Package Crypto :: Package Cipher :: Module AES
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Module AES

AES symmetric cipher

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
>>> from Crypto import Random
>>> key = b'Sixteen byte key'
>>> iv =
>>> cipher =, AES.MODE_CFB, iv)
>>> msg = iv + cipher.encrypt(b'Attack at dawn')
AES cipher object
new(key, *args, **kwargs)
Create a new AES cipher
  MODE_ECB = 1
Electronic Code Book (ECB). See blockalgo.MODE_ECB.
  MODE_CBC = 2
Cipher-Block Chaining (CBC). See blockalgo.MODE_CBC.
  MODE_CFB = 3
Cipher FeedBack (CFB). See blockalgo.MODE_CFB.
  MODE_PGP = 4
This mode should not be used.
  MODE_OFB = 5
Output FeedBack (OFB). See blockalgo.MODE_OFB.
  MODE_CTR = 6
CounTer Mode (CTR). See blockalgo.MODE_CTR.
OpenPGP Mode. See blockalgo.MODE_OPENPGP.
  block_size = 16
Size of a data block (in bytes)
  key_size = (16, 24, 32)
Size of a key (in bytes)
Function Details

new(key, *args, **kwargs)

Create a new AES cipher
  • key (byte string) - The secret key to use in the symmetric cipher. It must be 16 (AES-128), 24 (AES-192), or 32 (AES-256) bytes long.
  • mode (a MODE_* constant) - The chaining mode to use for encryption or decryption. Default is MODE_ECB.
  • IV (byte string) - The initialization vector to use for encryption or decryption.

    It is ignored for MODE_ECB and MODE_CTR.

    For MODE_OPENPGP, IV must be block_size bytes long for encryption and block_size +2 bytes for decryption (in the latter case, it is actually the encrypted IV which was prefixed to the ciphertext). It is mandatory.

    For all other modes, it must be block_size bytes longs. It is optional and when not present it will be given a default value of all zeroes.

  • counter (callable) - (Only MODE_CTR). A stateful function that returns the next counter block, which is a byte string of block_size bytes. For better performance, use Crypto.Util.Counter.
  • 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 0 or not specified, it will be assumed to be 8.
an AESCipher object