Awesome

As people keep opening issues, a few notes:

This code is for AVR based arduino chips. So esp8266 is not supported. However, Arduino ESP8266 has AES/CBC build in via BearSSL
I would advice switching to my portable Chacha20-Poly1305 library it should compile on most boards. Has better cryptographic properties, and is often faster.

Arduino AESLib

This project is just an Arduino ready extract from the AVR-Crypto-Lib.

It only packages the ASM implementations of AES into a library ready to use in Arduino IDE.

See the LICENSE file for details of the GPLv3 license in which the AVR-Crypo-Lib is licensed.

Installation

Download the files in this repository (using either clone or the download button)
Copy the AESLib folder into libraries folder (same level as your sketch folder)
add #include <AESLib.h> in your sketch.

Usage

At the moment only 128bit keys are supported, the blocksize is also fixed at 128bit. This means that the key array and possible iv array should contain exactly 16 bytes (uint8_t or byte). Moreover the amount of bytes to encrypt should be mod 16. (this means you have to take care of padding yourself).

The library supports 3 kinds of operations.

single block encryption/decryption

multiple block encryption/decryption using CBC (single call)
multiple block encryption/decryption using CBC (multiple calls)

The single block enc/decryption are the following methods:

void aes128_enc_single(const uint8_t* key, void* data);
void aes128_dec_single(const uint8_t* key, void* data);

Usage example:

Serial.begin(57600);
uint8_t key[] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};
char data[] = "0123456789012345"; //16 chars == 16 bytes
aes128_enc_single(key, data);
Serial.print("encrypted:");
Serial.println(data);
aes128_dec_single(key, data);
Serial.print("decrypted:");
Serial.println(data);

Usage example for AES256:

Serial.begin(57600);
uint8_t key[] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31};
char data[] = "0123456789012345";
aes256_enc_single(key, data);
Serial.print("encrypted:");
Serial.println(data);
aes256_dec_single(key, data);
Serial.print("decrypted:");
Serial.println(data);