, remoteID discover.NodeID, token []byte) (s secrets, err error) {
    h, err := newInitiatorHandshake(remoteID)
    if err != nil {
        return s, err
    }
    auth, err := h.authMsg(prv, token)
    if err != nil {
        return s, err
    }
    if _, err = conn.Write(auth); err != nil {
        return s, err
    }

    response := make([]byte, encAuthRespLen)
    if _, err = io.ReadFull(conn, response); err != nil {
        return s, err
    }
    if err := h.decodeAuthResp(response, prv); err != nil {
        return s, err
    }
    return h.secrets(auth, response)
}

func newInitiatorHandshake(remoteID discover.NodeID) (*encHandshake, error) {
    // generate random initiator nonce
    n := make([]byte, shaLen)
    if _, err := rand.Read(n); err != nil {
        return nil, err
    }
    // generate random keypair to use for signing
    randpriv, err := ecies.GenerateKey(rand.Reader, crypto.S256(), nil)
    if err != nil {
        return nil, err
    }
    rpub, err := remoteID.Pubkey()
    if err != nil {
        return nil, fmt.Errorf("bad remoteID: %v", err)
    }
    h := &encHandshake{
        initiator:     true,
        remoteID:      remoteID,
        remotePub:     ecies.ImportECDSAPublic(rpub),
        initNonce:     n,
        randomPrivKey: randpriv,
    }
    return h, nil
}

// authMsg creates an encrypted initiator handshake message.
func (h *encHandshake) authMsg(prv *ecdsa.PrivateKey, token []byte) ([]byte, error) {
    var tokenFlag byte
    if token == nil {
        // no session token found means we need to generate shared secret.
        // ecies shared secret is used as initial session token for new peers
        // generate shared key from prv and remote pubkey
        var err error
        if token, err = h.ecdhShared(prv); err != nil {
            return nil, err
        }
    } else {
        // for known peers, we use stored token from the previous session
        tokenFlag = 0x01
    }

    // sign known message:
    //   ecdh-shared-secret^nonce for new peers
    //   token^nonce for old peers
    signed := xor(token, h.initNonce)
    signature, err := crypto.Sign(signed, h.randomPrivKey.ExportECDSA())
    if err != nil {
        return nil, err
    }

    // encode auth message
    // signature || sha3(ecdhe-random-pubk) || pubk || nonce || token-flag
    msg := make([]byte, authMsgLen)
    n := copy(msg, signature)
    n += copy(msg[n:], crypto.Sha3(exportPubkey(&h.randomPrivKey.PublicKey)))
    n += copy(msg[n:], crypto.FromECDSAPub(&prv.PublicKey)[1:])
    n += copy(msg[n:], h.initNonce)
    msg[n] = tokenFlag

    // encrypt auth message using remote-pubk
    return ecies.Encrypt(rand.Reader, h.remotePub, msg, nil, nil)
}

// decodeAuthResp decode an encrypted authentication response message.
func (h *encHandshake) decodeAuthResp(auth []byte, prv *ecdsa.PrivateKey) error {
    msg, err := crypto.Decrypt(prv, auth)
    if err != nil {
        return fmt.Errorf("could not decrypt auth response (%v)", err)
    }
    h.respNonce = msg[pubLen : pubLen+shaLen]
    h.remoteRandomPub, err = importPublicKey(msg[:pubLen])
    if err != nil {
        return err
    }
    // ignore token flag for now
    return nil
}

// receiverEncHandshake negotiates a session token on conn.
// it should be called on the listening side of the connection.
//
// prv is the local client's private key.
// token is the token from a previous session with this node.
func receiverEncHandshake(conn io.ReadWriter, prv *ecdsa.PrivateKey, token []byte) (s secrets, err error) {
    // read remote auth sent by initiator.
    auth := make([]byte, encAuthMsgLen)
    if _, err := io.ReadFull(conn, auth); err != nil {
        return s, err
    }
    h, err := decodeAuthMsg(prv, token, auth)
    if err != nil {
        return s, err
    }

    // send auth response
    resp, err := h.authResp(prv, token)
    if err != nil {
        return s, err
    }
    if _, err = conn.Write(resp); err != nil {
        return s, err
    }

    return h.secrets(auth, resp)
}

func decodeAuthMsg(prv *ecdsa.PrivateKey, token []byte, auth []byte) (*encHandshake, error) {
    var err error
    h := new(encHandshake)
    // generate random keypair for session
    h.randomPrivKey, err = ecies.GenerateKey(rand.Reader, crypto.S256(), nil)
    if err != nil {
        return nil, err
    }
    // generate random nonce
    h.respNonce = make([]byte, shaLen)
    if _, err = rand.Read(h.respNonce); err != nil {
        return nil, err
    }

    msg, err := crypto.Decrypt(prv, auth)
    if err != nil {
        return nil, fmt.Errorf("could not decrypt auth message (%v)", err)
    }

