Rearrange the file tree.

[u/mdw/catacomb] / base / ct.c

diff --git a/base/ct.c b/base/ct.c
new file mode 100644 (file)
index 0000000..ef2adeb
--- /dev/null
+++ b/base/ct.c
@@ -0,0 +1,144 @@
+/* -*-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 -------------------------------------------------*/