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Source file src/crypto/subtle/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 and only if the two 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		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 is 1 and y if v is 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		z := ^(x ^ y)
    33		z &= z >> 4
    34		z &= z >> 2
    35		z &= z >> 1
    36	
    37		return int(z)
    38	}
    39	
    40	// ConstantTimeEq returns 1 if x == y and 0 otherwise.
    41	func ConstantTimeEq(x, y int32) int {
    42		z := ^(x ^ y)
    43		z &= z >> 16
    44		z &= z >> 8
    45		z &= z >> 4
    46		z &= z >> 2
    47		z &= z >> 1
    48	
    49		return int(z & 1)
    50	}
    51	
    52	// ConstantTimeCopy copies the contents of y into x (a slice of equal length)
    53	// if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
    54	// takes any other value.
    55	func ConstantTimeCopy(v int, x, y []byte) {
    56		if len(x) != len(y) {
    57			panic("subtle: slices have different lengths")
    58		}
    59	
    60		xmask := byte(v - 1)
    61		ymask := byte(^(v - 1))
    62		for i := 0; i < len(x); i++ {
    63			x[i] = x[i]&xmask | y[i]&ymask
    64		}
    65	}
    66	
    67	// ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
    68	// Its behavior is undefined if x or y are negative or > 2**31 - 1.
    69	func ConstantTimeLessOrEq(x, y int) int {
    70		x32 := int32(x)
    71		y32 := int32(y)
    72		return int(((x32 - y32 - 1) >> 31) & 1)
    73	}
    74	

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