That part is implementation-specific. Traditional RSA uses multiplication of prime numbers, Wikipedia's explanation is decent: https://en.m.wikipedia.org/wiki/R

That part is implementation-specific. Traditional RSA uses multiplication of prime numbers, Wikipedia's explanation is decent: https://en.m.wikipedia.org/wiki/RSA_cryptosystem https://en.m.wikipedia.org/wiki/RSA_cryptosystem In modern times other implementations are used such as elliptic curves (which have better performance, especially important for battery powered devices like phones) or lattice based cryptography (which are not vulnerable to quantum computers)