X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/ba6e6b64033b1f9de49feccb5c9cd438354481f7..0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a:/base/ct.c

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+/* -*-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 = U32(~(x ^ y));
+
+  a &= a >> 16;
+  a &= a >>  8;
+  a &= a >>  4;
+  a &= a >>  2;
+  a &= a >>  1;
+  return (a & 1);
+}
+
+/* --- @ct_intle@ --- *
+ *
+ * Arguments:	@uint32 x, y@ = two 32-bit unsigned integers
+ *
+ * Returns:	One if %$x \le y$% are equal, 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 xx = x >> 31, yy = y >> 31, zz = (y - x) >> 31;
+  return ((~xx&yy) | (~(xx^yy)&~zz)) & 1;
+}
+
+/* --- @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 -------------------------------------------------*/