Cryptography

Cryptography or cryptology, is the study of techniques to ensure secure communication between two parties when in the presence of a third party, called adversaries. Before the age of computers, cryptography meant the same as encryption which is the process of converting information (plain text) using a cipher, into another form (cipher text) that only authorized parties will be able to read by the use of decryption.

History
Cryptography was used for communication between high ranking officials, military generals and spies dating as far back as the Roman Empire. One classic cipher used to send secure messages was the transposition cipher. This would change the order of letters in words (changing "moving north" to something like "gomvni torhn"), making messages harder for the less literate people of the time to decipher. However, many diplomats or spies intercepting sensitive messages were literate, which required another method of encryption. A substitution cipher is another method of encoding messages that replaces letters in words with other letters a certain number of positions further down the alphabet. One of the most famous of these was the Caesar cipher, used by Julius Caesar when communicating with his generals. The Caesar cipher utilized the systematic replacement of a letter with another letter 3 positions up in the alphabet, making "E" become "B" and "D" become "A".

Cryptography Today
See: Encryption 

With the advent of computers, cryptographic techniques became much more sophisticated. Algorithms such as symmetric-key and public-key were created to make deciphering information without authorization a nearly impossible task.

Legality
Since encryption techniques have become so secure, private use of such techniques could be seen as criminal acts such as espionage or sedition. This has led governments of some governments to classify it as a weapon and even go as far as banning it's use or export. In the U.S. in 1999, controversy also arose over the Cyberspace Electronic Security Act.