| 1 | /* -*-c-*- |
| 2 | * |
| 3 | * Constant-time operations |
| 4 | * |
| 5 | * (c) 2013 Straylight/Edgeware |
| 6 | */ |
| 7 | |
| 8 | /*----- Licensing notice --------------------------------------------------* |
| 9 | * |
| 10 | * This file is part of Catacomb. |
| 11 | * |
| 12 | * Catacomb is free software; you can redistribute it and/or modify |
| 13 | * it under the terms of the GNU Library General Public License as |
| 14 | * published by the Free Software Foundation; either version 2 of the |
| 15 | * License, or (at your option) any later version. |
| 16 | * |
| 17 | * Catacomb is distributed in the hope that it will be useful, |
| 18 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 19 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 20 | * GNU Library General Public License for more details. |
| 21 | * |
| 22 | * You should have received a copy of the GNU Library General Public |
| 23 | * License along with Catacomb; if not, write to the Free |
| 24 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
| 25 | * MA 02111-1307, USA. |
| 26 | */ |
| 27 | |
| 28 | /*----- Header files ------------------------------------------------------*/ |
| 29 | |
| 30 | #include <mLib/bits.h> |
| 31 | |
| 32 | #include "ct.h" |
| 33 | |
| 34 | /*----- Main code ---------------------------------------------------------*/ |
| 35 | |
| 36 | #define MASK(a) (U32(~((a) - 1))) |
| 37 | |
| 38 | /* --- @ct_inteq@ --- * |
| 39 | * |
| 40 | * Arguments: @uint32 x, y@ = two 32-bit unsigned integers |
| 41 | * |
| 42 | * Returns: One if @x@ and @y@ are equal, zero if they differ. |
| 43 | * |
| 44 | * Use: Answers whether two integers are equal, in constant time. |
| 45 | */ |
| 46 | |
| 47 | int ct_inteq(uint32 x, uint32 y) |
| 48 | { |
| 49 | uint32 a = x ^ y; |
| 50 | |
| 51 | /* --- How it works --- * |
| 52 | * |
| 53 | * Obviously, %$a$% is nonzero if and only if %$x \ne y$%. Let %$N > 0$%. |
| 54 | * Suppose that %$0 < a < N$%. Then: |
| 55 | * |
| 56 | * %$N/2 = N - N/2 \le N - (a + 1)/2 = N - a/2 - 1/2$% |
| 57 | * %${} = N - a + a/2 - 1/2 = N - a + (a - 1)/2$% |
| 58 | * %${} \le N - a + \lfloor a/2 \rfloor < N$% |
| 59 | * |
| 60 | * Consequently, if %$N = 2^{32}$% and %$a$% is nonzero, then |
| 61 | * %$a - \lfloor a/2 \rfloor$% has its top bit set. Of course, if |
| 62 | * %$a = 0$% then %$a - \lfloor a/2 \rfloor = 0$%. |
| 63 | */ |
| 64 | |
| 65 | a = (a >> 1) - a; |
| 66 | return (U32(~a) >> 31); |
| 67 | } |
| 68 | |
| 69 | /* --- @ct_intle@ --- * |
| 70 | * |
| 71 | * Arguments: @uint32 x, y@ = two 32-bit unsigned integers |
| 72 | * |
| 73 | * Returns: One if %$x \le y$%, zero if @x@ is greater. |
| 74 | * |
| 75 | * Use: Answers whether two integers are ordered, in constant time. |
| 76 | */ |
| 77 | |
| 78 | int ct_intle(uint32 x, uint32 y) |
| 79 | { |
| 80 | /* --- This bit's a little fiddly --- * |
| 81 | * |
| 82 | * If the top bits of @x@ and @y@ are the same then %$x \le y$% if and only |
| 83 | * if %$y - x$% has its top bit clear; otherwise, %$x \le y$% if and only |
| 84 | * if (the top bit of @x@ is clear and) the top bit of @y@ is set. |
| 85 | * |
| 86 | * This assumes we can do subtraction in constant time, which seems like a |
| 87 | * safe enough bet. |
| 88 | */ |
| 89 | |
| 90 | uint32 diff = x^y; |
| 91 | uint32 low = y - x; |
| 92 | uint32 a = (y&diff) | (~low&~diff); |
| 93 | return (U32(a) >> 31); |
| 94 | } |
| 95 | |
| 96 | /* --- @ct_pick@ --- * |
| 97 | * |
| 98 | * Arguments: @uint32 a@ = a switch, either zero or one |
| 99 | * @uint32 x0, x1@ = two 32-bit unsigned integers |
| 100 | * |
| 101 | * Returns: @x0@ if @a@ is zero; @x1@ if @a@ is one. Other values of @a@ |
| 102 | * will give you unhelpful results. |
| 103 | * |
| 104 | * Use: Picks one of two results according to a switch variable, in |
| 105 | * constant time. |
| 106 | */ |
| 107 | |
| 108 | uint32 ct_pick(uint32 a, uint32 x0, uint32 x1) |
| 109 | { uint32 m = MASK(a); return (x0&~m) | (x1&m); } |
| 110 | |
| 111 | /* --- @ct_condcopy@ --- * |
| 112 | * |
| 113 | * Arguments: @uint32 a@ = a switch, either zero or one |
| 114 | * @void *d@ = destination pointer |
| 115 | * @const void *s@ = source pointer |
| 116 | * @size_t n@ amount to copy |
| 117 | * |
| 118 | * Returns: --- |
| 119 | * |
| 120 | * Use: If @a@ is one then copy the @n@ bytes starting at @s@ to |
| 121 | * @d@; if @a@ is zero then leave @d@ unchanged (but it will |
| 122 | * still be written). All of this is done in constant time. |
| 123 | */ |
| 124 | |
| 125 | void ct_condcopy(uint32 a, void *d, const void *s, size_t n) |
| 126 | { |
| 127 | octet *dd = d; |
| 128 | const octet *ss = s; |
| 129 | uint32 m = MASK(a); |
| 130 | |
| 131 | while (n--) { |
| 132 | *dd = (*ss++&m) | (*dd&~m); |
| 133 | dd++; |
| 134 | } |
| 135 | } |
| 136 | |
| 137 | /* --- @ct_memeq@ --- |
| 138 | * |
| 139 | * Arguments: @const void *p, *q@ = two pointers to buffers |
| 140 | * @size_t n@ = the (common) size of the buffers |
| 141 | * |
| 142 | * Returns: One if the two buffers are equal, zero if they aren't. |
| 143 | * |
| 144 | * Use: Compares two chunks of memory, in constant time. |
| 145 | */ |
| 146 | |
| 147 | int ct_memeq(const void *p, const void *q, size_t n) |
| 148 | { |
| 149 | const octet *pp = p, *qq = q; |
| 150 | octet a = 0; |
| 151 | |
| 152 | while (n--) a |= *pp++ ^ *qq++; |
| 153 | return (ct_inteq(a, 0)); |
| 154 | } |
| 155 | |
| 156 | /*----- That's all, folks -------------------------------------------------*/ |