Cryptography: four methods, from Caesar to Alice and Bob
Substitution, transposition, XOR and RSA: full encryption and decryption examples with an interactive laboratory.
ReadMathematics
Articles by topic
Clear, organized learning paths that build strong foundations in mathematics and computer science.
Number theory
The Chinese remainder theorem: from Sunzi to 30 problems
Sunzi’s block method, modern congruences, and 30 interactive challenges involving coins, soldiers, calendars, and trick questions.
ReadFrom 3 and 5 to primes in arithmetic progression
Fajtlowicz’s conjecture, two ways to generate primes from 3 and 5, and a laboratory that also accepts a new starting pair.
ReadModular arithmetic: the remainders laboratory
From clocks to enormous powers: an intuitive guide to congruences with 25 problems and an interactive laboratory.
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