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 // 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 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         if len(x) != len(y) {
  14                 panic("subtle: slices have different lengths")
  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 is 1 and y if v is 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         z := ^(x ^ y)
  33         z &= z >> 4
  34         z &= z >> 2
  35         z &= z >> 1
  36
  37         return int(z)
  38 }
  39
  40 // ConstantTimeEq returns 1 if x == y and 0 otherwise.
  41 func ConstantTimeEq(x, y int32) int {
  42         z := ^(x ^ y)
  43         z &= z >> 16
  44         z &= z >> 8
  45         z &= z >> 4
  46         z &= z >> 2
  47         z &= z >> 1
  48
  49         return int(z & 1)
  50 }
  51
  52 // ConstantTimeCopy copies the contents of y into x iff v == 1. If v == 0, x is left unchanged.
  53 // Its behavior is undefined if v takes any other value.
  54 func ConstantTimeCopy(v int, x, y []byte) {
  55         xmask := byte(v - 1)
  56         ymask := byte(^(v - 1))
  57         for i := 0; i < len(x); i++ {
  58                 x[i] = x[i]&xmask | y[i]&ymask
  59         }
  60         return
  61 }
  62
  63 // ConstantTimeLessOrEq returns 1 if x <= y and 0 otherwise.
  64 // Its behavior is undefined if x or y are negative or > 2**31 - 1.
  65 func ConstantTimeLessOrEq(x, y int) int {
  66         x32 := int32(x)
  67         y32 := int32(y)
  68         return int(((x32 - y32 - 1) >> 31) & 1)
  69 }