path: root/p2p/rlpx.go

// Copyright 2015 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 p2p

import (
    "bytes"
    "crypto/aes"
    "crypto/cipher"
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/hmac"
    "crypto/rand"
    "encoding/binary"
    "errors"
    "fmt"
    "hash"
    "io"
    "io/ioutil"
    mrand "math/rand"
    "net"
    "sync"
    "time"

    "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"
    "github.com/golang/snappy"
)

const (
    maxUint24 = ^uint32(0) >> 8

    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

    eciesOverhead = 65 /* pubkey */ + 16 /* IV */ + 32 /* MAC */

    encAuthMsgLen  = authMsgLen + eciesOverhead  // size of encrypted pre-EIP-8 initiator handshake
    encAuthRespLen = authRespLen + eciesOverhead // size of encrypted pre-EIP-8 handshake reply

    // total timeout for encryption handshake and protocol
    // handshake in both directions.
    handshakeTimeout = 5 * time.Second

    // This is the timeout for sending the disconnect reason.
    // This is shorter than the usual timeout because we don't want
    // to wait if the connection is known to be bad anyway.
    discWriteTimeout = 1 * time.Second
)

// errPlainMessageTooLarge is returned if a decompressed message length exceeds
// the allowed 24 bits (i.e. length >= 16MB).
var errPlainMessageTooLarge = errors.New("message length >= 16MB")

// rlpx is the transport protocol used by actual (non-test) connections.
// It wraps the frame encoder with locks and read/write deadlines.
type rlpx struct {
    fd net.Conn

    rmu, wmu sync.Mutex
    rw       *rlpxFrameRW
}

func newRLPX(fd net.Conn) transport {
    fd.SetDeadline(time.Now().Add(handshakeTimeout))
    return &rlpx{fd: fd}
}

func (t *rlpx) ReadMsg() (Msg, error) {
    t.rmu.Lock()
    defer t.rmu.Unlock()
    t.fd.SetReadDeadline(time.Now().Add(frameReadTimeout))
    return t.rw.ReadMsg()
}

func (t *rlpx) WriteMsg(msg Msg) error {
    t.wmu.Lock()
    defer t.wmu.Unlock()
    t.fd.SetWriteDeadline(time.Now().Add(frameWriteTimeout))
    return t.rw.WriteMsg(msg)
}

func (t *rlpx) close(err error) {
    t.wmu.Lock()
    defer t.wmu.Unlock()
    // Tell the remote end why we're disconnecting if possible.
    if t.rw != nil {
        if r, ok := err.(DiscReason); ok && r != DiscNetworkError {
            // rlpx tries to send DiscReason to disconnected peer
            // if the connection is net.Pipe (in-memory simulation)
            // it hangs forever, since net.Pipe does not implement
            // a write deadline. Because of this only try to send
            // the disconnect reason message if there is no error.
            if err := t.fd.SetWriteDeadline(time.Now().Add(discWriteTimeout)); err == nil {
                SendItems(t.rw, discMsg, r)
            }
        }
    }
    t.fd.Close()
}

func (t *rlpx) doProtoHandshake(our *protoHandshake) (their *protoHandshake, err error) {
    // Writing our handshake happens concurrently, 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, 1)
    go func() { werr <- Send(t.rw, handshakeMsg, our) }()
    if their, err = readProtocolHandshake(t.rw, our); err != nil {
        <-werr // make sure the write terminates too
        return nil, err
    }
    if err := <-werr; err != nil {
        return nil, fmt.Errorf("write error: %v", err)
    }
    // If the protocol version supports Snappy encoding, upgrade immediately
    t.rw.snappy = their.Version >= snappyProtocolVersion

    return their, nil
}

func readProtocolHandshake(rw MsgReader, our *protoHandshake) (*protoHandshake, error) {
    msg, err := rw.ReadMsg()
    if err != nil {
        return nil, err
    }
    if msg.Size > baseProtocolMaxMsgSize {
        return nil, fmt.Errorf("message too big")
    }
    if msg.Code == discMsg {
        // Disconnect before protocol handshake is valid according to the
        // spec and we send it ourself if the posthanshake checks fail.
        // We can't return the reason directly, though, because it is echoed
        // back otherwise. Wrap it in a string instead.
        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)
    }
    var hs protoHandshake
    if err := msg.Decode(&hs); err != nil {
        return nil, err
    }
    if (hs.ID == discover.NodeID{}) {
        return nil, DiscInvalidIdentity
    }
    return &hs, nil
}

