Check hash function

Requirements: pip3 install --user pycrypto

File `insecure_hash` implements the hash function `hash_string`.

- If a message `m` consists of two blocks `B1;B2` of 128 bits, the hash is
computed as `AES-decrypt(B1, B2)`: i.e. it decrypts the block `B1` using `B2`
as key.
- If the message consists of three blocks `B1;B2;B3`, the hash is computed as
`AES-decrypt(AES-decrypt(B1, B2), B3)`
- If the message consists of n blocks `B1;B2;B3;...Bn`, the hash is computed as
`AES-decrypt(... AES-decrypt(AES-decrypt(B1, B2), B3)..., Bn)`

The message is padded with " " to be block aligned, e.g. if the length of the
message is 200 bits, then 7 spaces are appended to the message before
computing the hash.

This hash function is not secure. In particular it is not weak collision
resistant. Complete the code in file `collision.py`, implementing the function
`find_collision(message)` that finds a collision.
DO NOT BRUTE FORCE.