cryptography

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Computers Any of various mathematical techniques for encrypting and decrypting data in order to keep it private when transmitted or stored electronically.
• noun The process or skill of communicating in or deciphering secret writings or ciphers.
• noun Secret writing.

from The Century Dictionary.

• noun The act or art of writing in secret characters.
• noun A system of secret or occult characters; that which is written in cipher.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act or art of writing in code or secret characters; also, secret characters, codes or ciphers, or messages written in a secret code.
• noun The science which studies methods for encoding messages so that they can be read only by a person who knows the secret information required for decoding, called the key; it includes cryptanalysis, the science of decoding encrypted messages without possessing the proper key, and has several other branches; see for example steganography.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The discipline concerned with communication security (eg, confidentiality of messages, integrity of messages, sender authentication, non-repudiation of messages, and many other related issues), regardless of the used medium such as pencil and paper or computers.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun act of writing in code or cipher
• noun the science of analyzing and deciphering codes and ciphers and cryptograms