// doEncHandshake runs the protocol handshake using authenticated
// messages. the protocol handshake is the first authenticated message
// and also verifies whether the encryption handshake 'worked' and the
// remote side actually provided the right public key.
func (t *rlpx) doEncHandshake(prv *ecdsa.PrivateKey, dial *discover.Node) (discover.NodeID, error) {
    var (
        sec secrets
        err error
    )
    if dial == nil {
        sec, err = receiverEncHandshake(t.fd, prv)
    } else {
        sec, err = initiatorEncHandshake(t.fd, prv, dial.ID)
    }
    if err != nil {
        return discover.NodeID{}, err
    }
    t.wmu.Lock()
    t.rw = newRLPXFrameRW(t.fd, sec)
    t.wmu.Unlock()
    return sec.RemoteID, 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 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
}

// RLPx v4 handshake auth (defined in EIP-8).
type authMsgV4 struct {
    gotPlain bool // whether read packet had plain format.

    Signature       [sigLen]byte
    InitiatorPubkey [pubLen]byte
    Nonce           [shaLen]byte
    Version         uint

    // Ignore additional fields (forward-compatibility)
    Rest []rlp.RawValue `rlp:"tail"`
}

// RLPx v4 handshake response (defined in EIP-8).
type authRespV4 struct {
    RandomPubkey [pubLen]byte
    Nonce        [shaLen]byte
    Version      uint

    // Ignore additional fields (forward-compatibility)
    Rest []rlp.RawValue `rlp:"tail"`
}

// 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.Keccak256(ecdheSecret, crypto.Keccak256(h.respNonce, h.initNonce))
    aesSecret := crypto.Keccak256(ecdheSecret, sharedSecret)
    s := secrets{
        RemoteID: h.remoteID,
        AES:      aesSecret,
        MAC:      crypto.Keccak256(ecdheSecret, aesSecret),
    }

    // 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
}

// staticSharedSecret returns the static shared secret, the result
// of key agreement between the local and remote static node key.
func (h *encHandshake) staticSharedSecret(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.
func initiatorEncHandshake(conn io.ReadWriter, prv *ecdsa.PrivateKey, remoteID discover.NodeID) (s secrets, err error) {
    h := &encHandshake{initiator: true, remoteID: remoteID}
    authMsg, err := h.makeAuthMsg(prv)
    if err != nil {
        return s, err
    }
    authPacket, err := sealEIP8(authMsg, h)
    if err != nil {
        return s, err
    }
    if _, err = conn.Write(authPacket); err != nil {
        return s, err
    }

    authRespMsg := new(authRespV4)
    authRespPacket, err := readHandshakeMsg(authRespMsg, encAuthRespLen, prv, conn)
    if err != nil {
        return s, err
    }
    if err := h.handleAuthResp(authRespMsg); err != nil {
        return s, err
    }
    return h.secrets(authPacket, authRespPacket)
}

// makeAuthMsg creates the initiator handshake message.
func (h *encHandshake) makeAuthMsg(prv *ecdsa.PrivateKey) (*authMsgV4, error) {
    rpub, err := h.remoteID.Pubkey()
    if err != nil {
        return nil, fmt.Errorf("bad remoteID: %v", err)
    }
    h.remotePub = ecies.ImportECDSAPublic(rpub)
    // Generate random initiator nonce.
    h.initNonce = make([]byte, shaLen)
    if _, err := rand.Read(h.initNonce); err != nil {
        return nil, err
    }
    // Generate random keypair to for ECDH.
    h.randomPrivKey, err = ecies.GenerateKey(rand.Reader, crypto.S256(), nil)
    if err != nil {
        return nil, err
    }

    // Sign known message: static-shared-secret ^ nonce
    token, err := h.staticSharedSecret(prv)
    if err != nil {
        return nil, err
    }
    signed := xor(token, h.initNonce)
    signature, err := crypto.Sign(signed, h.randomPrivKey.ExportECDSA())
    if err != nil {
        return nil, err
    }

    msg := new(authMsgV4)
    copy(msg.Signature[:], signature)
    copy(msg.InitiatorPubkey[:], crypto.FromECDSAPub(&prv.PublicKey)[1:])
    copy(msg.Nonce[:], h.initNonce)
    msg.Version = 4
    return msg, nil
}

func (h *encHandshake) handleAuthResp(msg *authRespV4) (err error) {
    h.respNonce = msg.Nonce[:]
    h.remoteRandomPub, err = importPublicKey(msg.RandomPubkey[:])
    return err
}

