Challenge Problem #1
### Make DES Weak

First read the details of DES.
Then, change the S-boxes in any way you'd like to produce WDES (weak DES).
Then demonstrate a method where, given any known plaintext-ciphertext pair,
you can recover all 56 bits of key.

Hint: make the cipher linear. Then express each round as an affine
transformation on 32 bits (that is, show that each round value can be
expressed as a 56 x 32 matrix of bits, times a key vector (which you will
later solve for), plus some 56 x 1 vector). Finally, solve for the
key vector at the end with Gaussian elimination. If you do things right, you
will get a rank 56 matrix, yielding all key bits.

**Extra Credit Points: ** 10