    // decode message parameters
    // signature || sha3(ecdhe-random-pubk) || pubk || nonce || token-flag
    h.initNonce = msg[authMsgLen-shaLen-1 : authMsgLen-1]
    copy(h.remoteID[:], msg[sigLen+shaLen:sigLen+shaLen+pubLen])
    rpub, err := h.remoteID.Pubkey()
    if err != nil {
        return nil, fmt.Errorf("bad remoteID: %#v", err)
    }
    h.remotePub = ecies.ImportECDSAPublic(rpub)

    // recover remote random pubkey from signed message.
    if token == nil {
        // TODO: it is an error if the initiator has a token and we don't. check that.

        // no session token means we need to generate shared secret.
        // ecies shared secret is used as initial session token for new peers.
        // generate shared key from prv and remote pubkey.
        if token, err = h.ecdhShared(prv); err != nil {
            return nil, err
        }
    }
    signedMsg := xor(token, h.initNonce)
    remoteRandomPub, err := secp256k1.RecoverPubkey(signedMsg, msg[:sigLen])
    if err != nil {
        return nil, err
    }
    h.remoteRandomPub, _ = importPublicKey(remoteRandomPub)
    return h, nil
}

// authResp generates the encrypted authentication response message.
func (h *encHandshake) authResp(prv *ecdsa.PrivateKey, token []byte) ([]byte, error) {
    // responder auth message
    // E(remote-pubk, ecdhe-random-pubk || nonce || 0x0)
    resp := make([]byte, authRespLen)
    n := copy(resp, exportPubkey(&h.randomPrivKey.PublicKey))
    n += copy(resp[n:], h.respNonce)
    if token == nil {
        resp[n] = 0
    } else {
        resp[n] = 1
    }
    // encrypt using remote-pubk
    return ecies.Encrypt(rand.Reader, h.remotePub, resp, nil, nil)
}

// importPublicKey unmarshals 512 bit public keys.
func importPublicKey(pubKey []byte) (*ecies.PublicKey, error) {
    var pubKey65 []byte
    switch len(pubKey) {
    case 64:
        // add 'uncompressed key' flag
        pubKey65 = append([]byte{0x04}, pubKey...)
    case 65:
        pubKey65 = pubKey
    default:
        return nil, fmt.Errorf("invalid public key length %v (expect 64/65)", len(pubKey))
    }
    // TODO: fewer pointless conversions
    return ecies.ImportECDSAPublic(crypto.ToECDSAPub(pubKey65)), nil
}

func exportPubkey(pub *ecies.PublicKey) []byte {
    if pub == nil {
        panic("nil pubkey")
    }
    return elliptic.Marshal(pub.Curve, pub.X, pub.Y)[1:]
}

func xor(one, other []byte) (xor []byte) {
    xor = make([]byte, len(one))
    for i := 0; i < len(one); i++ {
        xor[i] = one[i] ^ other[i]
    }
    return xor
}

func readProtocolHandshake(rw MsgReadWriter, wantID discover.NodeID, our *protoHandshake) (*protoHandshake, error) {
    msg, err := rw.ReadMsg()
    if err != nil {
        return nil, err
    }
    if msg.Code == discMsg {
        // disconnect before protocol handshake is valid according to the
        // spec and we send it ourself if Server.addPeer fails.
        var reason [1]DiscReason
        rlp.Decode(msg.Payload, &reason)
        return nil, reason[0]
    }
    if msg.Code != handshakeMsg {
        return nil, fmt.Errorf("expected handshake, got %x", msg.Code)
    }
    if msg.Size > baseProtocolMaxMsgSize {
        return nil, fmt.Errorf("message too big (%d > %d)", msg.Size, baseProtocolMaxMsgSize)
    }
    var hs protoHandshake
    if err := msg.Decode(&hs); err != nil {
        return nil, err
    }
    // validate handshake info
    if hs.Version != our.Version {
        SendItems(rw, discMsg, DiscIncompatibleVersion)
        return nil, fmt.Errorf("required version %d, received %d\n", baseProtocolVersion, hs.Version)
    }
    if (hs.ID == discover.NodeID{}) {
        SendItems(rw, discMsg, DiscInvalidIdentity)
        return nil, errors.New("invalid public key in handshake")
    }
    if hs.ID != wantID {
        SendItems(rw, discMsg, DiscUnexpectedIdentity)
        return nil, errors.New("handshake node ID does not match encryption handshake")
    }
    return &hs, nil
}