In the first round of AES, use a 256 element table and do the

rotations in the code. This reduces the number of cache lines
potentially accessed in the first round from 64 to 16 (assuming 64
byte cache lines). On average, about 10 cache lines will actually be
accessed, assuming a uniform distribution of the inputs, so there
definitely is still a timing channel here, just a somewhat smaller
one.

I experimented with using the 256 element table for all rounds but it
reduced performance significantly and I'm not sure if the benefit is
worth the cost or not.

1 files changed, 28 insertions, 9 deletions

diff --git a/src/block/aes/aes.cpp b/src/block/aes/aes.cpp
index bf9a4198b..1177a1461 100644
--- a/src/block/aes/aes.cpp
+++ b/src/block/aes/aes.cpp
@@ -8,6 +8,7 @@
 #include <botan/aes.h>
 #include <botan/loadstor.h>
 #include <botan/rotate.h>
+#include <botan/internal/prefetch.h>
 
 namespace Botan {
 
@@ -426,15 +427,33 @@ void AES::encrypt_n(const byte in[], byte out[], u32bit blocks) const
       u32bit T2 = load_be<u32bit>(in, 2) ^ EK[2];
       u32bit T3 = load_be<u32bit>(in, 3) ^ EK[3];
 
-      u32bit B0, B1, B2, B3;
-      B0 = TE0[get_byte(0, T0)] ^ TE1[get_byte(1, T1)] ^
-           TE2[get_byte(2, T2)] ^ TE3[get_byte(3, T3)] ^ EK[4];
-      B1 = TE0[get_byte(0, T1)] ^ TE1[get_byte(1, T2)] ^
-           TE2[get_byte(2, T3)] ^ TE3[get_byte(3, T0)] ^ EK[5];
-      B2 = TE0[get_byte(0, T2)] ^ TE1[get_byte(1, T3)] ^
-           TE2[get_byte(2, T0)] ^ TE3[get_byte(3, T1)] ^ EK[6];
-      B3 = TE0[get_byte(0, T3)] ^ TE1[get_byte(1, T0)] ^
-           TE2[get_byte(2, T1)] ^ TE3[get_byte(3, T2)] ^ EK[7];
+      /* Use only the first 256 entries of the TE table and do the
+      * rotations directly in the code. This reduces the number of
+      * cache lines potentially used in the first round from 64 to 16
+      * (assuming a typical 64 byte cache line), which makes timing
+      * attacks a little harder; the first round is particularly
+      * vulnerable.
+      */
+
+      u32bit B0 = TE[get_byte(0, T0)] ^
+                  rotate_right(TE[get_byte(1, T1)],  8) ^
+                  rotate_right(TE[get_byte(2, T2)], 16) ^
+                  rotate_right(TE[get_byte(3, T3)], 24) ^ EK[4];
+
+      u32bit B1 = TE[get_byte(0, T1)] ^
+                  rotate_right(TE[get_byte(1, T2)],  8) ^
+                  rotate_right(TE[get_byte(2, T3)], 16) ^
+                  rotate_right(TE[get_byte(3, T0)], 24) ^ EK[5];
+
+      u32bit B2 = TE[get_byte(0, T2)] ^
+                  rotate_right(TE[get_byte(1, T3)],  8) ^
+                  rotate_right(TE[get_byte(2, T0)], 16) ^
+                  rotate_right(TE[get_byte(3, T1)], 24) ^ EK[6];
+
+      u32bit B3 = TE[get_byte(0, T3)] ^
+                  rotate_right(TE[get_byte(1, T0)],  8) ^
+                  rotate_right(TE[get_byte(2, T1)], 16) ^
+                  rotate_right(TE[get_byte(3, T2)], 24) ^ EK[7];
 
       for(u32bit j = 2; j != ROUNDS; j += 2)
          {