Encryption dates back through the ages. Ancient Hebrews used a basic cryptographic system called ATBASH that worked by replacing each letter used with another letter the same distance away from the end of the alphabet; A was sent as a Z, and B was sent as a Y.
The Spartans also had their own form of encryption, called scytale. This system functioned by wrapping a strip of papyrus around a rod of fixed diameter on which a message was written. The recipient used a rod of the same diameter on which he wrapped the paper to read the message. If anyone intercepted the paper, it appeared as a meaningless letter.
The Caesar cipher used the alphabet but swapped one letter for another by incrementing by three characters. In this system, Caesar wrote D instead of A and E instead of B.
More complicated substitution ciphers were developed through the middle ages as individuals became better at breaking simple encryption systems. In the ninth century, Abu al-Kindi published what is considered to be the first paper that discusses how to break cryptographic systems, titled "A Manuscript on Deciphering Cryptographic Messages." It deals with using frequency analysis to break cryptographic codes. Frequency analysis is the study of how frequent letters or groups of letters appear in cipher text. Uncovered patterns can aid individuals in determining patterns and breaking the cipher text.
All three encryption techniques discussed are considered substitution ciphers, which operate by replacing bits, bytes, or characters with alternate bits, bytes, or characters. Substitution ciphers are vulnerable to frequency analysis and are no longer used.
Around the beginning of the twentieth century, mechanical devices such as the German Enigma machine, which used a series of internal rotors to perform the encryption, and the Japanese Purple Machine were developed to counter the weaknesses of substitution ciphers. Today, in the United States the National Security Agency (NSA) is responsible for coding and code breaking. It helped lead the implementation of the Data Encryption Standard (DES).
Modern cryptographic systems no longer use substitution and transposition. Today block ciphers and stream ciphers are used. A block cipher, such as DES, operates on fixed-length groups of bits. A stream cipher inputs and encrypts one digit at a time.