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File Content: Counter.cpython-37.pyc

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dS�)a���Fast counter functions for CTR cipher modes.

CTR is a chaining mode for symmetric block encryption or decryption.
Messages are divideded into blocks, and the cipher operation takes
place on each block using the secret key and a unique *counter block*.

The most straightforward way to fulfil the uniqueness property is
to start with an initial, random *counter block* value, and increment it as
the next block is processed.

The block ciphers from `Crypto.Cipher` (when configured in *MODE_CTR* mode)
invoke a callable object (the *counter* parameter) to get the next *counter block*.
Unfortunately, the Python calling protocol leads to major performance degradations.

The counter functions instantiated by this module will be invoked directly
by the ciphers in `Crypto.Cipher`. The fact that the Python layer is bypassed
lead to more efficient (and faster) execution of CTR cipher modes.

An example of usage is the following:

>>> from Crypto.Cipher import AES
    >>> from Crypto.Util import Counter
    >>>
    >>> pt = b'�'*1000000
    >>> ctr = Counter.new(128)
    >>> cipher = AES.new(b'�'*16, AES.MODE_CTR, counter=ctr)
    >>> ct = cipher.encrypt(pt)

:undocumented: __package__
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|rrtjt|�t|�|
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||d�S�d	S�)
ac��Create a stateful counter block function suitable for CTR encryption modes.

Each call to the function returns the next counter block.
    Each counter block is made up by three parts::
 
      prefix || counter value || postfix

The counter value is incremented by one at each call.

:Parameters:
      nbits : integer
        Length of the desired counter, in bits. It must be a multiple of 8.
      prefix : byte string
        The constant prefix of the counter block. By default, no prefix is
        used.
      suffix : byte string
        The constant postfix of the counter block. By default, no suffix is
        used.
      initial_value : integer
        The initial value of the counter. Default value is 1.
      little_endian : boolean
        If True, the counter number will be encoded in little endian format.
        If False (default), in big endian format.
      allow_wraparound : boolean
        If True, the function will raise an *OverflowError* exception as soon
        as the counter wraps around. If False (default), the counter will
        simply restart from zero.
      disable_shortcut : boolean
        If True, do not make ciphers from `Crypto.Cipher` bypass the Python
        layer when invoking the counter block function.
        If False (default), bypass the Python layer.
    :Returns:
      The counter block function.
    ����r���z%nbits must be a multiple of 8; got %dr���znbits too smalli����znbits too large)�allow_wraparound�disable_shortcutN)�divmod�
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r���c�������������C���s`���g�}t�|��}�xDt|�D�]8}|r2|�t|�d@����n|�dt|�d@����|�dL�}�qW�td��|�S�)N����r���r���r���)�int�range�appendZbchr�insert�b�join)�nr���r���Zretval�ir���r���r���r���t���s����r���)F)�__doc__�sys�version_infoZCrypto.Util.py21compatZCrypto.Util.py3compatZCrypto.Utilr����structr���r���r���r���r���r���r����<module>5���s��� 5

Name	Size	Permission
Counter.cpython-37.opt-1.pyc	3815 bytes	0644
Counter.cpython-37.pyc	3815 bytes	0644
RFC1751.cpython-37.opt-1.pyc	23656 bytes	0644
RFC1751.cpython-37.pyc	23656 bytes	0644
__init__.cpython-37.opt-1.pyc	690 bytes	0644
__init__.cpython-37.pyc	690 bytes	0644
_number_new.cpython-37.opt-1.pyc	2614 bytes	0644
_number_new.cpython-37.pyc	2703 bytes	0644
asn1.cpython-37.opt-1.pyc	11607 bytes	0644
asn1.cpython-37.pyc	11607 bytes	0644
number.cpython-37.opt-1.pyc	59476 bytes	0644
number.cpython-37.pyc	59515 bytes	0644
py3compat.cpython-37.opt-1.pyc	3270 bytes	0644
py3compat.cpython-37.pyc	3270 bytes	0644
randpool.cpython-37.opt-1.pyc	2583 bytes	0644
randpool.cpython-37.pyc	2583 bytes	0644
winrandom.cpython-37.opt-1.pyc	231 bytes	0644
winrandom.cpython-37.pyc	231 bytes	0644