constant_time.go

     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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