In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.
Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input. A code maps one meaning with another. Words and phrases can be coded as letters or numbers. Codes typically have direct meaning from input to key. Codes primarily function to save time. Ciphers are algorithmic. The given input must follow the cipher's process to be solved. Ciphers are commonly used to encrypt written information.
Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase. For example, "UQJHSE" could be the code for "Proceed to the following coordinates." When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it.
The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a cryptovariable). The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext.
Most modern ciphers can be categorized in several ways:
Originating from the Arabic word for zero صفر (sifr), the word "cipher" spread to Europe as part of the Arabic numeral system during the Middle Ages. The Roman numeral system lacked the concept of zero, and this limited advances in mathematics. In this transition, the word was adopted into Medieval Latin as cifra, and then into Middle French as cifre. This eventually led to the English word cipher (minority spelling cypher). One theory for how the term came to refer to encoding is that the concept of zero was confusing to Europeans, and so the term came to refer to a message or communication that was not easily understood.[1]
The term cipher was later also used to refer to any Arabic digit, or to calculation using them, so encoding text in the form of Arabic numerals is literally converting the text to "ciphers".
Computational power available, i.e., the computing power which can be brought to bear on the problem. It is important to note that average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of for a key (i.e., "brute force" attack) substantially.
exhaustive search
i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of exhaustive search to the point where it becomes impractical to crack encryption directly.
Key size
In a pure mathematical attack, (i.e., lacking any other information to help break a cipher) two factors above all count:
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly.
An example of this process can be found at Key Length which uses multiple reports to suggest that a symmetrical cipher with 128 bits, an asymmetric cipher with 3072 bit keys, and an elliptic curve cipher with 256 bits, all have similar difficulty at present.
Claude Shannon proved, using information theory considerations, that any theoretically unbreakable cipher must have keys which are at least as long as the plaintext, and used only once: one-time pad.[6]
Autokey cipher
Cover-coding
Encryption software
List of ciphertexts
Steganography
Telegraph code
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Helen Fouché Gaines, "Cryptanalysis", 1939, Dover. 0-486-20097-3
ISBN
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Ibrahim A. Al-Kadi
The Codebreakers - The Story of Secret Writing (ISBN0-684-83130-9) (1967)
Caldwell, William Casey. "Shakespeare's Henry V and the Ciphers of History." SEL Studies in English Literature, 1500-1900, vol. 61, no. 2, 2021, pp. 241–68. EBSCOhost, :10.1353/sel.2022.0003.
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Luciano, Dennis, and Gordon Prichett. "Cryptology: From Caesar Ciphers to Public-Key Cryptosystems." The College Mathematics Journal, vol. 18, no. 1, 1987, pp. 2–17. JSTOR, https://doi.org/10.2307/2686311. Accessed 19 Feb. 2023.
Ho Yean Li, et al. "Heuristic Cryptanalysis of Classical and Modern Ciphers." 2005 13th IEEE International Conference on Networks Jointly Held with the 2005 IEEE 7th Malaysia International Conf on Communic, Networks, 2005. Jointly Held with the 2005 IEEE 7th Malaysia International Conference on Communication., 2005 13th IEEE International Conference on, Networks and Communications, vol. 2, Jan. 2005. EBSCOhost, :10.1109/ICON.2005.1635595.
