github.com/varialus/godfly@v0.0.0-20130904042352-1934f9f095ab/src/pkg/crypto/subtle/constant_time.go (about)

     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 iff the two equal length slices, x
    10  // and y, have equal contents. The time taken is a function of the length of
    11  // the slices and is independent of the contents.
    12  func ConstantTimeCompare(x, y []byte) int {
    13  	var v byte
    14  
    15  	for i := 0; i < len(x); i++ {
    16  		v |= x[i] ^ y[i]
    17  	}
    18  
    19  	return ConstantTimeByteEq(v, 0)
    20  }
    21  
    22  // ConstantTimeSelect returns x if v is 1 and y if v is 0.
    23  // Its behavior is undefined if v takes any other value.
    24  func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
    25  
    26  // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
    27  func ConstantTimeByteEq(x, y uint8) int {
    28  	z := ^(x ^ y)
    29  	z &= z >> 4
    30  	z &= z >> 2
    31  	z &= z >> 1
    32  
    33  	return int(z)
    34  }
    35  
    36  // ConstantTimeEq returns 1 if x == y and 0 otherwise.
    37  func ConstantTimeEq(x, y int32) int {
    38  	z := ^(x ^ y)
    39  	z &= z >> 16
    40  	z &= z >> 8
    41  	z &= z >> 4
    42  	z &= z >> 2
    43  	z &= z >> 1
    44  
    45  	return int(z & 1)
    46  }
    47  
    48  // ConstantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged.
    49  // Its behavior is undefined if v takes any other value.
    50  func ConstantTimeCopy(v int, x, y []byte) {
    51  	xmask := byte(v - 1)
    52  	ymask := byte(^(v - 1))
    53  	for i := 0; i < len(x); i++ {
    54  		x[i] = x[i]&xmask | y[i]&ymask
    55  	}
    56  	return
    57  }
    58  
    59  // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
    60  // Its behavior is undefined if x or y are negative or > 2**31 - 1.
    61  func ConstantTimeLessOrEq(x, y int) int {
    62  	x32 := int32(x)
    63  	y32 := int32(y)
    64  	return int(((x32 - y32 - 1) >> 31) & 1)
    65  }