Modular Arithmetic & Cryptography Basics: From Caesar to Toy RSA
MOD and clock arithmetic, Caesar and affine ciphers, frequency analysis, Euclid and the extended Euclid inverse, square-and-multiply, a complete toy RSA key and ISBN/Luhn check digits: 8 graded labs (toy keys only, never for real security).
The first lesson is free — no account needed.
About this course
A modular arithmetic and cryptography basics course where every puzzle of one fictional escape-room designer is solved step by step in a spreadsheet, for students, puzzle fans and developers who want to see how the math works. You split times into hours and minutes with QUOTIENT and MOD and wrap a clock and a letter wheel, encrypt and decrypt with Caesar and affine ciphers, break a Caesar cipher with frequency analysis and COUNTIF, run Euclid's algorithm in a step table, find a modular inverse with the extended Euclidean algorithm, compute large powers mod n with square-and-multiply, build a complete toy RSA key, encrypt and decrypt a message and calculate ISBN-13 and Luhn check digits. Toy keys for learning only, never for real security. The first lab is free.
Lessons
- MOD & Clock Arithmetic: Remainders That Wrap AroundFree
- Caesar & Affine Ciphers: Letters as Numbers Pro
- Frequency Analysis: Breaking a Caesar Cipher with COUNTIF Pro
- GCD & Euclid's Algorithm: A Step Table Pro
- Modular Inverse with the Extended Euclidean Algorithm Pro
- Fast Modular Exponentiation: Square-and-Multiply Pro
- Toy RSA: Keys, Encryption & Decryption Pro
- Check Digits & Hashing Ideas: ISBN-13 and Luhn Pro