Wikipedia: Cryptography

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Cryptography (from Greek kryptós, "hidden", and gráphein, "to write") is the study of the principles and techniques by which information can be concealed in ciphers and later revealed to legitimate users employing the encryption key (see /Key), but in which it is either impossible or computationally infeasible for an unauthorized person to do so.

Cryptography has four main goals:

message confidentiality: Only those authorised to read a message should be able to do so. It should be impossible(or at least very difficult) for unauthorised persons to read the message contents or obtain information about the contents of the message.
message integrity: The receiver should be able to determine if the message has been altered during transmission.
authentication: The receiver should be able to identify the sender. Furthermore, they should be able to verify that the sender did actually send the message.
non-repudiation: The sender should not be able to later deny sending the message.

However, not all cryptographic systems achieve all of the above goals. Although possible, most have not been practically feasible until the recent development of complex algorithms and machinery to carry out such algorithms.

Although cryptography has a long and complex history, it is only in the 20th century that it has developed into a rigorous science with a sound mathematical basis. Even then, it has taken the ubiquity of the internet to fully deploy cryptography in a public manner.

Classic Cryptography

The earliest use of cryptography is probably found with the use of non-standard hieroglyphics? by the Egyptians? around 1900 BCE. Hebrew scholars also made use of simple [substitution ciphers]? (such as the [Atbash cipher]?) around 500 to 600 BCE.

World War 2 Cryptography

Modern Cryptography

The era of modern cryptography started with Claude Shannon, arguably the father of mathematical cryptography. In 1949 he published the paper [Communication Theory of Secrecy Systems]. This, in addition to his other works on information and communication theory would establish a strong mathematical basis for cryptography.

1976 saw two major advances. First was the DES (Data Encryption Standard) developed by IBM and the NSA in an effort to develop secure banking facilities (DES was later published as a Federal Information Processing Standard in 1977). DES, and more secure variants of it, are still used today.

Secondly, and more importantly, was the publication of the paper [New Directions in Cryptography] by [Whitfield Diffie]? and [Martin Hellman]?. This paper introduced a radical new method of distrubting cryptographic keys, known as [public key cryptography].