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