From Greek κρυπτός, (kryptos) “hidden, secret”; and γράφειν (graphein) “writing” (or cryptology, -λογία, (-logia), “the study”, cryptography is a branch of information processing which studies techniques for secure communications over an untrusted channel, and of “cracking” such communications.
Modern cryptography includes: encryption and decryption; authentication & authorisation; data integrity and confidentiality.
“Classic” cryptography almost entirely revolved around encryption and decryption: creating cyphers (paired algorithms, usually with a key) to turn plaintext into cyphertext; some early work also involved hiding the message (steganography).
Modern cryptology encompasses cryptography (creating encryption techniques) and cryptanalysis (breaking encrypted messages without the key).
Attacks On The 1-time Pad
c1 = m1 ⊕ PRG(k)
c2 = m2 ⊕ PRG(k)
c1 ⊕ c2 = m1 ⊕ m2
and there is enough redundancy in English (and ASCII) to recover m1, m2 (and therefore PRG(k)).