Simplify the XTEA key schedule code - there really is no reason to

precompute the deltas when they are just a few additions; removing the
additions from the encrypt/decrypt rounds seems enough to me.

1 files changed, 13 insertions, 29 deletions

diff --git a/src/block/xtea/xtea.cpp b/src/block/xtea/xtea.cpp
index 1b24dd159..5047f6594 100644
--- a/src/block/xtea/xtea.cpp
+++ b/src/block/xtea/xtea.cpp
@@ -1,6 +1,6 @@
 /*
 * XTEA
-* (C) 1999-2007 Jack Lloyd
+* (C) 1999-2009 Jack Lloyd
 *
 * Distributed under the terms of the Botan license
 */
@@ -34,10 +34,10 @@ void XTEA::dec(const byte in[], byte out[]) const
    {
    u32bit L = load_be<u32bit>(in, 0), R = load_be<u32bit>(in, 1);
 
-   for(u32bit j = 32; j > 0; --j)
+   for(u32bit j = 0; j != 32; ++j)
       {
-      R -= (((L << 4) ^ (L >> 5)) + L) ^ EK[2*j - 1];
-      L -= (((R << 4) ^ (R >> 5)) + R) ^ EK[2*j - 2];
+      R -= (((L << 4) ^ (L >> 5)) + L) ^ EK[63 - 2*j];
+      L -= (((R << 4) ^ (R >> 5)) + R) ^ EK[62 - 2*j];
       }
 
    store_be(out, L, R);
@@ -48,33 +48,17 @@ void XTEA::dec(const byte in[], byte out[]) const
 */
 void XTEA::key_schedule(const byte key[], u32bit)
    {
-   static const u32bit DELTAS[64] = {
-      0x00000000, 0x9E3779B9, 0x9E3779B9, 0x3C6EF372, 0x3C6EF372, 0xDAA66D2B,
-      0xDAA66D2B, 0x78DDE6E4, 0x78DDE6E4, 0x1715609D, 0x1715609D, 0xB54CDA56,
-      0xB54CDA56, 0x5384540F, 0x5384540F, 0xF1BBCDC8, 0xF1BBCDC8, 0x8FF34781,
-      0x8FF34781, 0x2E2AC13A, 0x2E2AC13A, 0xCC623AF3, 0xCC623AF3, 0x6A99B4AC,
-      0x6A99B4AC, 0x08D12E65, 0x08D12E65, 0xA708A81E, 0xA708A81E, 0x454021D7,
-      0x454021D7, 0xE3779B90, 0xE3779B90, 0x81AF1549, 0x81AF1549, 0x1FE68F02,
-      0x1FE68F02, 0xBE1E08BB, 0xBE1E08BB, 0x5C558274, 0x5C558274, 0xFA8CFC2D,
-      0xFA8CFC2D, 0x98C475E6, 0x98C475E6, 0x36FBEF9F, 0x36FBEF9F, 0xD5336958,
-      0xD5336958, 0x736AE311, 0x736AE311, 0x11A25CCA, 0x11A25CCA, 0xAFD9D683,
-      0xAFD9D683, 0x4E11503C, 0x4E11503C, 0xEC48C9F5, 0xEC48C9F5, 0x8A8043AE,
-      0x8A8043AE, 0x28B7BD67, 0x28B7BD67, 0xC6EF3720 };
-
-   static const byte KEY_INDEX[64] = {
-      0x00, 0x03, 0x01, 0x02, 0x02, 0x01, 0x03, 0x00, 0x00, 0x00, 0x01, 0x03,
-      0x02, 0x02, 0x03, 0x01, 0x00, 0x00, 0x01, 0x00, 0x02, 0x03, 0x03, 0x02,
-      0x00, 0x01, 0x01, 0x01, 0x02, 0x00, 0x03, 0x03, 0x00, 0x02, 0x01, 0x01,
-      0x02, 0x01, 0x03, 0x00, 0x00, 0x03, 0x01, 0x02, 0x02, 0x01, 0x03, 0x01,
-      0x00, 0x00, 0x01, 0x03, 0x02, 0x02, 0x03, 0x02, 0x00, 0x01, 0x01, 0x00,
-      0x02, 0x03, 0x03, 0x02 };
-
    SecureBuffer<u32bit, 4> UK;
-   for(u32bit j = 0; j != 4; ++j)
-      UK[j] = load_be<u32bit>(key, j);
+   for(u32bit i = 0; i != 4; ++i)
+      UK[i] = load_be<u32bit>(key, i);
 
-   for(u32bit j = 0; j != 64; ++j)
-      EK[j] = DELTAS[j] + UK[KEY_INDEX[j]];
+   u32bit D = 0;
+   for(u32bit i = 0; i != 64; i += 2)
+      {
+      EK[i  ] = D + UK[D % 4];
+      D += 0x9E3779B9;
+      EK[i+1] = D + UK[(D >> 11) % 4];
+      }
    }
 
 }