well-goknown/vendor/github.com/gobwas/ws/cipher.go

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package ws
import (
"encoding/binary"
)
// 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 16 bytes in each iteration of main loop.
rn := (n - ln) % 16
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]
}
// NOTE: we use here binary.LittleEndian regardless of what is real
// endianness on machine is. To do so, we have to use binary.LittleEndian in
// the masking loop below as well.
var (
m = binary.LittleEndian.Uint32(mask[:])
m2 = uint64(m)<<32 | uint64(m)
)
// Skip already processed right part.
// Get number of uint64 parts remaining to process.
n = (n - ln - rn) >> 4
j := ln
for i := 0; i < n; i++ {
chunk := payload[j : j+16]
p := binary.LittleEndian.Uint64(chunk) ^ m2
p2 := binary.LittleEndian.Uint64(chunk[8:]) ^ m2
binary.LittleEndian.PutUint64(chunk, p)
binary.LittleEndian.PutUint64(chunk[8:], p2)
j += 16
}
}
// 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}