// 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.
func receiverEncHandshake(conn io.ReadWriter, prv *ecdsa.PrivateKey) (s secrets, err error) {
    authMsg := new(authMsgV4)
    authPacket, err := readHandshakeMsg(authMsg, encAuthMsgLen, prv, conn)
    if err != nil {
        return s, err
    }
    h := new(encHandshake)
    if err := h.handleAuthMsg(authMsg, prv); err != nil {
        return s, err
    }

    authRespMsg, err := h.makeAuthResp()
    if err != nil {
        return s, err
    }
    var authRespPacket []byte
    if authMsg.gotPlain {
        authRespPacket, err = authRespMsg.sealPlain(h)
    } else {
        authRespPacket, err = sealEIP8(authRespMsg, h)
    }
    if err != nil {
        return s, err
    }
    if _, err = conn.Write(authRespPacket); err != nil {
        return s, err
    }
    return h.secrets(authPacket, authRespPacket)
}

func (h *encHandshake) handleAuthMsg(msg *authMsgV4, prv *ecdsa.PrivateKey) error {
    // Import the remote identity.
    h.initNonce = msg.Nonce[:]
    h.remoteID = msg.InitiatorPubkey
    rpub, err := h.remoteID.Pubkey()
    if err != nil {
        return fmt.Errorf("bad remoteID: %#v", err)
    }
    h.remotePub = ecies.ImportECDSAPublic(rpub)

    // Generate random keypair for ECDH.
    // If a private key is already set, use it instead of generating one (for testing).
    if h.randomPrivKey == nil {
        h.randomPrivKey, err = ecies.GenerateKey(rand.Reader, crypto.S256(), nil)
        if err != nil {
            return err
        }
    }

    // Check the signature.
    token, err := h.staticSharedSecret(prv)
    if err != nil {
        return err
    }
    signedMsg := xor(token, h.initNonce)
    remoteRandomPub, err := secp256k1.RecoverPubkey(signedMsg, msg.Signature[:])
    if err != nil {
        return err
    }
    h.remoteRandomPub, _ = importPublicKey(remoteRandomPub)
    return nil
}

func (h *encHandshake) makeAuthResp() (msg *authRespV4, err error) {
    // Generate random nonce.
    h.respNonce = make([]byte, shaLen)
    if _, err = rand.Read(h.respNonce); err != nil {
        return nil, err
    }

    msg = new(authRespV4)
    copy(msg.Nonce[:], h.respNonce)
    copy(msg.RandomPubkey[:], exportPubkey(&h.randomPrivKey.PublicKey))
    msg.Version = 4
    return msg, nil
}

func (msg *authMsgV4) sealPlain(h *encHandshake) ([]byte, error) {
    buf := make([]byte, authMsgLen)
    n := copy(buf, msg.Signature[:])
    n += copy(buf[n:], crypto.Keccak256(exportPubkey(&h.randomPrivKey.PublicKey)))
    n += copy(buf[n:], msg.InitiatorPubkey[:])
    n += copy(buf[n:], msg.Nonce[:])
    buf[n] = 0 // token-flag
    return ecies.Encrypt(rand.Reader, h.remotePub, buf, nil, nil)
}

func (msg *authMsgV4) decodePlain(input []byte) {
    n := copy(msg.Signature[:], input)
    n += shaLen // skip sha3(initiator-ephemeral-pubk)
    n += copy(msg.InitiatorPubkey[:], input[n:])
    copy(msg.Nonce[:], input[n:])
    msg.Version = 4
    msg.gotPlain = true
}

func (msg *authRespV4) sealPlain(hs *encHandshake) ([]byte, error) {
    buf := make([]byte, authRespLen)
    n := copy(buf, msg.RandomPubkey[:])
    copy(buf[n:], msg.Nonce[:])
    return ecies.Encrypt(rand.Reader, hs.remotePub, buf, nil, nil)
}

func (msg *authRespV4) decodePlain(input []byte) {
    n := copy(msg.RandomPubkey[:], input)
    copy(msg.Nonce[:], input[n:])
    msg.Version = 4
}

var padSpace = make([]byte, 300)

