**Consider the following block cipher.**Suppose that each block cipher T simply reverses the order of the eight input bits (so that, for example 11110000 becomes 00001111).

Further suppose that the 64-bit scrambler does not modify any bits.
With n = 3 iterations and the original 64-bit input equal to 10100000
repeated eight times, what is the value of the output?

Now change the last bit of the original 64-bit input from 0 to a 1.
Now suppose that the 64-bit scrambler inverses the order of the 64
bits.

**Solution in python:**

def chunks( l, n ): """accept a list and chuck size, return chunks""" return [ l[ i:i+n ] for i in range( 0, len(l), n ) ] def T( blocks ): """for each block, reverse block, return blocks""" result = [] for block in blocks: result.append( ''.join( [bit for bit in reversed( block )] ) ) return result def scrambler( input ): """inverse the order of input""" return ''.join( [i for i in reversed( input ) ] ) def cipher1( input, n = 3, chunk_length = 8 ): """make chucks out of input, reverse each chunk return result""" blocks = chunks( input, chunk_length ) for i in range( 0, n ): blocks = T( blocks ) return ''.join( blocks ) def cipher2( input, n = 3, chunk_length = 8 ): """same as cipher1 but with scrambler""" blocks = chunks( input, chunk_length ) for i in range( 0, n ): blocks = T( blocks ) blocks = chunks( scrambler( ''.join( blocks ) ), chunk_length ) return ''.join( blocks ) if __name__ == "__main__": input = "1010000010100000101000001010000010100000101000001010000010100000" print cipher1( input ) # output: 0000010100000101000001010000010100000101000001010000010100000101 input = "1010000010100000101000001010000010100000101000001010000010100001" print cipher1( input ) # output: 0000010100000101000001010000010100000101000001010000010110000101 input = "1010000010100000101000001010000010100000101000001010000010100000" print cipher2( input ) # output: 1010000010100000101000001010000010100000101000001010000010100000 input = "1010000010100000101000001010000010100000101000001010000010100001" print cipher2( input ) # output: 1010000110100000101000001010000010100000101000001010000010100000

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