path: root/p2p/rlpx.go

package p2p

import (
    "bytes"
    "crypto/aes"
    "crypto/cipher"
    "crypto/hmac"
    "errors"
    "hash"
    "io"

    "github.com/ethereum/go-ethereum/rlp"
)

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)

    maxUint24 = ^uint32(0) >> 8
)

// 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
}

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)

    // 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
    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)
}