The process of turning ciphertext back into plaintext is called decryption . Cryptography deals with making communications secure. Cryptoanalysis deals with breaking ciphertext, that is, recovering plaintext without knowing the key.
Cryptology is a branch of mathematics which deals with both cryptography and cryptoanalysis.
A cryptographic algorithm , also known as a cipher , is a mathematical function which uses plaintext as the input and produces ciphertext as the output and vice versa. All modern ciphers use keys together with plaintext as the input to produce ciphertext. The same or a different key is supplied to the decryption function to recover plaintext from ciphertext. The details of a cryptographic algorithm are usually made public. It is the key that the security of a modern cipher lies in, not the details of the cipher.