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
     6  
     7  // ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
     8  // and 0 otherwise. The time taken is a function of the length of the slices and
     9  // is independent of the contents. If the lengths of x and y do not match it
    10  // returns 0 immediately.
    11  func ConstantTimeCompare(x, y []byte) int {
    12  	if len(x) != len(y) {
    13  		return 0
    14  	}
    15  
    16  	var v byte
    17  
    18  	for i := 0; i < len(x); i++ {
    19  		v |= x[i] ^ y[i]
    20  	}
    21  
    22  	return ConstantTimeByteEq(v, 0)
    23  }
    24  
    25  // ConstantTimeSelect returns x if v == 1 and y if v == 0.
    26  // Its behavior is undefined if v takes any other value.
    27  func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
    28  
    29  // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
    30  func ConstantTimeByteEq(x, y uint8) int {
    31  	return int((uint32(x^y) - 1) >> 31)
    32  }
    33  
    34  // ConstantTimeEq returns 1 if x == y and 0 otherwise.
    35  func ConstantTimeEq(x, y int32) int {
    36  	return int((uint64(uint32(x^y)) - 1) >> 63)
    37  }
    38  
    39  // ConstantTimeCopy copies the contents of y into x (a slice of equal length)
    40  // if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
    41  // takes any other value.
    42  func ConstantTimeCopy(v int, x, y []byte) {
    43  	if len(x) != len(y) {
    44  		panic("subtle: slices have different lengths")
    45  	}
    46  
    47  	xmask := byte(v - 1)
    48  	ymask := byte(^(v - 1))
    49  	for i := 0; i < len(x); i++ {
    50  		x[i] = x[i]&xmask | y[i]&ymask
    51  	}
    52  }
    53  
    54  // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
    55  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
    56  func ConstantTimeLessOrEq(x, y int) int {
    57  	x32 := int32(x)
    58  	y32 := int32(y)
    59  	return int(((x32 - y32 - 1) >> 31) & 1)
    60  }
    61
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