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

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 %}

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