Tiny AES128 in C

This is a small and portable implementation of the AES128 ECB and CBC encryption algorithms written in C.

This is meant to be the back-end of whatever error checking you need to validate your input. There is no built-in protection for buffer overflows if malicious input is given, or the size of buffers are not chosen carefully.

The API is very simple and looks like this (I am using C99 <stdint.h>-style annotated types):

void AES128_ECB_encrypt(uint8_t* input, const uint8_t* key, uint8_t* output);
void AES128_ECB_decrypt(uint8_t* input, const uint8_t* key, uint8_t* output);
void AES128_CBC_encrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);
void AES128_CBC_decrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);

You can choose to use one or both of the modes-of-operation, by defining the symbols CBC and ECB. See the header file for clarification.

The module uses around 200 bytes of RAM and 2.5K ROM when compiled for ARM (~2K for Thumb but 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 ECB mode is compiled for ARM:

$ arm-none-eabi-gcc -Os -c aes.c -DCBC=0
$ size aes.o
   text    data     bss     dec     hex filename
   2323       0     184    2507     9cb aes.o

.. and when compiling for the THUMB instruction set, we end up around 2K in code size.

$ arm-none-eabi-gcc -mthumb -Os -c aes.c -DCBC=0
$ size aes.o
   text    data     bss     dec     hex filename
   1775       0     184    1959     7a7 aes.o

I am using Mentor Graphics free ARM toolchain:

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.8.4 20140526 (release) [ARM/embedded-4_8-branch revision 211358]
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.

This 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.

All material in this repository is in the public domain.