Algebraic Attacks on the Courtois Toy Cipher

Block ciphers are fundamental building block of modern cryptography. Recently, a new technique to attack block ciphers has emerged called “algebraic attacks”. These attacks work by expressing block ciphers as quadratic equation systems and solving those systems of equations. In May 2006 Nicolas Courtois – author of many influential research papers on algebraic attacks – presented a toy cipher called CTC and claimed to have broken this cipher in a configuration where the block size is 255-bit and the number of rounds is six. This thesis presents, discusses, and implements some of the most important algebraic attack algorithms (F4, DR, XL) and employs them against Courtois’ toy cipher. Also CTC is attacked using more specialized algorithms and the experimental results of these attacks are presented.