constant_time.go

Documentation: crypto/subtle

		 1  // Copyright 2009 The Go Authors. All rights reserved.
		 2  // Use of this source code is governed by a BSD-style
		 3  // license that can be found in the LICENSE file.
		 4  
		 5  // Package subtle implements functions that are often useful in cryptographic
		 6  // code but require careful thought to use correctly.
		 7  package subtle
		 8  
		 9  // ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
		10  // and 0 otherwise. The time taken is a function of the length of the slices and
		11  // is independent of the contents.
		12  func ConstantTimeCompare(x, y []byte) int {
		13  	if len(x) != len(y) {
		14  		return 0
		15  	}
		16  
		17  	var v byte
		18  
		19  	for i := 0; i < len(x); i++ {
		20  		v |= x[i] ^ y[i]
		21  	}
		22  
		23  	return ConstantTimeByteEq(v, 0)
		24  }
		25  
		26  // ConstantTimeSelect returns x if v == 1 and y if v == 0.
		27  // Its behavior is undefined if v takes any other value.
		28  func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
		29  
		30  // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
		31  func ConstantTimeByteEq(x, y uint8) int {
		32  	return int((uint32(x^y) - 1) >> 31)
		33  }
		34  
		35  // ConstantTimeEq returns 1 if x == y and 0 otherwise.
		36  func ConstantTimeEq(x, y int32) int {
		37  	return int((uint64(uint32(x^y)) - 1) >> 63)
		38  }
		39  
		40  // ConstantTimeCopy copies the contents of y into x (a slice of equal length)
		41  // if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
		42  // takes any other value.
		43  func ConstantTimeCopy(v int, x, y []byte) {
		44  	if len(x) != len(y) {
		45  		panic("subtle: slices have different lengths")
		46  	}
		47  
		48  	xmask := byte(v - 1)
		49  	ymask := byte(^(v - 1))
		50  	for i := 0; i < len(x); i++ {
		51  		x[i] = x[i]&xmask | y[i]&ymask
		52  	}
		53  }
		54  
		55  // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
		56  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
		57  func ConstantTimeLessOrEq(x, y int) int {
		58  	x32 := int32(x)
		59  	y32 := int32(y)
		60  	return int(((x32 - y32 - 1) >> 31) & 1)
		61  }
		62
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