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(What is modular arithmetic in cryptography?)
Modular arithmetic is the mathematics of working with remainders after division by a fixed number called the modulus. In cryptography, it underpins many core constructions because it defines arithmetic in finite sets (rings and fields) where values ''wrap around,'' enabling stable, repeatable operations with bounded results. Public-key systems like RSA rely on modular exponentiation (raising integers to powers modulo a composite number), while Diffie--Hellman and many elliptic-curve schemes operate in groups defined by modular arithmetic properties. Encryption and key exchange use modular operations because they allow efficient computation forward (e.g., exponentiation modulo a large number) while making certain inverse problems computationally hard without secret information (e.g., factoring or discrete logarithms). Modular reduction also helps keep intermediate values manageable and supports group properties needed for proofs of security. Although modular arithmetic is not ''encryption by itself,'' it is a foundational method used inside encryption algorithms and protocols. Therefore, among the options, describing it as a method used for encryption via modular operations best matches cryptographic usage.