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What is a cipher?
In cryptology, a cipher (or ciphertext) refers to the transformed result of concealing information known as plaintext. This transformation, using a secret key, is called encryption, while the reverse operation is decryption. Ciphers can be produced through transpositions, which rearrange the symbols in the plaintext, or substitutions, which replace plaintext elements with other symbols.
Cipher systems transform fixed-size pieces of plaintext into ciphertext. Block ciphers break plaintext into blocks of the same size for encryption with a common key, while stream ciphers break plaintext into units and encrypt each unit with a corresponding unit of a key stream.
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What is cryptology?
Cryptology is the science of secure communication and data storage, typically in a secret form. It includes cryptography, which is how information is concealed and encrypted, and cryptanalysis, which is how encrypted information is revealed when the secret key is unknown. Cryptology's purpose is to transform information, using secret keys, into ciphers that authorized users can decrypt to recover hidden information or authenticate the source.
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How is a cipher used to keep messages secret?
A cipher is a method of encryption that transforms information to hide its meaning. Ciphers replace pieces of information with other objects based on a secret key. This key is known only to the sender and receiver, so those without the key cannot decrypt the message. The transformation of the original message, or plaintext, into an unreadable form is encryption. Decryption is the reverse, changing the cipher back to plaintext using the key.
Ciphers use mathematical operations, such as transpositions and substitutions, to conceal the message. Transposition rearranges the order of symbols. Substitution replaces symbols with other symbols.
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What is the difference between a code and a cipher?
The terms code and cipher are often misused, even by experts. A code uses an unvarying rule to replace a piece of information (like a letter or word) with something else, like in Morse code or the ASCII American Standard Code for Information Interchange. A cipher also replaces elements of a message, but it does so following a rule defined by a secret key, known only to the sender and receiver, with the expectation that someone without the key will not be able to decrypt it. In modern communication, information is often both encoded and encrypted, making it important to understand the distinction.
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How do people break ciphers and solve encrypted messages?
People break ciphers and solve encrypted messages through cryptanalysis, which involves deciphering communications secured by cryptography. This can be done through recognizing patterns or structures in the cipher, often relying on intuition and recognizing structures at a subliminal level.
There are three types of cryptanalysis, characterized by the information known to the cryptanalyst: ciphertext only; known ciphertext/plaintext pairs; and chosen plaintext or chosen ciphertext.
Modern cryptanalysis uses mathematics and computing power. For example, in monoalphabetic substitution ciphers, the frequency of letters in the plaintext and ciphertext is identical, which the cryptanalyst can use to find equivalences of letters to be tried.
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What are some famous ciphers or encryption methods used in history?
Throughout history, various ciphers and encryption methods have been employed to secure communications. The Spartans used the scytale, a transposition cipher, for military correspondence as early as 400 BCE.
Julius Caesar employed a simple substitution cipher where each letter was replaced by a letter a fixed number of positions away in the alphabet. For example, A would become D using a right letter shift of four.
The Vigenère cipher is a polyalphabetic substitution method using alphabets derived from a pair of primary alphabets by sliding.
During World War II, the German military utilized the Enigma machine, which combined electrical and mechanical components to create a complex, ever-changing pattern of connections that made it extremely difficult to break.
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cipher, any method of transforming a message to conceal its meaning. The term is also used synonymously with ciphertext or cryptogram in reference to the encrypted form of the message. A brief treatment of ciphers follows. For full treatment, see cryptology.
All ciphers involve either transposition or substitution, or a combination of these two mathematical operations—i.e., product ciphers. In transposition cipher systems, elements of the plaintext (e.g., a letter, word, or string of symbols) are rearranged without any change in the identity of the elements. In substitution systems, such elements are replaced by other objects or groups of objects without a change in their sequence. In systems involving product ciphers, transposition and substitution are cascaded; for example, in a system of this type called a fractionation system, a substitution is first made from symbols in the plaintext to multiple symbols in the ciphertext, which is then superencrypted by a transposition. All operations or steps involved in the transformation of a message are carried out in accordance to a rule defined by a secret key known only to the sender of the message and the intended receiver.
Cipher devices or machines have commonly been used to encipher and decipher messages. The first cipher device appears to have been employed by the ancient Greeks around 400 bce for secret communications between military commanders. This device, called the scytale, consisted of a tapered baton around which was spirally wrapped a piece of parchment inscribed with the message. When unwrapped the parchment bore an incomprehensible set of letters, but when wrapped around another baton of identical proportions, the original text reappeared. Other simple devices known as cipher disks were used by European governments for diplomatic communications by the late 1400s. These devices consisted of two rotating concentric circles, both bearing a sequence of 26 letters. One disk was used to select plaintext letters, while the other was used for the corresponding cipher component.
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In 1891 Étienne Bazeries, a French cryptologist, invented a more sophisticated cipher device based on principles formulated by Thomas Jefferson of the United States nearly a century earlier. Bazeries’s so-called cylindrical cryptograph was made up of 20 numbered rotatable disks, each with a different alphabet engraved on its periphery. The disks were arranged in an agreed-upon order on a central shaft and rotated so that the first 20 letters of the message plaintext appeared in a row; the ciphertext was then formed by arbitrarily taking off any other row. The remaining letters of the message were treated in the same way, 20 letters at a time.
Advances in radio communications and electromechanical technology in the 1920s brought about a revolution in cryptodevices—the development of the rotor cipher machine. One common type of rotor system implemented product ciphers with simple monoalphabetic substitution ciphers as factors. The rotors in this machine consisted of disks with electrical contacts on each side that were hardwired to realize an arbitrary set of one-to-one connections (monoalphabetic substitution) between the contacts on opposite sides of the rotor.
The rotor cipher machine was used extensively by both the Allied and the Axis powers during World War II, with the most notable such device being the German Enigma machine. The application of electronic components in subsequent years resulted in significant increases in operation speed though no major changes in basic design. Since the early 1970s, cryptologists have adapted major developments in microcircuitry and computer technology to create new, highly sophisticated forms of cryptodevices and cryptosystems, as exemplified by the Fibonacci generator and the implementation of the Data Encryption Standard (DES) through the use of microprocessors.