path: root/README.md

### Tiny AES in C

This is a small and portable implementation of the AES [ECB](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Electronic_Codebook_.28ECB.29), [CTR](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Counter_.28CTR.29) and [CBC](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_.28CBC.29) encryption algorithms written in C.

You can override the default key-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):

```C
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 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](http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf) 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.