func sealEIP8(msg interface{}, h *encHandshake) ([]byte, error) {
    buf := new(bytes.Buffer)
    if err := rlp.Encode(buf, msg); err != nil {
        return nil, err
    }
    // pad with random amount of data. the amount needs to be at least 100 bytes to make
    // the message distinguishable from pre-EIP-8 handshakes.
    pad := padSpace[:mrand.Intn(len(padSpace)-100)+100]
    buf.Write(pad)
    prefix := make([]byte, 2)
    binary.BigEndian.PutUint16(prefix, uint16(buf.Len()+eciesOverhead))

    enc, err := ecies.Encrypt(rand.Reader, h.remotePub, buf.Bytes(), nil, prefix)
    return append(prefix, enc...), err
}

type plainDecoder interface {
    decodePlain([]byte)
}

func readHandshakeMsg(msg plainDecoder, plainSize int, prv *ecdsa.PrivateKey, r io.Reader) ([]byte, error) {
    buf := make([]byte, plainSize)
    if _, err := io.ReadFull(r, buf); err != nil {
        return buf, err
    }
    // Attempt decoding pre-EIP-8 "plain" format.
    key := ecies.ImportECDSA(prv)
    if dec, err := key.Decrypt(buf, nil, nil); err == nil {
        msg.decodePlain(dec)
        return buf, nil
    }
    // Could be EIP-8 format, try that.
    prefix := buf[:2]
    size := binary.BigEndian.Uint16(prefix)
    if size < uint16(plainSize) {
        return buf, fmt.Errorf("size underflow, need at least %d bytes", plainSize)
    }
    buf = append(buf, make([]byte, size-uint16(plainSize)+2)...)
    if _, err := io.ReadFull(r, buf[plainSize:]); err != nil {
        return buf, err
    }
    dec, err := key.Decrypt(buf[2:], nil, prefix)
    if err != nil {
        return buf, err
    }
    // Can't use rlp.DecodeBytes here because it rejects
    // trailing data (forward-compatibility).
    s := rlp.NewStream(bytes.NewReader(dec), 0)
    return buf, s.Decode(msg)
}

// 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
    pub, err := crypto.UnmarshalPubkey(pubKey65)
    if err != nil {
        return nil, err
    }
    return ecies.ImportECDSAPublic(pub), 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
}

var (
    // this is used in place of actual frame header data.
    // TODO: replace this when Msg contains the protocol type code.
    zeroHeader = []byte{0xC2, 0x80, 0x80}
    // sixteen zero bytes
    zero16 = make([]byte, 16)
)

// rlpxFrameRW implements a simplified version of RLPx framing.
// chunked messages are not supported and all headers are equal to
// zeroHeader.
//
// rlpxFrameRW is not safe for concurrent use from multiple goroutines.
type rlpxFrameRW struct {
    conn io.ReadWriter
    enc  cipher.Stream
    dec  cipher.Stream

    macCipher  cipher.Block
    egressMAC  hash.Hash
    ingressMAC hash.Hash

    snappy bool
}

func newRLPXFrameRW(conn io.ReadWriter, s secrets) *rlpxFrameRW {
    macc, err := aes.NewCipher(s.MAC)
    if err != nil {
        panic("invalid MAC secret: " + err.Error())
    }
    encc, err := aes.NewCipher(s.AES)
    if err != nil {
        panic("invalid AES secret: " + err.Error())
    }
    // we use an all-zeroes IV for AES because the key used
    // for encryption is ephemeral.
    iv := make([]byte, encc.BlockSize())
    return &rlpxFrameRW{
        conn:       conn,
        enc:        cipher.NewCTR(encc, iv),
        dec:        cipher.NewCTR(encc, iv),
        macCipher:  macc,
        egressMAC:  s.EgressMAC,
        ingressMAC: s.IngressMAC,
    }
}

func (rw *rlpxFrameRW) WriteMsg(msg Msg) error {
    ptype, _ := rlp.EncodeToBytes(msg.Code)

    // if snappy is enabled, compress message now
    if rw.snappy {
        if msg.Size > maxUint24 {
            return errPlainMessageTooLarge
        }
        payload, _ := ioutil.ReadAll(msg.Payload)
        payload = snappy.Encode(nil, payload)

        msg.Payload = bytes.NewReader(payload)
        msg.Size = uint32(len(payload))
    }
    // write header
    headbuf := make([]byte, 32)
    fsize := uint32(len(ptype)) + msg.Size
    if fsize > maxUint24 {
        return errors.New("message size overflows uint24")
    }
    putInt24(fsize, headbuf) // TODO: check overflow
    copy(headbuf[3:], zeroHeader)
    rw.enc.XORKeyStream(headbuf[:16], headbuf[:16]) // first half is now encrypted

