Module Counter
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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new(nbits,
prefix='',
suffix='',
initial_value=1,
overflow=0,
little_endian=False,
allow_wraparound=False,
disable_shortcut=False)
Create a stateful counter block function suitable for CTR encryption modes. |
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new(nbits,
prefix='',
suffix='',
initial_value=1,
overflow=0,
little_endian=False,
allow_wraparound=False,
disable_shortcut=False)
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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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