From 30b6de9dd377259685cfd0aedabd1f891fcf0d44 Mon Sep 17 00:00:00 2001
From: Kévin Le Gouguec <kevin.legouguec@airbus.com>
Date: Tue, 27 Nov 2018 14:08:56 +0100
Subject: Implémentation du mode SCT-2
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---
 src/ae-common.h | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

(limited to 'src/ae-common.h')

diff --git a/src/ae-common.h b/src/ae-common.h
index da5d04d..561854e 100644
--- a/src/ae-common.h
+++ b/src/ae-common.h
@@ -65,26 +65,23 @@ static inline void pad10(size_t X_len, const uint8_t X[X_len], uint8_t padded[BL
     }
 }
 
-static inline void _fill_ad_tweak(
+static inline void fill_index_tweak(
     uint8_t  prefix,
-    uint64_t block_nb,
+    uint64_t block_index,
     uint8_t  tweak[TWEAK_BYTES]
 )
 {
     /* The t-bit tweak is filled as follows:
      *
-     * - bits [  1, t-4]: block number
-     *        [  1,  64]: actual 64-bit block number
+     * - bits [  1, t-4]: block index
+     *        [  1,  64]: actual 64-bit block index
      *        [ 65, t-4]: 0-padding
      * - bits [t-3,   t]: constant 4-bit prefix
      */
 
-    for (size_t i=0; i<sizeof(block_nb); i++)
+    for (size_t i=0; i<sizeof(block_index); i++)
     {
-        uint64_t mask = (uint64_t)0xff << 8*i;
-        uint8_t b = (mask & block_nb) >> 8*i;
-
-        tweak[i] = b;
+        tweak[i] = block_index >> 8*i & 0xff;
     }
 
     /* Assume padding bytes have already been memset to 0. */
@@ -110,7 +107,7 @@ static void process_associated_data(
 
     for (size_t i=0; i<l_a; i++)
     {
-        _fill_ad_tweak(0x2, i, tweak);
+        fill_index_tweak(0x2, i, tweak);
         encrypt(key, tweak, &A[i*BLOCK_BYTES], Ek_Ai);
         xor_into(Auth, Ek_Ai);
     }
@@ -119,7 +116,7 @@ static void process_associated_data(
     {
         uint8_t A_rest[BLOCK_BYTES];
         pad10(rest, &A[l_a*BLOCK_BYTES], A_rest);
-        _fill_ad_tweak(0x6, l_a, tweak);
+        fill_index_tweak(0x6, l_a, tweak);
         encrypt(key, tweak, A_rest, Ek_Ai);
         xor_into(Auth, Ek_Ai);
     }
-- 
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