    // write header MAC
    copy(headbuf[16:], updateMAC(rw.egressMAC, rw.macCipher, headbuf[:16]))
    if _, err := rw.conn.Write(headbuf); err != nil {
        return err
    }

    // write encrypted frame, updating the egress MAC hash with
    // the data written to conn.
    tee := cipher.StreamWriter{S: rw.enc, W: io.MultiWriter(rw.conn, rw.egressMAC)}
    if _, err := tee.Write(ptype); err != nil {
        return err
    }
    if _, err := io.Copy(tee, msg.Payload); err != nil {
        return err
    }
    if padding := fsize % 16; padding > 0 {
        if _, err := tee.Write(zero16[:16-padding]); err != nil {
            return err
        }
    }

    // write frame MAC. egress MAC hash is up to date because
    // frame content was written to it as well.
    fmacseed := rw.egressMAC.Sum(nil)
    mac := updateMAC(rw.egressMAC, rw.macCipher, fmacseed)
    _, err := rw.conn.Write(mac)
    return err
}

func (rw *rlpxFrameRW) ReadMsg() (msg Msg, err error) {
    // read the header
    headbuf := make([]byte, 32)
    if _, err := io.ReadFull(rw.conn, headbuf); err != nil {
        return msg, err
    }
    // verify header mac
    shouldMAC := updateMAC(rw.ingressMAC, rw.macCipher, headbuf[:16])
    if !hmac.Equal(shouldMAC, headbuf[16:]) {
        return msg, errors.New("bad header MAC")
    }
    rw.dec.XORKeyStream(headbuf[:16], headbuf[:16]) // first half is now decrypted
    fsize := readInt24(headbuf)
    // ignore protocol type for now

    // read the frame content
    var rsize = fsize // frame size rounded up to 16 byte boundary
    if padding := fsize % 16; padding > 0 {
        rsize += 16 - padding
    }
    framebuf := make([]byte, rsize)
    if _, err := io.ReadFull(rw.conn, framebuf); err != nil {
        return msg, err
    }

    // read and validate frame MAC. we can re-use headbuf for that.
    rw.ingressMAC.Write(framebuf)
    fmacseed := rw.ingressMAC.Sum(nil)
    if _, err := io.ReadFull(rw.conn, headbuf[:16]); err != nil {
        return msg, err
    }
    shouldMAC = updateMAC(rw.ingressMAC, rw.macCipher, fmacseed)
    if !hmac.Equal(shouldMAC, headbuf[:16]) {
        return msg, errors.New("bad frame MAC")
    }

    // decrypt frame content
    rw.dec.XORKeyStream(framebuf, framebuf)

    // decode message code
    content := bytes.NewReader(framebuf[:fsize])
    if err := rlp.Decode(content, &msg.Code); err != nil {
        return msg, err
    }
    msg.Size = uint32(content.Len())
    msg.Payload = content

    // if snappy is enabled, verify and decompress message
    if rw.snappy {
        payload, err := ioutil.ReadAll(msg.Payload)
        if err != nil {
            return msg, err
        }
        size, err := snappy.DecodedLen(payload)
        if err != nil {
            return msg, err
        }
        if size > int(maxUint24) {
            return msg, errPlainMessageTooLarge
        }
        payload, err = snappy.Decode(nil, payload)
        if err != nil {
            return msg, err
        }
        msg.Size, msg.Payload = uint32(size), bytes.NewReader(payload)
    }
    return msg, nil
}

// updateMAC reseeds the given hash with encrypted seed.
// it returns the first 16 bytes of the hash sum after seeding.
func updateMAC(mac hash.Hash, block cipher.Block, seed []byte) []byte {
    aesbuf := make([]byte, aes.BlockSize)
    block.Encrypt(aesbuf, mac.Sum(nil))
    for i := range aesbuf {
        aesbuf[i] ^= seed[i]
    }
    mac.Write(aesbuf)
    return mac.Sum(nil)[:16]
}

func readInt24(b []byte) uint32 {
    return uint32(b[2]) | uint32(b[1])<<8 | uint32(b[0])<<16
}

func putInt24(v uint32, b []byte) {
    b[0] = byte(v >> 16)
    b[1] = byte(v >> 8)
    b[2] = byte(v)
}