cipher.go

package ws

import (
	"encoding/binary"
	"unsafe"
)

// Cipher applies XOR cipher to the payload using mask.
// Offset is used to cipher chunked data (e.g. in io.Reader implementations).
//
// To convert masked data into unmasked data, or vice versa, the following
// algorithm is applied.  The same algorithm applies regardless of the
// direction of the translation, e.g., the same steps are applied to
// mask the data as to unmask the data.
func Cipher(payload []byte, mask [4]byte, offset int) {
	n := len(payload)
	if n < 8 {
		for i := 0; i < n; i++ {
			payload[i] ^= mask[(offset+i)%4]
		}
		return
	}

	// Calculate position in mask due to previously processed bytes number.
	mpos := offset % 4
	// Count number of bytes will processed one by one from the beginning of payload.
	ln := remain[mpos]
	// Count number of bytes will processed one by one from the end of payload.
	// This is done to process payload by 8 bytes in each iteration of main loop.
	rn := (n - ln) % 8

	for i := 0; i < ln; i++ {
		payload[i] ^= mask[(mpos+i)%4]
	}
	for i := n - rn; i < n; i++ {
		payload[i] ^= mask[(mpos+i)%4]
	}

	// We should cast mask to uint32 with unsafe instead of encoding.BigEndian
	// to avoid care of os dependent byte order. That is, on any endianess mask
	// and payload will be presented with the same order. In other words, we
	// could not use encoding.BigEndian on xoring payload as uint64.
	m := *(*uint32)(unsafe.Pointer(&mask))
	m2 := uint64(m)<<32 | uint64(m)

	// Skip already processed right part.
	// Get number of uint64 parts remaining to process.
	n = (n - ln - rn) >> 3
	for i := 0; i < n; i++ {
		idx := ln + (i << 3)
		p := binary.LittleEndian.Uint64(payload[idx : idx+8])
		p = p ^ m2
		binary.LittleEndian.PutUint64(payload[idx:idx+8], p)
	}
}

// remain maps position in masking key [0,4) to number
// of bytes that need to be processed manually inside Cipher().
var remain = [4]int{0, 3, 2, 1}