 After Bob obtains Alice's public key, he can send a message to Alice.To do it, he first turns (strictly speaking, the un-padded plaintext) into an integer (strictly speaking, the padded plaintext), such that by using an agreed-upon reversible protocol known as a padding scheme.He then computes the ciphertext , using Alice's public key , corresponding to

This can be done reasonably quickly, even for very large numbers, using modular exponentiation.