You can override the default key-size of 128 bit with 192 or 256 bit by defining the symbols AES192 or AES256 in
The API is very simple and looks like this (I am using C99
<stdint.h>-style annotated types):
/* Initialize context calling one of: */ void AES_init_ctx(struct AES_ctx* ctx, const uint8_t* key); void AES_init_ctx_iv(struct AES_ctx* ctx, const uint8_t* key, const uint8_t* iv); /* ... or reset IV at random point: */ void AES_ctx_set_iv(struct AES_ctx* ctx, const uint8_t* iv); /* Then start encrypting and decrypting with the functions below: */ void AES_ECB_encrypt(const struct AES_ctx* ctx, uint8_t* buf); void AES_ECB_decrypt(const struct AES_ctx* ctx, uint8_t* buf); void AES_CBC_encrypt_buffer(struct AES_ctx* ctx, uint8_t* buf, size_t length); void AES_CBC_decrypt_buffer(struct AES_ctx* ctx, uint8_t* buf, size_t length); /* Same function for encrypting as for decrypting in CTR mode */ void AES_CTR_xcrypt_buffer(struct AES_ctx* ctx, uint8_t* buf, size_t length);
You can choose to use any or all of the modes-of-operations, by defining the symbols CBC, CTR or ECB in
aes.h (read the comments 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 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 implementations 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 1171 0 0 1171 493 aes.o
.. and when compiling for the THUMB instruction set, we end up well below 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 903 0 0 903 387 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.