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Cryptography prior to the modern age was effectively synonymous with encryption, the conversion of information from a readable state to apparent nonsense. The originator of an encrypted message shared the decoding technique needed to recover the original information only with intended recipients, thereby precluding unwanted persons from doing the same. It is theoretically possible to break such a system, but it is infeasible to do so by any known practical means. The growth of cryptographic technology has raised a number of legal issues in the information age. Cryptography’s potential for use as a tool for espionage and sedition has led many governments to classify it as a weapon and to limit or even prohibit its use and export. Alphabet shift ciphers are believed to have been used by Julius Caesar over 2,000 years ago.
In other words, the letters in the alphabet are shifted three in one direction to encrypt and three in the other direction to decrypt. 19th century – it originated in The Gold-Bug, a novel by Edgar Allan Poe. In colloquial use, the term “code” is often used to mean any method of encryption or concealment of meaning. However, in cryptography, code has a more specific meaning. In India, the 2000-year-old Kamasutra of Vātsyāyana speaks of two different kinds of ciphers called Kautiliyam and Mulavediya. In the Kautiliyam, the cipher letter substitutions are based on phonetic relations, such as vowels becoming consonants. In the Mulavediya, the cipher alphabet consists of pairing letters and using the reciprocal ones.
Language letter frequencies may offer little help for some extended historical encryption techniques such as homophonic cipher that tend to flatten the frequency distribution. Essentially all ciphers remained vulnerable to cryptanalysis using the frequency analysis technique until the development of the polyalphabetic cipher, most clearly by Leon Battista Alberti around the year 1467, though there is some indication that it was already known to Al-Kindi. Although frequency analysis can be a powerful and general technique against many ciphers, encryption has still often been effective in practice, as many a would-be cryptanalyst was unaware of the technique. Breaking a message without using frequency analysis essentially required knowledge of the cipher used and perhaps of the key involved, thus making espionage, bribery, burglary, defection, etc.
Different physical devices and aids have been used to assist with ciphers. In medieval times, other aids were invented such as the cipher grille, which was also used for a kind of steganography. Cryptanalysis of the new mechanical devices proved to be both difficult and laborious. In the United Kingdom, cryptanalytic efforts at Bletchley Park during WWII spurred the development of more efficient means for carrying out repetitious tasks. Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers. Computer use has thus supplanted linguistic cryptography, both for cipher design and cryptanalysis.
As well as being aware of cryptographic history, cryptographic algorithm and system designers must also sensibly consider probable future developments while working on their designs. Essentially, prior to the early 20th century, cryptography was chiefly concerned with linguistic and lexicographic patterns. The modern field of cryptography can be divided into several areas of study. This was the only kind of encryption publicly known until June 1976. Symmetric key ciphers are implemented as either block ciphers or stream ciphers. A block cipher enciphers input in blocks of plaintext as opposed to individual characters, the input form used by a stream cipher. Stream ciphers, in contrast to the ‘block’ type, create an arbitrarily long stream of key material, which is combined with the plaintext bit-by-bit or character-by-character, somewhat like the one-time pad.
In a stream cipher, the output stream is created based on a hidden internal state that changes as the cipher operates. That internal state is initially set up using the secret key material. Cryptographic hash functions are a third type of cryptographic algorithm. For good hash functions, an attacker cannot find two messages that produce the same hash. Symmetric-key cryptosystems use the same key for encryption and decryption of a message, though a message or group of messages may have a different key than others.