utils/split-pieces (QfConvert): Construct an instance of the right class.

[catacomb] / base / ct.c

/* -*-c-*-
 *
 * Constant-time operations
 *
 * (c) 2013 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of Catacomb.
 *
 * Catacomb is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * Catacomb is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with Catacomb; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include <mLib/bits.h>

#include "ct.h"

/*----- Main code ---------------------------------------------------------*/

#define MASK(a) (U32(~((a) - 1)))

/* --- @ct_inteq@ --- *
 *
 * Arguments:	@uint32 x, y@ = two 32-bit unsigned integers
 *
 * Returns:	One if @x@ and @y@ are equal, zero if they differ.
 *
 * Use:		Answers whether two integers are equal, in constant time.
 */

int ct_inteq(uint32 x, uint32 y)
{
  uint32 a = x ^ y;

  /* --- How it works --- *
   *
   * Obviously, %$a$% is nonzero if and only if %$x \ne y$%.  Let %$N > 0$%.
   * Suppose that %$0 < a < N$%.  Then:
   *
   *	%$N/2 =  N - N/2 \le N - (a + 1)/2 = N - a/2 - 1/2$%
   *	 %${} =  N - a + a/2 - 1/2 = N - a + (a - 1)/2$%
   *    %${} \le N - a + \lfloor a/2 \rfloor < N$%
   *
   * Consequently, if %$N = 2^{32}$% and %$a$% is nonzero, then
   * %$a - \lfloor a/2 \rfloor$% has its top bit set.  Of course, if
   * %$a = 0$% then %$a - \lfloor a/2 \rfloor = 0$%.
   */

  a = (a >> 1) - a;
  return (U32(~a) >> 31);
}

/* --- @ct_intle@ --- *
 *
 * Arguments:	@uint32 x, y@ = two 32-bit unsigned integers
 *
 * Returns:	One if %$x \le y$%, zero if @x@ is greater.
 *
 * Use:		Answers whether two integers are ordered, in constant time.
 */

int ct_intle(uint32 x, uint32 y)
{
  /* --- This bit's a little fiddly --- *
   *
   * If the top bits of @x@ and @y@ are the same then %$x \le y$% if and only
   * if %$y - x$% has its top bit clear; otherwise, %$x \le y$% if and only
   * if (the top bit of @x@ is clear and) the top bit of @y@ is set.
   *
   * This assumes we can do subtraction in constant time, which seems like a
   * safe enough bet.
   */

  uint32 diff = x^y;
  uint32 low = y - x;
  uint32 a = (y&diff) | (~low&~diff);
  return (U32(a) >> 31);
}

/* --- @ct_pick@ --- *
 *
 * Arguments:	@uint32 a@ = a switch, either zero or one
 *		@uint32 x0, x1@ = two 32-bit unsigned integers
 *
 * Returns:	@x0@ if @a@ is zero; @x1@ if @a@ is one.  Other values of @a@
 *		will give you unhelpful results.
 *
 * Use:		Picks one of two results according to a switch variable, in
 *		constant time.
 */

int ct_pick(uint32 a, uint32 x0, uint32 x1)
  { uint32 m = MASK(a); return (x0&~m) | (x1&m); }

/* --- @ct_condcopy@ --- *
 *
 * Arguments:	@uint32 a@ = a switch, either zero or one
 *		@void *d@ = destination pointer
 *		@const void *s@ = source pointer
 *		@size_t n@ amount to copy
 *
 * Returns:	---
 *
 * Use:		If @a@ is one then copy the @n@ bytes starting at @s@ to
 *		@d@; if @a@ is zero then leave @d@ unchanged (but it will
 *		still be written).  All of this is done in constant time.
 */

void ct_condcopy(uint32 a, void *d, const void *s, size_t n)
{
  octet *dd = d;
  const octet *ss = s;
  uint32 m = MASK(a);

  while (n--) {
    *dd = (*ss++&m) | (*dd&~m);
    dd++;
  }
}

/* --- @ct_memeq@ ---
 *
 * Arguments:	@const void *p, *q@ = two pointers to buffers
 *		@size_t n@ = the (common) size of the buffers
 *
 * Returns:	One if the two buffers are equal, zero if they aren't.
 *
 * Use:		Compares two chunks of memory, in constant time.
 */

int ct_memeq(const void *p, const void *q, size_t n)
{
  const octet *pp = p, *qq = q;
  octet a = 0;

  while (n--) a |= *pp++ ^ *qq++;
  return (ct_inteq(a, 0));
}

/*----- That's all, folks -------------------------------------------------*/