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� Z d S( s� 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)
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S( sc 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.
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