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### 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):
```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 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](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.
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