B ?[$@szdZddlZejddkr0ejddkr0ddlTddlTddlmZddlZededdddddfd d Z d d d Z dS)aFast counter functions for CTR cipher modes. CTR is a chaining mode for symmetric block encryption or decryption. Messages are divideded into blocks, and the cipher operation takes place on each block using the secret key and a unique *counter block*. The most straightforward way to fulfil the uniqueness property is to start with an initial, random *counter block* value, and increment it as the next block is processed. The block ciphers from `Crypto.Cipher` (when configured in *MODE_CTR* mode) invoke a callable object (the *counter* parameter) to get the next *counter block*. Unfortunately, the Python calling protocol leads to major performance degradations. The counter functions instantiated by this module will be invoked directly by the ciphers in `Crypto.Cipher`. The fact that the Python layer is bypassed lead to more efficient (and faster) execution of CTR cipher modes. An example of usage is the following: >>> from Crypto.Cipher import AES >>> from Crypto.Util import Counter >>> >>> pt = b''*1000000 >>> ctr = Counter.new(128) >>> cipher = AES.new(b''*16, AES.MODE_CTR, counter=ctr) >>> ct = cipher.encrypt(pt) :undocumented: __package__ N)*)_counterFc Cst|d\}} | dkr$td|f|dkr6tdn|dkrFtdt|||} |rrtjt|t|| ||dStjt|t|| ||dSd S) acCreate 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. rz%nbits must be a multiple of 8; got %drznbits too smalliznbits too large)allow_wraparounddisable_shortcutN)divmod ValueError_encoderZ_newLEZbstrZ_newBE) ZnbitsprefixsuffixZ initial_valueZoverflow little_endianrr nbytesZ remainderZinitvalr2lib/python3.7/site-packages/Crypto/Util/Counter.pynew?s%  rcCs`g}t|}xDt|D]8}|r2|t|d@n|dt|d@|dL}qWtd|S)Nrrr)intrangeappendZbchrinsertbjoin)nrrZretvalirrrr ts r )F) __doc__sys version_infoZCrypto.Util.py21compatZCrypto.Util.py3compatZ Crypto.UtilrZstructrrr rrrr5s  5