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

AES is very fast and secure, and it is the de facto standard for symmetric encryption.

As an example, encryption can be done as follows:

>>> from Crypto.Cipher import AES
>>> key = b'Sixteen byte key'
>>> cipher =, AES.MODE_EAX)
>>> nonce = cipher.nonce
>>> ciphertext, tag = cipher.encrypt_and_digest(data)

The recipient can obtain the original message using the same key and the incoming triple (nonce, ciphertext, tag):

>>> from Crypto.Cipher import AES
>>> key = b'Sixteen byte key'
>>> cipher =, AES.MODE_EAX, nonce=nonce)
>>> plaintext = cipher.decrypt(ciphertext)
>>> try:
>>>     cipher.verify(tag)
>>>     print("The message is authentic:", plaintext)
>>> except ValueError:
>>>     print("Key incorrect or message corrupted")

Module’s constants for the modes of operation supported with AES:

var MODE_ECB:Electronic Code Book (ECB)
var MODE_CBC:Cipher-Block Chaining (CBC)
var MODE_CFB:Cipher FeedBack (CFB)
var MODE_OFB:Output FeedBack (OFB)
var MODE_CTR:CounTer Mode (CTR)
 OpenPGP Mode
var MODE_CCM:Counter with CBC-MAC (CCM) Mode
var MODE_GCM:Galois Counter Mode (GCM)
var MODE_SIV:Syntethic Initialization Vector (SIV)
var MODE_OCB:Offset Code Book (OCB), mode, *args, **kwargs)

Create a new AES cipher.

  • key (bytes/bytearray/memoryview) –

    The secret key to use in the symmetric cipher.

    It must be 16, 24 or 32 bytes long (respectively for AES-128, AES-192 or AES-256).

    For MODE_SIV only, it doubles to 32, 48, or 64 bytes.

  • mode (One of the supported MODE_* constants) – The chaining mode to use for encryption or decryption. If in doubt, use MODE_EAX.
Keyword Arguments:
  • iv (bytes, bytearray, memoryview) – (Only applicable for MODE_CBC, MODE_CFB, MODE_OFB, and MODE_OPENPGP modes).

    The initialization vector to use for encryption or decryption.

    For MODE_CBC, MODE_CFB, and MODE_OFB it must be 16 bytes long.

    For MODE_OPENPGP mode only, it must be 16 bytes long for encryption and 18 bytes for decryption (in the latter case, it is actually the encrypted IV which was prefixed to the ciphertext).

    If not provided, a random byte string is generated (you must then read its value with the iv attribute).

  • nonce (bytes, bytearray, memoryview) – (Only applicable for MODE_CCM, MODE_EAX, MODE_GCM, MODE_SIV, MODE_OCB, and MODE_CTR).

    A value that must never be reused for any other encryption done with this key (except possibly for MODE_SIV, see below).

    For MODE_EAX, MODE_GCM and MODE_SIV there are no restrictions on its length (recommended: 16 bytes).

    For MODE_CCM, its length must be in the range [7..13]. Bear in mind that with CCM there is a trade-off between nonce length and maximum message size. Recommendation: 11 bytes.

    For MODE_OCB, its length must be in the range [1..15] (recommended: 15).

    For MODE_CTR, its length must be in the range [0..15] (recommended: 8).

    For MODE_SIV, the nonce is optional, if it is not specified, then no nonce is being used, which renders the encryption deterministic.

    If not provided, for modes other than MODE_SIV`, a random byte string of the recommended length is used (you must then read its value with the nonce attribute).

  • segment_size (integer) – (Only MODE_CFB).The number of bits the plaintext and ciphertext are segmented in. It must be a multiple of 8. If not specified, it will be assumed to be 8.

  • mac_len : (integer) – (Only MODE_EAX, MODE_GCM, MODE_OCB, MODE_CCM) Length of the authentication tag, in bytes.

    It must be even and in the range [4..16]. The recommended value (and the default, if not specified) is 16.

  • msg_len : (integer) – (Only MODE_CCM). Length of the message to (de)cipher. If not specified, encrypt must be called with the entire message. Similarly, decrypt can only be called once.

  • assoc_len : (integer) – (Only MODE_CCM). Length of the associated data. If not specified, all associated data is buffered internally, which may represent a problem for very large messages.

  • initial_value : (integer or bytes/bytearray/memoryview) – (Only MODE_CTR). The initial value for the counter. If not present, the cipher will start counting from 0. The value is incremented by one for each block. The counter number is encoded in big endian mode.

  • counter : (object) – Instance of Crypto.Util.Counter, which allows full customization of the counter block. This parameter is incompatible to both nonce and initial_value.

  • use_aesni : (boolean) – Use Intel AES-NI hardware extensions (default: use if available).


an AES object, of the applicable mode.