1. IntroductionCryptography is a subject that has been studied and applied since ancient Romantimes, and research into better encryption methods continues to this day. Cryptography is theart of encoding and decoding messages so that messages can be securely transmitted from asender to a receiver without fear of an outside party intercepting and reading or altering themessage's contents. The purpose of this paper is to describe the history and evolution ofcryptography starting from the beginning of the 20th century and continuing into the presentday. Specifically, the following five topics will be addressed: the cryptography used from1900 until the end of World War II, the history of the politics involved with governmentcontrol over cryptography, the history and current status of asymmetric cryptography, thehistory and current status of symmetric cryptography, and the future of the field ofcryptography using quantum physics.2. A Brief History of CryptographyThe history of cryptography goes back thousands of years. Until recently, this historyhas consisted of the classical components of cryptography – ciphers written with pen onpaper or other simple mechanical services. The turn of the 20th century however, broughtabout several advances in this subject. The invention of electromagnetic machines such asthe German Enigma provided a much more efficient method of encrypting messages, andsubsequently led to the introduction of cryptography into the emerging field of computing.“The fundamental objective of cryptography is to enable two people, usually referred to asAlice and Bob, to communicate over an insecure channel in such a way that an opponent,Oscar, cannot understand what is being said” (Zotos). A cipher is an algorithm forperforming encryption (and in the reverse, decryption); it is in effect a series of well-definedsteps that can be followed as a procedure. The original information is usually known asplaintext and the encrypted messages are known as cipher text.2.1 Early CryptographyOne of the oldest cipher styles is the substitution cipher, whose first examples aredated back thousands of years. Substitution is a method of encrypting by which units ofplaintext are substituted with cipher text according to a regular system. An example of this isthe Atbash cipher, which dates from 500 BC (Kartalopoulos). This cipher is based on theHebrew alphabet, and is regulated where the first letter is substituted by the last letter, thesecond letter by the second to last letter and so on. Because it is a monoalphabetic cipher andcan have only one possible key, this cipher is relatively weak; however this was not a viableconcern during its time as literacy was not common. These types of ciphers were usedfrequently in religious texts and writings, and it was probably this use that developed themethod of frequency analysis to analyze the messages (Kartalopoulos). Frequency analysis iswhere one examines the frequency of substituted letters, from which they can estimate certainletters which appear repeatedly in the plaintext language.