Rename and document new configuration option for packing AES tables
This commit renames the new AES table packing option introduced in the
previous MBEDTLS_AES_PACK_TABLES and documents its use and memory vs.
speed tradeoff. It also enhances the documentation of the other
AES-related option MBEDTLS_AES_ROM_TABLES.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 44def95..37a9d07 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -381,20 +381,41 @@
/**
* \def MBEDTLS_AES_ROM_TABLES
*
- * Store the AES tables in ROM.
+ * Use precomputed AES tables stored in ROM.
*
- * Uncomment this macro to store the AES tables in ROM.
+ * Uncomment this macro to use precomputed AES tables stored in ROM.
+ * Comment this macro to generate AES tables in RAM at runtime.
+ *
+ * Tradeoff: Using precomputed ROM tables reduces the time to setup
+ * an AES context but comes at the cost of additional 8192b ROM use
+ * (resp. 2048b if \c MBEDTLS_AES_FEWER_TABLES below is used).
+ *
+ * This option is independent of \c MBEDTLS_AES_FEWER_TABLES.
+ *
*/
//#define MBEDTLS_AES_ROM_TABLES
/**
- * \def MBEDTLS_AES_SMALL_TABLES
+ * \def MBEDTLS_AES_FEWER_TABLES
*
- * Use less ROM/RAM for the AES implementation (saves about 6144 bytes).
+ * Use less ROM/RAM for AES tables.
*
- * Uncomment this macro to use less memory for AES.
+ * Uncommenting this macro omits 75% of the AES tables from
+ * ROM / RAM (depending on the value of \c MBEDTLS_AES_ROM_TABLES)
+ * by computing their values on the fly during operations
+ * (the tables are entry-wise rotations of one another).
+ *
+ * Tradeoff: Uncommenting this reduces the RAM / ROM footprint
+ * by 6144b but at the cost of more arithmetic operations during
+ * runtime. Specifically, one has to compare 4 accesses within
+ * different tables to 4 accesses with additional arithmetic
+ * operations within the same table. The performance gain/loss
+ * depends on the system and memory details.
+ *
+ * This option is independent of \c MBEDTLS_AES_ROM_TABLES.
+ *
*/
-//#define MBEDTLS_AES_SMALL_TABLES
+//#define MBEDTLS_AES_FEWER_TABLES
/**
* \def MBEDTLS_CAMELLIA_SMALL_MEMORY