Cloak/internal/multiplex/obfs.go

package multiplex

import (
	"crypto/aes"
	"crypto/cipher"
	"encoding/binary"
	"errors"
	"fmt"
	"github.com/cbeuw/Cloak/internal/common"
	"golang.org/x/crypto/chacha20poly1305"
	"golang.org/x/crypto/salsa20"
)

type Obfser func(*Frame, []byte, int) (int, error)
type Deobfser func(*Frame, []byte) error

var u32 = binary.BigEndian.Uint32
var u64 = binary.BigEndian.Uint64
var putU32 = binary.BigEndian.PutUint32
var putU64 = binary.BigEndian.PutUint64

const frameHeaderLength = 14
const salsa20NonceSize = 8

const (
	EncryptionMethodPlain = iota
	EncryptionMethodAESGCM
	EncryptionMethodChaha20Poly1305
)

// Obfuscator is responsible for serialisation, obfuscation, and optional encryption of data frames.
type Obfuscator struct {
	// Used in Stream.Write. Add multiplexing headers, encrypt and add TLS header
	Obfs Obfser
	// Remove TLS header, decrypt and unmarshall frames
	Deobfs     Deobfser
	SessionKey [32]byte

	maxOverhead int
}

// MakeObfs returns a function of type Obfser. An Obfser takes three arguments:
// a *Frame with all the field set correctly, a []byte as buffer to put encrypted
// message in, and an int called payloadOffsetInBuf to be used when *Frame.payload
// is in the byte slice used as buffer (2nd argument). payloadOffsetInBuf specifies
// the index at which data belonging to *Frame.Payload starts in the buffer.
func MakeObfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Obfser {
	// The method here is to use the first payloadCipher.NonceSize() bytes of the serialised frame header
	// as iv/nonce for the AEAD cipher to encrypt the frame payload. Then we use
	// the authentication tag produced appended to the end of the ciphertext (of size payloadCipher.Overhead())
	// as nonce for Salsa20 to encrypt the frame header. Both with SessionKey as keys.
	//
	// Several cryptographic guarantees we have made here: that payloadCipher, as an AEAD, is given a unique
	// iv/nonce each time, relative to its key; that the frame header encryptor Salsa20 is given a unique
	// nonce each time, relative to its key; and that the authenticity of frame header is checked.
	//
	// The payloadCipher is given a unique iv/nonce each time because it is derived from the frame header, which
	// contains the monotonically increasing stream id (uint32) and frame sequence (uint64). There will be a nonce
	// reuse after 2^64-1 frames sent (sent, not received because frames going different ways are sequenced
	// independently) by a stream, or after 2^32-1 streams created in a single session. We consider these number
	// to be large enough that they may never happen in reasonable time frames. Of course, different sessions
	// will produce the same combination of stream id and frame sequence, but they will have different session keys.
	//
	// Salsa20 is assumed to be given a unique nonce each time because we assume the tags produced by payloadCipher
	// AEAD is unique each time, as payloadCipher itself is given a unique iv/nonce each time due to points made above.
	// This is relatively a weak guarantee as we are assuming AEADs to produce different tags given different iv/nonces.
	// This is almost certainly true but I cannot find a source that outright states this.
	//
	// Because the frame header, before it being encrypted, is fed into the AEAD, it is also authenticated.
	// (rfc5116 s.2.1 "The nonce is authenticated internally to the algorithm").
	//
	// In case the user chooses to not encrypt the frame payload, payloadCipher will be nil. In this scenario,
	// we pad the frame payload with random bytes until it reaches Salsa20's nonce size (8 bytes). Then we simply
	// encrypt the frame header with the last 8 bytes of frame payload as nonce.
	// If the payload provided by the user is greater than 8 bytes, then we use entirely the user input as nonce.
	// We can't ensure its uniqueness ourselves, which is why plaintext mode must only be used when the user input
	// is already random-like. For Cloak it would normally mean that the user is using a proxy protocol that sends
	// encrypted data.
	obfs := func(f *Frame, buf []byte, payloadOffsetInBuf int) (int, error) {
		payloadLen := len(f.Payload)
		if payloadLen == 0 {
			return 0, errors.New("payload cannot be empty")
		}
		var extraLen int
		if payloadCipher == nil {
			extraLen = salsa20NonceSize - payloadLen
			if extraLen < 0 {
				// if our payload is already greater than 8 bytes
				extraLen = 0
			}
		} else {
			extraLen = payloadCipher.Overhead()
			if extraLen < salsa20NonceSize {
				return 0, errors.New("AEAD's Overhead cannot be fewer than 8 bytes")
			}
		}

