The classic algorithm applied to modern cryptography from the given options is modular arithmetic. Modular arithmetic is a fundamental concept used in various cryptographic algorithms, such as RSA and Diffie-Hellman key exchange.
Pseudorandom number generators are also used in cryptography but can be considered more a technique than a classic algorithm itself compared to modular arithmetic. Binary sort and linear search are algorithms primarily related to sorting and searching data, respectively, rather than cryptography.