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Source file src/crypto/subtle/constant_time.go

Documentation: crypto/subtle

  // Copyright 2009 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 subtle implements functions that are often useful in cryptographic
  // code but require careful thought to use correctly.
  package subtle
  
  // ConstantTimeCompare returns 1 if and only if the two slices, x
  // and y, have equal contents. The time taken is a function of the length of
  // the slices and is independent of the contents.
  func ConstantTimeCompare(x, y []byte) int {
  	if len(x) != len(y) {
  		return 0
  	}
  
  	var v byte
  
  	for i := 0; i < len(x); i++ {
  		v |= x[i] ^ y[i]
  	}
  
  	return ConstantTimeByteEq(v, 0)
  }
  
  // ConstantTimeSelect returns x if v is 1 and y if v is 0.
  // Its behavior is undefined if v takes any other value.
  func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
  
  // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
  func ConstantTimeByteEq(x, y uint8) int {
  	z := ^(x ^ y)
  	z &= z >> 4
  	z &= z >> 2
  	z &= z >> 1
  
  	return int(z)
  }
  
  // ConstantTimeEq returns 1 if x == y and 0 otherwise.
  func ConstantTimeEq(x, y int32) int {
  	z := ^(x ^ y)
  	z &= z >> 16
  	z &= z >> 8
  	z &= z >> 4
  	z &= z >> 2
  	z &= z >> 1
  
  	return int(z & 1)
  }
  
  // ConstantTimeCopy copies the contents of y into x (a slice of equal length)
  // if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
  // takes any other value.
  func ConstantTimeCopy(v int, x, y []byte) {
  	if len(x) != len(y) {
  		panic("subtle: slices have different lengths")
  	}
  
  	xmask := byte(v - 1)
  	ymask := byte(^(v - 1))
  	for i := 0; i < len(x); i++ {
  		x[i] = x[i]&xmask | y[i]&ymask
  	}
  }
  
  // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
  func ConstantTimeLessOrEq(x, y int) int {
  	x32 := int32(x)
  	y32 := int32(y)
  	return int(((x32 - y32 - 1) >> 31) & 1)
  }
  

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