Cryptography has four main goals:
Not all cryptographic systems and algorithms achieve all of the above goals. Some are not practical (or necessary) in some contexts and require sophisticated algorithms needing many computations.
Although cryptography has a long and complex history, it wasn't until the 20th centry that it developed into a rigorous science supported by mathematics. Even then, it has taken the popularity of the internet to require it's widespread use and to distribute it.
The earliest use of cryptography can be 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.
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 established 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.
See also:
Public key cryptosystems (asymmetric algorithms):
Secret key cryptosystems (symmetric algorithms):
Terminology:
Echelon, Enigma, Ultra, Security engineering, Steganography, Cryptographers