Public-key cryptosystem: choose primes p,q; n=pq, φ(n)=(p−1)(q−1); choose e coprime to φ(n); d ≡ e⁻¹ (mod φ(n)). Encrypt: c ≡ m^e (mod n). Decrypt: m ≡ c^d (mod n) = m^(ed) ≡ m (mod n) by Euler.. The key formula is c = m^e mod n, m = c^d mod n. This concept is typically introduced in College. Understanding this concept builds a strong foundation for more advanced mathematics.