Merge remote-tracking branch 'upstream-restricted/pr/363' into development-proposed

include/mbedtls/config.h

@@ -440,12 +440,45 @@
 /**
  * \def MBEDTLS_AES_ROM_TABLES
  *
- * Store the AES tables in ROM.
+ * Use precomputed AES tables stored 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 RAM usage by ~8kb
+ * (or ~2kb if \c MBEDTLS_AES_FEWER_TABLES is used) and reduces the
+ * initialization time before the first AES operation can be performed.
+ * It comes at the cost of additional ~8kb ROM use (resp. ~2kb if \c
+ * MBEDTLS_AES_FEWER_TABLES below is used), and potentially degraded
+ * performance if ROM access is slower than RAM access.
+ *
+ * This option is independent of \c MBEDTLS_AES_FEWER_TABLES.
  *
- * Uncomment this macro to store the AES tables in ROM.
  */
 //#define MBEDTLS_AES_ROM_TABLES
 
+/**
+ * \def MBEDTLS_AES_FEWER_TABLES
+ *
+ * Use less ROM/RAM for AES tables.
+ *
+ * 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 ~6kb 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_FEWER_TABLES
+
 /**
  * \def MBEDTLS_CAMELLIA_SMALL_MEMORY
  *