libgo/go/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 // This package 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 iff the two equal length slices, x
  10 // and y, have equal contents. The time taken is a function of the length of
  11 // the slices and is independent of the contents.
  12 func ConstantTimeCompare(x, y []byte) int {
  13         var v byte
  14
  15         for i := 0; i < len(x); i++ {
  16                 v |= x[i] ^ y[i]
  17         }
  18
  19         return ConstantTimeByteEq(v, 0)
  20 }
  21
  22 // ConstantTimeSelect returns x if v is 1 and y if v is 0.
  23 // Its behavior is undefined if v takes any other value.
  24 func ConstantTimeSelect(v, x, y int) int { return ^(v-1)&x | (v-1)&y }
  25
  26 // ConstantTimeByteEq returns 1 if x == y and 0 otherwise.
  27 func ConstantTimeByteEq(x, y uint8) int {
  28         z := ^(x ^ y)
  29         z &= z >> 4
  30         z &= z >> 2
  31         z &= z >> 1
  32
  33         return int(z)
  34 }
  35
  36 // ConstantTimeEq returns 1 if x == y and 0 otherwise.
  37 func ConstantTimeEq(x, y int32) int {
  38         z := ^(x ^ y)
  39         z &= z >> 16
  40         z &= z >> 8
  41         z &= z >> 4
  42         z &= z >> 2
  43         z &= z >> 1
  44
  45         return int(z & 1)
  46 }
  47
  48 // ConstantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged.
  49 // Its behavior is undefined if v takes any other value.
  50 func ConstantTimeCopy(v int, x, y []byte) {
  51         xmask := byte(v - 1)
  52         ymask := byte(^(v - 1))
  53         for i := 0; i < len(x); i++ {
  54                 x[i] = x[i]&xmask | y[i]&ymask
  55         }
  56         return
  57 }