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encryption.go
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encryption.go
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package mux
import (
"crypto/cipher"
"crypto/ed25519"
"encoding/binary"
"errors"
"fmt"
"io"
"net"
"golang.org/x/crypto/blake2b"
"golang.org/x/crypto/chacha20poly1305"
"golang.org/x/crypto/curve25519"
"lukechampine.com/frand"
)
const (
cipherChaCha20Poly1305 = "Chacha20P1305\x00\x00\x00" // padded to 16 bytes
chachaPoly1305NonceSize = 12
chachaPoly1305TagSize = 16
chachaOverhead = chachaPoly1305NonceSize + chachaPoly1305TagSize
)
func generateX25519KeyPair() (xsk, xpk [32]byte) {
xsk = frand.Entropy256()
pk, _ := curve25519.X25519(xsk[:], curve25519.Basepoint)
copy(xpk[:], pk)
return
}
func deriveSharedSecret(xsk, xpk [32]byte) ([32]byte, error) {
secret, err := curve25519.X25519(xsk[:], xpk[:])
var dst [32]byte
copy(dst[:], secret)
return dst, err
}
func encryptInPlace(buf []byte, aead cipher.AEAD) {
nonce, plaintext := buf[:chachaPoly1305NonceSize], buf[chachaPoly1305NonceSize:len(buf)-chachaPoly1305TagSize]
frand.Read(nonce)
aead.Seal(plaintext[:0], nonce, plaintext, nil)
}
func decryptInPlace(buf []byte, aead cipher.AEAD) ([]byte, error) {
nonce, ciphertext := buf[:chachaPoly1305NonceSize], buf[chachaPoly1305NonceSize:]
return aead.Open(ciphertext[:0], nonce, ciphertext, nil)
}
func encryptFrame(buf []byte, h frameHeader, payload []byte, packetSize int, aead cipher.AEAD) []byte {
// pad frame to packet boundary
numPackets := (encryptedHeaderSize + (len(payload) + chachaOverhead) + (packetSize - 1)) / packetSize
frame := buf[:numPackets*packetSize]
// encode + encrypt header
encodeFrameHeader(frame[chachaPoly1305NonceSize:][:frameHeaderSize], h)
encryptInPlace(frame[:encryptedHeaderSize], aead)
// pad + encrypt payload
copy(frame[encryptedHeaderSize+chachaPoly1305NonceSize:], payload)
encryptInPlace(frame[encryptedHeaderSize:], aead)
return frame
}
func decryptFrameHeader(buf []byte, aead cipher.AEAD) (frameHeader, error) {
buf, err := decryptInPlace(buf, aead)
if err != nil {
return frameHeader{}, err
}
return decodeFrameHeader(buf), nil
}
func readEncryptedFrame(r io.Reader, buf []byte, packetSize int, aead cipher.AEAD) (frameHeader, []byte, error) {
// read, decrypt, and decode header
if _, err := io.ReadFull(r, buf[:encryptedHeaderSize]); err != nil {
return frameHeader{}, nil, err
}
h, err := decryptFrameHeader(buf[:encryptedHeaderSize], aead)
if err != nil {
return frameHeader{}, nil, fmt.Errorf("could not decrypt header: %w", err)
}
numPackets := (encryptedHeaderSize + (int(h.length) + chachaOverhead) + (packetSize - 1)) / packetSize
paddedSize := numPackets*packetSize - encryptedHeaderSize
if h.length > uint32(len(buf)) || paddedSize > len(buf) {
return frameHeader{}, nil, errors.New("peer sent too-large frame")
}
// read (padded) payload
if _, err := io.ReadFull(r, buf[:paddedSize]); err != nil {
return frameHeader{}, nil, err
}
// decrypt payload
payload, err := decryptInPlace(buf[:paddedSize], aead)
if err != nil {
return frameHeader{}, nil, fmt.Errorf("could not decrypt payload: %w", err)
}
return h, payload[:h.length], nil
}
func initiateEncryptionHandshake(conn net.Conn, theirKey ed25519.PublicKey) (cipher.AEAD, error) {
xsk, xpk := generateX25519KeyPair()
// write request
buf := make([]byte, 112) // large enough to hold request + response
frameBuf := buf[:frameHeaderSize+32+8+16]
payload := frameBuf[frameHeaderSize:]
encodeFrameHeader(frameBuf, frameHeader{
id: idEstablishEncryption,
length: uint32(len(payload)),
})
copy(payload[:32], xpk[:])
binary.LittleEndian.PutUint64(payload[32:], 1) // number of ciphers we're offering
copy(payload[40:], cipherChaCha20Poly1305)
if _, err := conn.Write(frameBuf); err != nil {
return nil, err
}
// read response
h, payload, err := readFrame(conn, buf)
if err != nil {
return nil, err
} else if h.id != idEstablishEncryption {
return nil, errors.New("invalid handshake ID")
} else if h.length < 32+64+16 {
return nil, errors.New("handshake payload is too short")
} else if string(payload[32+64:]) != cipherChaCha20Poly1305 {
return nil, errors.New("invalid cipher selected")
}
var rxpk [32]byte
copy(rxpk[:], payload[:32])
sig := payload[32:96]
// verify signature
sigHash := blake2b.Sum256(append(rxpk[:], xpk[:]...))
if !ed25519.Verify(theirKey, sigHash[:], sig) {
return nil, errors.New("invalid signature")
}
// derive encryption key
cipherKey, err := deriveSharedSecret(xsk, rxpk)
if err != nil {
return nil, err
}
return chacha20poly1305.New(cipherKey[:])
}
func acceptEncryptionHandshake(conn net.Conn, ourKey ed25519.PrivateKey) (cipher.AEAD, error) {
xsk, xpk := generateX25519KeyPair()
// read request
buf := make([]byte, 1024) // large enough to hold many ciphers
h, payload, err := readFrame(conn, buf)
if err != nil {
return nil, err
} else if h.id != idEstablishEncryption {
return nil, errors.New("invalid handshake ID")
} else if h.length < 8+32+16 {
return nil, errors.New("handshake payload is too short")
}
// parse pubkey
var rxpk [32]byte
copy(rxpk[:], payload[:32])
// select cipher
numCiphers := binary.LittleEndian.Uint64(payload[32:])
if uint64(h.length-40)/16 < numCiphers {
return nil, errors.New("invalid cipher encoding")
}
var supportsChaCha bool
for i := uint64(0); i < numCiphers; i++ {
supportsChaCha = supportsChaCha || string(payload[40+16*i:][:16]) == cipherChaCha20Poly1305
}
if !supportsChaCha {
return nil, errors.New("no cipher overlap")
}
// write response
sigHash := blake2b.Sum256(append(xpk[:], rxpk[:]...))
sig := ed25519.Sign(ourKey, sigHash[:])
frameBuf := buf[:frameHeaderSize+32+64+16]
payload = frameBuf[frameHeaderSize:]
encodeFrameHeader(frameBuf, frameHeader{
id: idEstablishEncryption,
length: uint32(len(payload)),
})
copy(payload[:32], xpk[:])
copy(payload[32:96], sig)
copy(payload[96:], cipherChaCha20Poly1305)
if _, err := conn.Write(frameBuf); err != nil {
return nil, err
}
// derive encryption key
cipherKey, err := deriveSharedSecret(xsk, rxpk)
if err != nil {
return nil, err
}
return chacha20poly1305.New(cipherKey[:])
}