aboutsummaryrefslogtreecommitdiffstats
path: root/contrib/go/_std_1.22/src/crypto/subtle/constant_time.go
blob: 4e0527f9d5a61c0862454f9610d2adbe7390fa0c (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package subtle implements functions that are often useful in cryptographic
// code but require careful thought to use correctly.
package subtle

// ConstantTimeCompare returns 1 if the two slices, x and y, have equal contents
// and 0 otherwise. The time taken is a function of the length of the slices and
// is independent of the contents. If the lengths of x and y do not match it
// returns 0 immediately.
func ConstantTimeCompare(x, y []byte) int {
	if len(x) != len(y) {
		return 0
	}

	var v byte

	for i := 0; i < len(x); i++ {
		v |= x[i] ^ y[i]
	}

	return ConstantTimeByteEq(v, 0)
}

// ConstantTimeSelect returns x if v == 1 and y if v == 0.
// Its behavior is undefined if v takes any other value.
func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }

// ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
func ConstantTimeByteEq(x, y uint8) int {
	return int((uint32(x^y) - 1) >> 31)
}

// ConstantTimeEq returns 1 if x == y and 0 otherwise.
func ConstantTimeEq(x, y int32) int {
	return int((uint64(uint32(x^y)) - 1) >> 63)
}

// ConstantTimeCopy copies the contents of y into x (a slice of equal length)
// if v == 1. If v == 0, x is left unchanged. Its behavior is undefined if v
// takes any other value.
func ConstantTimeCopy(v int, x, y []byte) {
	if len(x) != len(y) {
		panic("subtle: slices have different lengths")
	}

	xmask := byte(v - 1)
	ymask := byte(^(v - 1))
	for i := 0; i < len(x); i++ {
		x[i] = x[i]&xmask | y[i]&ymask
	}
}

// ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
// Its behavior is undefined if x or y are negative or > 2**31 - 1.
func ConstantTimeLessOrEq(x, y int) int {
	x32 := int32(x)
	y32 := int32(y)
	return int(((x32 - y32 - 1) >> 31) & 1)
}