62 lines
1.8 KiB
Go
62 lines
1.8 KiB
Go
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package ws
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import (
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"encoding/binary"
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)
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// Cipher applies XOR cipher to the payload using mask.
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// Offset is used to cipher chunked data (e.g. in io.Reader implementations).
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//
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// To convert masked data into unmasked data, or vice versa, the following
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// algorithm is applied. The same algorithm applies regardless of the
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// direction of the translation, e.g., the same steps are applied to
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// mask the data as to unmask the data.
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func Cipher(payload []byte, mask [4]byte, offset int) {
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n := len(payload)
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if n < 8 {
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for i := 0; i < n; i++ {
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payload[i] ^= mask[(offset+i)%4]
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}
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return
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}
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// Calculate position in mask due to previously processed bytes number.
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mpos := offset % 4
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// Count number of bytes will processed one by one from the beginning of payload.
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ln := remain[mpos]
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// Count number of bytes will processed one by one from the end of payload.
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// This is done to process payload by 8 bytes in each iteration of main loop.
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rn := (n - ln) % 8
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for i := 0; i < ln; i++ {
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payload[i] ^= mask[(mpos+i)%4]
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}
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for i := n - rn; i < n; i++ {
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payload[i] ^= mask[(mpos+i)%4]
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}
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// NOTE: we use here binary.LittleEndian regardless of what is real
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// endianess on machine is. To do so, we have to use binary.LittleEndian in
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// the masking loop below as well.
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var (
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m = binary.LittleEndian.Uint32(mask[:])
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m2 = uint64(m)<<32 | uint64(m)
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)
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// Skip already processed right part.
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// Get number of uint64 parts remaining to process.
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n = (n - ln - rn) >> 3
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for i := 0; i < n; i++ {
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var (
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j = ln + (i << 3)
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chunk = payload[j : j+8]
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)
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p := binary.LittleEndian.Uint64(chunk)
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p = p ^ m2
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binary.LittleEndian.PutUint64(chunk, p)
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}
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}
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// remain maps position in masking key [0,4) to number
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// of bytes that need to be processed manually inside Cipher().
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var remain = [4]int{0, 3, 2, 1}
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