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.

AES is implemented using the ESP32 hardware module.

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:])


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



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


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


  • 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_8, AES.SEGMENT_128: Length of the segment for AES.MODE_CFB

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.

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