toc AES
Firmware & API Reference > Pycom Modules > AES

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.

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:])
print(original)

Constructors

class ucrypto.AES(key, [mode=AES.MODE_ECB, 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. Can be the following values:
    • AES.MODE_ECB: Electronic Code Book. Simplest encryption mode.
    • 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 as 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.
  • IV (byte string) initialisation vector. Should be 16 bytes long. It is ignored in modes AES.MODE_ECB and AES.MODE_CRT.

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.

  • 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
    • 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, AES.MODE_CBC, AES.MODE_CFB, AES.MODE_CTR
  • AES.SEGMENT_8, AES.SEGMENT_128

 

Previous Next