[riot-devel] AES crypto optimizations, speed and size

Oleg Artamonov oleg at unwds.com
Tue Jan 2 17:08:44 CET 2018

* oops, sorry, here is the plain text version, no HTML *

Hello all,

Recently we did some AES encoding/decoding optimizations and experiments, and here I want to share results.

System: STM32L151CCU6 @ 32 MHz, arm-none-eabi-gcc 6.3.1, -O2

What we tried:
* existing option: FULL_UNROLL to unroll loops in aes.c
* new option: AES_CALCULATE_TABLES to calculate AES T-tables in runtime if possible instead of storing everything in the flash memory
* new implementation: AES_ASM (AES-128 only) using "All the AES You Need on Cortex-M3 and M4" code by Peter Schwabe and Ko Stoffelen, see https://github.com/Ko-/aes-armcortexm and https://eprint.iacr.org/2016/714.pdf

What we measured:
* firmware size (we have quite a big fw, but only AES options were changed between build)
* time to encode/decode a single AES-128 block, using xtimer_now (1 us default tick )

Ok, so here it comes (encoding / decoding, firmware size):
* default RIOT AES implementation (no unroll, T-tables in flash): 74 / 131 us, 104.8 KB
* no unroll, run-time T-tables calc: 77 / 139 us, 97.1 KB

* full unroll, T-tables in flash: 64 / 108 us, 110.1 KB
* full unroll, run-time T-tables calc: 56 / 108 us, 102.1 KB

* assembler: 45 / 87 us, 104.6 KB

It seems that storing all T-tables in flash is quite useless, at least on Cortex-M MCUs: takes a lot of flash and performance improvement is insignificant or even negative. Unrolling loops with run-time T-tables provides 25-30 % speed improvement over default settings _and_ firmware size reduction by 2.7 KB at the same time, so if you need fast and portable implementation — look no further.

Code: https://github.com/unwireddevices/RIOT/tree/loralan-public/sys/crypto (aes.c, aes_asm_cortexm.c)

I'm not sure about other hardware platforms, but as Cortex-M3/M4 is the most popular processor core now, I think default AES implementation in RIOT should be changed to run-time T-tables calculation to save flash.

P.S. https://docs.google.com/spreadsheets/d/1RwxomeVPoE-SngUHeRgPoI3ObQVgENsQQuIliMXDZNg/edit?usp=sharing

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