Tiny AES128 in C

This is a small and portable implementation of the AES128 algorithm implemented in C.

The module uses just a bit more than 200 bytes of RAM and 1K ROM when compiled for ARM (YMMV).

It is the smallest implementation in C I've seen yet, but do contact me if you know of something smaller (or have improvements to the code here).

I've successfully used the code on 64bit x86, 32bit ARM and 8 bit AVR platforms.

GCC size output when compiled for ARM:

$ arm-none-eabi-gcc -Os -c aes.c -o aes.o
$ size aes.o
   text    data     bss     dec     hex filename
   1079       0     204    1283     503 aes.o
   
$ arm-none-eabi-gcc -O2 -c aes.c -o aes.o
$ size aes.o
   text    data     bss     dec     hex filename
   1024       0     204    1228     4cc aes.o

And if we're compiling for the Thumb ISA:

$ arm-none-eabi-gcc -O2 -c -mthumb aes.c -o aes.o
$ size aes.o
   text    data     bss     dec     hex filename
    856       0     204    1060     424 aes.o
$ arm-none-eabi-gcc -Os -c -mthumb aes.c -o aes.o
$ size aes.o
   text    data     bss     dec     hex filename
    867       0     204    1071     42f aes.o

Somehow, -O2 is a bit smaller than -Os with this compiler. I am using Mentor Graphics ARM toolchain:

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.8.3 20131129 (release) [ARM/embedded-4_8-branch revision 205641]
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The implementation is verified against the data in:

National Institute of Standards and Technology Special Publication 800-38A 2001 ED Appendix F: Example Vectors for Modes of Operation of the AES.