Tiny AES in C

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

You can override the default block-size of 128 bit with 192 or 256 bit by defining the symbols AES192 or AES256 in aes.h.

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

void AES_ECB_encrypt(uint8_t* input, const uint8_t* key, uint8_t* output);
void AES_ECB_decrypt(uint8_t* input, const uint8_t* key, uint8_t* output);

void AES_CBC_encrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);
void AES_CBC_decrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* iv);

/* Same function for encrypting as for decrypting. Note no IV/nonce should ever be reused with the same key */
void AES_CTR_xcrypt_buffer(uint8_t* output, uint8_t* input, uint32_t length, const uint8_t* key, const uint8_t* nonce);

You can choose to use any or all of the modes-of-operations, by defining the symbols CBC, CTR or ECB. See the header file for clarification.

There is no built-in error checking or protection from out-of-bounds memory access errors as a result of malicious input. The two functions AES_ECB_xxcrypt() do most of the work, and they expect inputs of 128 bit length.

The module uses less than 200 bytes of RAM and 1-2K ROM when compiled for ARM, but YMMV depending on which modes are enabled.

It is one of 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 only CTR mode is compiled for ARM:

$ arm-none-eabi-gcc -Os -DCBC=0 -DECB=0 -DCTR=1 -c aes.c
$ size aes.o
   text    data     bss     dec     hex filename
   1155       0     184    1339     53b aes.o

.. and when compiling for the THUMB instruction set, we end up just above 1K in code size.

$ arm-none-eabi-gcc -Os -mthumb -DCBC=0 -DECB=0 -DCTR=1 -c aes.c
$ size aes.o
   text    data     bss     dec     hex filename
    855       0     184    1039     40f aes.o

I am using the Free Software Foundation, ARM GCC compiler:

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (4.8.4-1+11-1) 4.8.4 20141219 (release)
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.

The other appendices in the document are valuable for implementation details on e.g. padding, generation of IVs and nonces in CTR-mode etc.

A heartfelt thank-you to all the nice people out there who have contributed to this project.

All material in this repository is in the public domain.