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# Source file src/crypto/subtle/constant_time.go

## Documentation: crypto/subtle

```     1  // Copyright 2009 The Go Authors. All rights reserved.
2  // Use of this source code is governed by a BSD-style
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++ {
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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