Source file src/pkg/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 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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