src/pkg/crypto/subtle/constant_time.go

// 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.

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

// ConstantTimeCompare returns 1 iff the two equal length slices, x
// and y, have equal contents. The time taken is a function of the length of
// the slices and is independent of the contents.
func ConstantTimeCompare(x, y []byte) int {
	var v byte;

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

	return ConstantTimeByteEq(v, 0);
}

// ConstantTimeSelect returns x if v is 1 and y if v is 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 == x and 0 otherwise.
func ConstantTimeByteEq(x, y uint8) int {
	z := ^(x^y);
	z &= z>>4;
	z &= z>>2;
	z &= z>>1;

	return int(z);
}

// ConstantTimeEq returns 1 if x == y and 0 otherwise.
func ConstantTimeEq(x, y int32) int {
	z := ^(x^y);
	z &= z>>16;
	z &= z>>8;
	z &= z>>4;
	z &= z>>2;
	z &= z>>1;

	return int(z&1);
}

// ConstantTimeCopy copies the contents of y into x iff 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) {
	xmask := byte(v-1);
	ymask := byte(^(v-1));
	for i := 0; i < len(x); i++ {
		x[i] = x[i]&xmask | y[i]&ymask;
	}
	return;
}