Use a new multiplication function from an Ascom white paper to resist

timing attacks.

diff --git a/idea.c b/idea.c
index 5f88948..cde3af4 100644 (file)
--- a/idea.c
+++ b/idea.c
@@ -1,6 +1,6 @@
 /* -*-c-*-
  *
 /* -*-c-*-
  *

- * $Id: idea.c,v 1.2 2000/06/17 11:24:08 mdw Exp $
+ * $Id: idea.c,v 1.3 2000/07/02 18:24:39 mdw Exp $

  *
  * Implementation of the IDEA cipher
  *
  *
  * Implementation of the IDEA cipher
  *


@@ -30,6 +30,10 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: idea.c,v $
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: idea.c,v $

+ * Revision 1.3  2000/07/02 18:24:39  mdw
+ * Use a new multiplication function from an Ascom white paper to resist
+ * timing attacks.
+ *

  * Revision 1.2  2000/06/17 11:24:08  mdw
  * New key size interface.
  *
  * Revision 1.2  2000/06/17 11:24:08  mdw
  * New key size interface.
  *


@@ -90,22 +94,21 @@ static uint16 inv(uint16 n)
  * Arguments @x@ and @y@ are two 32-bit values to multiply.  On exit, @x@ is
  * the product of the two arguments.  The result is not normalized back to 16
  * bits; the arguments are not expected to be normalized.
  * Arguments @x@ and @y@ are two 32-bit values to multiply.  On exit, @x@ is
  * the product of the two arguments.  The result is not normalized back to 16
  * bits; the arguments are not expected to be normalized.

+ *
+ * This code is from `Side Channel Attack Hardening of the IDEA Cipher',
+ * published by Ascom Tech.

  */
 
 #define MUL(x, y) do {                                                 \
  */
 
 #define MUL(x, y) do {                                                 \

-  uint32 _mx, _my = (y);                                               \
-  if ((_mx = U16(x)) == 0)                                             \
-    (x) = 1 - _my;                                                     \
-  else if (_my == 0)                                                   \
-    (x) = 1 - _mx;                                                     \
-  else {                                                               \
-    _my *= _mx;                                                                \
-    _mx = U16(_my); _my >>= 16;                                                \
-    if (_mx < _my)                                                     \
-      (x) = _mx - _my + 1;                                             \
-    else                                                               \
-      (x) = _mx - _my;                                                 \
-  }                                                                    \
+  unsigned _t;                                                         \
+  uint32 _tt;                                                          \
+                                                                       \
+  x = U16(x - 1);                                                      \
+  _t = U16(y - 1);                                                     \
+  _tt = (uint32)x * (uint32)_t + (uint32)x + (uint32)_t + 1;           \
+  x = U16(_tt);                                                                \
+  _t = U16(_tt >> 16);                                                 \
+  x = x - _t + (x <= _t);                                              \

 } while (0)
 
 /* --- @idea_init@ --- *
 } while (0)
 
 /* --- @idea_init@ --- *


@@ -209,28 +212,28 @@ void idea_init(idea_ctx *k, const void *buf, size_t sz)
 /* --- @ROUND@ --- */
 
 #define MIX(k, a, b, c, d) do {                                                \
 /* --- @ROUND@ --- */
 
 #define MIX(k, a, b, c, d) do {                                                \

-  MUL(a, (k)[0]);                                                      \
-  (b) += (k)[1];                                                       \
-  (c) += (k)[2];                                                       \
-  MUL(d, (k)[3]);                                                      \
+  MUL(a, k[0]);                                                                \
+  b += k[1];                                                           \
+  c += k[2];                                                           \
+  MUL(d, k[3]);                                                                \

 } while (0)
 
 #define MA(k, a, b, c, d) do {                                         \
 } while (0)
 
 #define MA(k, a, b, c, d) do {                                         \

-  unsigned _u = (a) ^ (c);                                             \
-  unsigned _v = (b) ^ (d);                                             \
-  MUL(_u, (k)[4]);                                                     \
+  unsigned _u = a ^ c;                                                 \
+  unsigned _v = b ^ d;                                                 \
+  MUL(_u, k[4]);                                                       \

   _v += _u;                                                            \
   _v += _u;                                                            \

-  MUL(_v, (k)[5]);                                                     \
+  MUL(_v, k[5]);                                                       \

   _u += _v;                                                            \
   _u += _v;                                                            \

-  (a) ^= _v;                                                           \
-  (b) ^= _u;                                                           \
-  (c) ^= _v;                                                           \
-  (d) ^= _u;                                                           \
+  a ^= _v;                                                             \
+  b ^= _u;                                                             \
+  c ^= _v;                                                             \
+  d ^= _u;                                                             \

 } while (0);
 
 #define ROUND(k, a, b, c, d) do {                                      \
 } while (0);
 
 #define ROUND(k, a, b, c, d) do {                                      \

-  MIX((k), (a), (b), (c), (d));                                                \
-  MA((k), (a), (b), (c), (d));                                         \
+  MIX(k, a, b, c, d);                                                  \
+  MA(k, a, b, c, d);                                                   \

   (k) += 6;                                                            \
 } while (0)
 
   (k) += 6;                                                            \
 } while (0)
 


@@ -252,8 +255,8 @@ void idea_init(idea_ctx *k, const void *buf, size_t sz)
   ROUND(_k, _a, _b, _c, _d);                                           \
   ROUND(_k, _a, _c, _b, _d);                                           \
   MIX  (_k, _a, _c, _b, _d);                                           \
   ROUND(_k, _a, _b, _c, _d);                                           \
   ROUND(_k, _a, _c, _b, _d);                                           \
   MIX  (_k, _a, _c, _b, _d);                                           \

-  (c) = (U16(_a) << 16) | U16(_c);                                     \
-  (d) = (U16(_b) << 16) | U16(_d);                                     \
+  c = (U16(_a) << 16) | U16(_c);                                       \
+  d = (U16(_b) << 16) | U16(_d);                                       \

 } while (0)
 
 #define DBLK(k, a, b) EBLK((k), (a), (b))
 } while (0)
 
 #define DBLK(k, a, b) EBLK((k), (a), (b))