Six historical ciphers for hiding messages: from the reversed Hebrew alphabet to the American telegraphers' zigzag. Ends with 3 camp cryptographer missions.

Learn the RSA algorithm step by step: Caesar cipher, modular arithmetic and Euler's φ function.

A line meets the curve y² = x³ + ax + b in three points: from two of them you build the third. Drag points, add with chords and tangents, multiply a point by n and hunt rational points like Diophantus and Bachet. Discover torsion, the points that come back to the start, and Mazur’s theorem, which allows only 15 shapes. Finally, the same curve in modular arithmetic: multiplying is lightning fast and undoing the multiplication is almost impossible. The cryptography that protects the internet rests on that.