# 5.2.3 AES

AES \(Advanced Encryption Standard\) is a symmetric block cipher standardised by NIST. It has a fixed data block size of 16 bytes. Its keys can be 128, 192, or 256 bits long.

{% hint style="danger" %}
AES is implemented using the ESP32 hardware module.
{% endhint %}

## Quick Usage Example

```python
from crypto import AES
import crypto
key = b'notsuchsecretkey' # 128 bit (16 bytes) key
iv = crypto.getrandbits(128) # hardware generated random IV (never reuse it)

cipher = AES(key, AES.MODE_CFB, iv)
msg = iv + cipher.encrypt(b'Attack at dawn')

# ... after properly sent the encrypted message somewhere ...

cipher = AES(key, AES.MODE_CFB, msg[:16]) # on the decryption side
original = cipher.decrypt(msg[16:])
print(original)
```

## Constructors

### class ucrypto.AES\(key, mode, IV, \* , counter, segment\_size\)

Create an AES object that will let you encrypt and decrypt messages.

The arguments are:

* `key` \(byte string\) is the secret key to use. It must be 16 \(AES-128\), 24 \(AES-192\), or 32 \(AES-256\) bytes long.
* `mode` is the chaining mode to use for encryption and decryption. Default is AES.MODE\_ECB.
* `IV` \(byte string\) initialisation vector. Should be 16 bytes long. It is ignored in modes AES.MODE\_ECB and AES.MODE\_CRT.
* `counter` \(byte string\) used only for AES.MODE\_CTR. Should be 16 bytes long. Should not be reused.
* `segment_size` is the number of bits `plaintext` and `ciphertext` are segmented in. Is only used in AES.MODE\_CFB. Supported values are AES.SEGMENT\_8 and AES.SEGMENT\_128.

## Methods

### ucrypto.encrypt\(\)

Encrypt data with the key and the parameters set at initialisation.

### ucrypto.decrypt\(\)

Decrypt data with the key and the parameters set at initialisation.

## Constants

AES.MODE\_ECB

Electronic Code Book. Simplest encryption mode. It does not hide data patterns well \(see this article for more info\).

AES.MODE\_CBC

Cipher-Block Chaining. An Initialisation Vector \(IV\) is required.

AES.MODE\_CFB

Cipher feedback. `plaintext` and `ciphertext` are processed in segments of `segment_size` bits. Works a stream cipher.

AES.MODE\_CTR

Counter mode. Each message block is associated to a counter which must be unique across all messages that get encrypted with the same key.

AES.SEGMENT\_8AES.SEGMENT\_128

Length of the segment for AES.MODE\_CFB.

{% hint style="danger" %}
To avoid security issues, IV should always be a random number and should never be reused to encrypt two different messages. The same applies to the counter in CTR mode. You can use crypto.getrandbits\(\) for this purpose.
{% endhint %}