Ë
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¬`iÜ ã ó ddZ y)ó c ó | dz dk7 r t d« |j « }|| kD rt d|| fz « | dz ||||dS )a Create a stateful counter block function suitable for CTR encryption modes.
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Each call to the function returns the next counter block.
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Each counter block is made up by three parts:
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+------+--------------+-------+
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|prefix| counter value|postfix|
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+------+--------------+-------+
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The counter value is incremented by 1 at each call.
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Args:
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nbits (integer):
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Length of the desired counter value, in bits. It must be a multiple of 8.
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prefix (byte string):
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The constant prefix of the counter block. By default, no prefix is
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used.
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suffix (byte string):
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The constant postfix of the counter block. By default, no suffix is
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used.
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initial_value (integer):
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The initial value of the counter. Default value is 1.
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Its length in bits must not exceed the argument ``nbits``.
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little_endian (boolean):
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If ``True``, the counter number will be encoded in little endian format.
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If ``False`` (default), in big endian format.
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allow_wraparound (boolean):
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This parameter is ignored.
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An ``OverflowError`` exception is always raised when the counter wraps
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around to zero.
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Returns:
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An object that can be passed with the :data:`counter` parameter to a CTR mode
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cipher.
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It must hold that *len(prefix) + nbits//8 + len(suffix)* matches the
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block size of the underlying block cipher.
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é é z 'nbits' must be a multiple of 8zGInitial value takes %d bits but it is longer than the counter (%d bits))Ú counter_lenÚprefixÚsuffixÚ initial_valueÚ little_endian)Ú
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ValueErrorÚ
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