Source: constant_time.go in package crypto/subtle

Source File
	constant_time.go

Belonging Package
	crypto/subtle

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.

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)


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