AES¶
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.new(key, 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.new(key, 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) |
var MODE_OPENPGP: | |
OpenPGP Mode | |
var MODE_CCM: | Counter with CBC-MAC (CCM) Mode |
var MODE_EAX: | EAX Mode |
var MODE_GCM: | Galois Counter Mode (GCM) |
var MODE_SIV: | Syntethic Initialization Vector (SIV) |
var MODE_OCB: | Offset Code Book (OCB) |
-
Crypto.Cipher.AES.
new
(key, mode, *args, **kwargs)¶ Create a new AES cipher.
Parameters: - 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, useMODE_EAX
.
Keyword Arguments: iv (bytes, bytearray, memoryview) – (Only applicable for
MODE_CBC
,MODE_CFB
,MODE_OFB
, andMODE_OPENPGP
modes).The initialization vector to use for encryption or decryption.
For
MODE_CBC
,MODE_CFB
, andMODE_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
, andMODE_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
andMODE_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 thenonce
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 bothnonce
andinitial_value
.use_aesni : (boolean) – Use Intel AES-NI hardware extensions (default: use if available).
Return: an AES object, of the applicable mode.
- key (bytes/bytearray/memoryview) –