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. If the lengths of x and y do not match it
    12  // returns 0 immediately.
    13  func ConstantTimeCompare(x, y []byte) int {
    14  	if len(x) != len(y) {
    15  		return 0
    16  	}
    17  
    18  	var v byte
    19  
    20  	for i := 0; i < len(x); i++ {
    21  		v |= x[i] ^ y[i]
    22  	}
    23  
    24  	return ConstantTimeByteEq(v, 0)
    25  }
    26  
    27  // ConstantTimeSelect returns x if v == 1 and y if v == 0.
    28  // Its behavior is undefined if v takes any other value.
    29  func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
    30  
    31  // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
    32  func ConstantTimeByteEq(x, y uint8) int {
    33  	return int((uint32(x^y) - 1) >> 31)
    34  }
    35  
    36  // ConstantTimeEq returns 1 if x == y and 0 otherwise.
    37  func ConstantTimeEq(x, y int32) int {
    38  	return int((uint64(uint32(x^y)) - 1) >> 63)
    39  }
    40  
    41  // ConstantTimeCopy copies the contents of y into x (a slice of equal length)
    42  // if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
    43  // takes any other value.
    44  func ConstantTimeCopy(v int, x, y []byte) {
    45  	if len(x) != len(y) {
    46  		panic("subtle: slices have different lengths")
    47  	}
    48  
    49  	xmask := byte(v - 1)
    50  	ymask := byte(^(v - 1))
    51  	for i := 0; i < len(x); i++ {
    52  		x[i] = x[i]&xmask | y[i]&ymask
    53  	}
    54  }
    55  
    56  // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
    57  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
    58  func ConstantTimeLessOrEq(x, y int) int {
    59  	x32 := int32(x)
    60  	y32 := int32(y)
    61  	return int(((x32 - y32 - 1) >> 31) & 1)
    62  }
    63
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