		usefulLen := frameHeaderLength + payloadLen + extraLen
		if len(buf) < usefulLen {
			return 0, errors.New("obfs buffer too small")
		}
		// we do as much in-place as possible to save allocation
		payload := buf[frameHeaderLength : frameHeaderLength+payloadLen]
		if payloadOffsetInBuf != frameHeaderLength {
			// if payload is not at the correct location in buffer
			copy(payload, f.Payload)
		}

		header := buf[:frameHeaderLength]
		putU32(header[0:4], f.StreamID)
		putU64(header[4:12], f.Seq)
		header[12] = f.Closing
		header[13] = byte(extraLen)

		if payloadCipher == nil {
			if extraLen != 0 { // read nonce
				extra := buf[usefulLen-extraLen : usefulLen]
				common.CryptoRandRead(extra)
			}
		} else {
			payloadCipher.Seal(payload[:0], header[:payloadCipher.NonceSize()], payload, nil)
		}

		nonce := buf[usefulLen-salsa20NonceSize : usefulLen]
		salsa20.XORKeyStream(header, header, nonce, &salsaKey)

		return usefulLen, nil
	}
	return obfs
}

// MakeDeobfs returns a function Deobfser. A Deobfser takes in a single byte slice,
// containing the message to be decrypted, and returns a *Frame containing the frame
// information and plaintext
func MakeDeobfs(salsaKey [32]byte, payloadCipher cipher.AEAD) Deobfser {
	// frame header length + minimum data size (i.e. nonce size of salsa20)
	const minInputLen = frameHeaderLength + salsa20NonceSize
	deobfs := func(f *Frame, in []byte) error {
		if len(in) < minInputLen {
			return fmt.Errorf("input size %v, but it cannot be shorter than %v bytes", len(in), minInputLen)
		}

		header := in[:frameHeaderLength]
		pldWithOverHead := in[frameHeaderLength:] // payload + potential overhead

		nonce := in[len(in)-salsa20NonceSize:]
		salsa20.XORKeyStream(header, header, nonce, &salsaKey)

		streamID := u32(header[0:4])
		seq := u64(header[4:12])
		closing := header[12]
		extraLen := header[13]

		usefulPayloadLen := len(pldWithOverHead) - int(extraLen)
		if usefulPayloadLen < 0 || usefulPayloadLen > len(pldWithOverHead) {
			return errors.New("extra length is negative or extra length is greater than total pldWithOverHead length")
		}

		var outputPayload []byte

		if payloadCipher == nil {
			if extraLen == 0 {
				outputPayload = pldWithOverHead
			} else {
				outputPayload = pldWithOverHead[:usefulPayloadLen]
			}
		} else {
			_, err := payloadCipher.Open(pldWithOverHead[:0], header[:payloadCipher.NonceSize()], pldWithOverHead, nil)
			if err != nil {
				return err
			}
			outputPayload = pldWithOverHead[:usefulPayloadLen]
		}

		f.StreamID = streamID
		f.Seq = seq
		f.Closing = closing
		f.Payload = outputPayload
		return nil
	}
	return deobfs
}

func MakeObfuscator(encryptionMethod byte, sessionKey [32]byte) (obfuscator Obfuscator, err error) {
	obfuscator = Obfuscator{
		SessionKey: sessionKey,
	}
	var payloadCipher cipher.AEAD
	switch encryptionMethod {
	case EncryptionMethodPlain:
		payloadCipher = nil
		obfuscator.maxOverhead = salsa20NonceSize
	case EncryptionMethodAESGCM:
		var c cipher.Block
		c, err = aes.NewCipher(sessionKey[:])
		if err != nil {
			return
		}
		payloadCipher, err = cipher.NewGCM(c)
		if err != nil {
			return
		}
		obfuscator.maxOverhead = payloadCipher.Overhead()
	case EncryptionMethodChaha20Poly1305:
		payloadCipher, err = chacha20poly1305.New(sessionKey[:])
		if err != nil {
			return
		}
		obfuscator.maxOverhead = payloadCipher.Overhead()
	default:
		return obfuscator, errors.New("Unknown encryption method")
	}

	if payloadCipher != nil {
		if payloadCipher.NonceSize() > frameHeaderLength {
			return obfuscator, errors.New("payload AEAD's nonce size cannot be greater than size of frame header")
		}
	}

	obfuscator.Obfs = MakeObfs(sessionKey, payloadCipher)
	obfuscator.Deobfs = MakeDeobfs(sessionKey, payloadCipher)
	return
}