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Source file src/crypto/cipher/xor.go

Documentation: crypto/cipher

  // Copyright 2013 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package cipher
  
  import (
  	"runtime"
  	"unsafe"
  )
  
  const wordSize = int(unsafe.Sizeof(uintptr(0)))
  const supportsUnaligned = runtime.GOARCH == "386" || runtime.GOARCH == "amd64" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOARCH == "s390x"
  
  // fastXORBytes xors in bulk. It only works on architectures that
  // support unaligned read/writes.
  func fastXORBytes(dst, a, b []byte) int {
  	n := len(a)
  	if len(b) < n {
  		n = len(b)
  	}
  
  	w := n / wordSize
  	if w > 0 {
  		dw := *(*[]uintptr)(unsafe.Pointer(&dst))
  		aw := *(*[]uintptr)(unsafe.Pointer(&a))
  		bw := *(*[]uintptr)(unsafe.Pointer(&b))
  		for i := 0; i < w; i++ {
  			dw[i] = aw[i] ^ bw[i]
  		}
  	}
  
  	for i := (n - n%wordSize); i < n; i++ {
  		dst[i] = a[i] ^ b[i]
  	}
  
  	return n
  }
  
  func safeXORBytes(dst, a, b []byte) int {
  	n := len(a)
  	if len(b) < n {
  		n = len(b)
  	}
  	for i := 0; i < n; i++ {
  		dst[i] = a[i] ^ b[i]
  	}
  	return n
  }
  
  // xorBytes xors the bytes in a and b. The destination is assumed to have enough
  // space. Returns the number of bytes xor'd.
  func xorBytes(dst, a, b []byte) int {
  	if supportsUnaligned {
  		return fastXORBytes(dst, a, b)
  	} else {
  		// TODO(hanwen): if (dst, a, b) have common alignment
  		// we could still try fastXORBytes. It is not clear
  		// how often this happens, and it's only worth it if
  		// the block encryption itself is hardware
  		// accelerated.
  		return safeXORBytes(dst, a, b)
  	}
  }
  
  // fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
  // The arguments are assumed to be of equal length.
  func fastXORWords(dst, a, b []byte) {
  	dw := *(*[]uintptr)(unsafe.Pointer(&dst))
  	aw := *(*[]uintptr)(unsafe.Pointer(&a))
  	bw := *(*[]uintptr)(unsafe.Pointer(&b))
  	n := len(b) / wordSize
  	for i := 0; i < n; i++ {
  		dw[i] = aw[i] ^ bw[i]
  	}
  }
  
  func xorWords(dst, a, b []byte) {
  	if supportsUnaligned {
  		fastXORWords(dst, a, b)
  	} else {
  		safeXORBytes(dst, a, b)
  	}
  }
  

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