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Offset Codebook (OCB) mode.
OCB is Authenticated Encryption with Associated Data (AEAD) cipher mode
designed by Prof. Phillip Rogaway and specified in `RFC7253`_.
The algorithm provides both authenticity and privacy, it is very efficient,
it uses only one key and it can be used in online mode (so that encryption
or decryption can start before the end of the message is available).
This module implements the third and last variant of OCB (OCB3) and it only
works in combination with a 128-bit block symmetric cipher, like AES.
OCB is patented in US but `free licenses`_ exist for software implementations
meant for non-military purposes.
Example:
>>> from Crypto.Cipher import AES
>>> from Crypto.Random import get_random_bytes
>>>
>>> key = get_random_bytes(32)
>>> cipher = AES.new(key, AES.MODE_OCB)
>>> plaintext = b"Attack at dawn"
>>> ciphertext, mac = cipher.encrypt_and_digest(plaintext)
>>> # Deliver cipher.nonce, ciphertext and mac
...
>>> cipher = AES.new(key, AES.MODE_OCB, nonce=nonce)
>>> try:
>>> plaintext = cipher.decrypt_and_verify(ciphertext, mac)
>>> except ValueError:
>>> print "Invalid message"
>>> else:
>>> print plaintext
:undocumented: __package__
.. _RFC7253: http://www.rfc-editor.org/info/rfc7253
.. _free licenses: http://web.cs.ucdavis.edu/~rogaway/ocb/license.htm
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bytes_to_long)strxor)BLAKE2s)get_random_bytes)load_pycryptodome_raw_lib VoidPointercreate_string_bufferget_raw_buffer SmartPointerc_size_t c_uint8_ptr is_bufferzCrypto.Cipher._raw_ocba
int OCB_start_operation(void *cipher,
const uint8_t *offset_0,
size_t offset_0_len,
void **pState);
int OCB_encrypt(void *state,
const uint8_t *in,
uint8_t *out,
size_t data_len);
int OCB_decrypt(void *state,
const uint8_t *in,
uint8_t *out,
size_t data_len);
int OCB_update(void *state,
const uint8_t *in,
size_t data_len);
int OCB_digest(void *state,
uint8_t *tag,
size_t tag_len);
int OCB_stop_operation(void *state);
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c_ ||d }t | j dk r| S d| j c| _ }| | t | dz dz }t |d| | _ | || | S )a Process the associated data.
If there is any associated data, the caller has to invoke
this method one or more times, before using
``decrypt`` or ``encrypt``.
By *associated data* it is meant any data (e.g. packet headers) that
will not be encrypted and will be transmitted in the clear.
However, the receiver shall still able to detect modifications.
If there is no associated data, this method must not be called.
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invoke this method multiple times, each time with the next segment.
:Parameters:
assoc_data : bytes/bytearray/memoryview
A piece of associated data.
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the final piece of ciphertext.
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:Parameters:
plaintext : bytes/bytearray/memoryview
The next piece of data to encrypt or ``None`` to signify
that encryption has finished and that any remaining ciphertext
has to be produced.
:Return:
the ciphertext, as a byte string.
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:Parameters:
ciphertext : bytes/bytearray/memoryview
The next piece of data to decrypt or ``None`` to signify
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the plaintext, as a byte string.
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r c d| j vrt d t | j dk sJ dg| _ | j | | j S )a# Compute the *binary* MAC tag.
Call this method after the final `encrypt` (the one with no arguments)
to obtain the MAC tag.
The MAC tag is needed by the receiver to determine authenticity
of the message.
:Return: the MAC, as a byte string.
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:Return: the MAC, as a hexadecimal string.
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d }t j d|| j }t j d|| }| | k rt d dS ) a Validate the *binary* MAC tag.
Call this method after the final `decrypt` (the one with no arguments)
to check if the message is authentic and valid.
:Parameters:
received_mac_tag : bytes/bytearray/memoryview
This is the *binary* MAC, as received from the sender.
:Raises ValueError:
if the MAC does not match. The message has been tampered with
or the key is incorrect.
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= ! ! # # #!"%%{sT]KKK{s=MNNN ;;==DKKMM ) ) /00 0 * )r c J | t | dS )aW Validate the *printable* MAC tag.
This method is like `verify`.
:Parameters:
hex_mac_tag : string
This is the *printable* MAC, as received from the sender.
:Raises ValueError:
if the MAC does not match. The message has been tampered with
or the key is incorrect.
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Ik**+++++r c ~ | | | z | fS )a& Encrypt the message and create the MAC tag in one step.
:Parameters:
plaintext : bytes/bytearray/memoryview
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:Return:
a tuple with two byte strings:
- the encrypted data
- the MAC
)r r rm s rT encrypt_and_digestzOcbMode.encrypt_and_digest s0 ||I&& 7 FFr c | | | z }| | |S )a Decrypted the message and verify its authenticity in one step.
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ciphertext : bytes/bytearray/memoryview
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received_mac_tag : byte string
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:Return: the decrypted data (byte string).
:Raises ValueError:
if the MAC does not match. The message has been tampered with
or the key is incorrect.
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:Parameters:
factory : module
A symmetric cipher module from `Crypto.Cipher`
(like `Crypto.Cipher.AES`).
:Keywords:
nonce : bytes/bytearray/memoryview
A value that must never be reused for any other encryption.
Its length can vary from 1 to 15 bytes.
If not specified, a random 15 bytes long nonce is generated.
mac_len : integer
Length of the MAC, in bytes.
It must be in the range ``[8..16]``.
The default is 16 (128 bits).
Any other keyword will be passed to the underlying block cipher.
See the relevant documentation for details (at least ``key`` will need
to be present).
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