 Ciphertext

This article is about encrypted information. For an overview of cryptographic technology in general, see Cryptography.
In cryptography, ciphertext (or cyphertext) is the result of encryption performed on plaintext using an algorithm, called a cipher.^{[1]} Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. Decryption, the inverse of encryption, is the process of turning ciphertext into readable plaintext. Ciphertext is not to be confused with codetext because the latter is a result of a Code, not a cipher.
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Symmetric key example
Let be the plaintext message that Alice wants to secretly transmit to Bob and let be the encryption cipher, where is a secret key. Alice must first transform the plaintext into ciphertext, , in order to securely send the message to Bob.
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Both Alice and Bob must know the choice of key, , or else the ciphertext is useless. Once the message is encrypted as ciphertext, Alice can safely transmit it to Bob (assuming no one else knows the key). In order to read Alice's message, Bob must decrypt the ciphertext using which is known as the decryption cipher, .
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Types of ciphers
Main article: CipherThe history of cryptography begins thousands of years ago and contains a variety of different types of encryption. Earlier algorithms were performed by hand and are substantially different from modern algorithms, which are generally executed by a machine.
Historical ciphers
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include:
 Substitution cipher: the units of plaintext are replaced with ciphertext (Caesar cipher and Onetime pad)
 Transposition cipher: the ciphertext is a permutation of the plaintext (Rail fence cipher)
 Polyalphabetic substitution cipher: a substitution cipher using multiple substitution alphabets (Vigenère cipher and Enigma machine)
 Permutation cipher: a transposition cipher in which the key is a permutation
Historical ciphers are not generally used as a standalone encryption solution because they are quite easy to crack. Many of the classical ciphers can be cracked using brute force or by analyzing only ciphertext with the exception of the onetime pad.
Modern ciphers
Modern ciphers are more secure than classical ciphers and are designed to withstand a wide range of attacks. An attacker should not be able to find the key used in a modern cipher, even if he knows any amount of plaintext and corresponding ciphertext. Modern encryption methods can be divided into the following categories:
 Privatekey cryptography (symmetric key algorithm): the same key is used for encryption and decryption
 Publickey cryptography (asymmetric key algorithm): two different keys are used for encryption and decryption
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Symmetric key ciphers can be divided into block ciphers and stream ciphers. Block ciphers operate on fixedlength groups of bits, called blocks, with an unvarying transformation. Stream ciphers encrypt plaintext digits one at a time on a continuous stream of data and the transformation of successive digits varies during the encryption process.
Cryptanalysis
Main article: CryptanalysisCryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key. Cryptanalysis is also referred to as codebreaking or cracking the code. Ciphertext is generally the easiest part of a cryptosystem to obtain and therefore is a important part of cryptanalysis. Depending on what information is available and what type of cipher is being analyzed, crypanalysts can follow one or more attack models to crack a cipher.
Attack Models
 Ciphertextonly: the cryptanalyst has access only to a collection of ciphertexts or codetexts.
 Knownplaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext.
 Chosenplaintext attack: the attacker can obtain the ciphertexts corresponding to an arbitrary set of plaintexts of his own choosing.
 Batch chosenplaintext attack: where the cryptanalyst chooses all plaintexts before any of them are encrypted. This is often the meaning of an unqualified use of "chosenplaintext attack".
 Adaptive chosenplaintext attack: where the cryptanalyst makes a series of interactive queries, choosing subsequent plaintexts based on the information from the previous encryptions.
 Chosenciphertext attack: the attacker can obtain the plaintexts corresponding to an arbitrary set of ciphertexts of his own choosing.
 Adaptive chosenciphertext attack
 Indifferent chosenciphertext attack
 Relatedkey attack: like a chosenplaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys. The keys are unknown, but the relationship between them is known; for example, two keys that differ in the one bit.
The ciphertextonly attack model is the weakest attack because it implies that the cryptanalyst has nothing but ciphertext. Modern ciphers rarely fail under this attack.^{[3]}
Famous ciphertexts
Main article: List of ciphertexts The Babington Plot ciphers
 The Shugborough inscription
 The Zimmermann Telegram
 The Magic Words are Squeamish Ossifrage
 The cryptogram in "The GoldBug"
 Beale ciphers
 Kryptos
 Zodiac Killer ciphers
See also
 RED/BLACK concept
 Frequency analysis
 Books on cryptography
 Cryptographic hash function
 Category:Uncracked codes and ciphers
References
 ^ Berti, Hansche, Hare (2003). Official (ISC)² Guide to the CISSP Exam. Auerbach Publications. pp. 379. ISBN 084931707X.
 ^ ^{a} ^{b} van Tilborg, Henk C.A. (2000). Fundamentals of Cryptology. Kluwer Academic Publishers. pp. 3. ISBN 0792386753.
 ^ Schneier, Bruce. Secrets & Lies. Wiley Computer Publishing Inc. pp. 90–91. ISBN 0471253111.
Further reading
 Helen Fouché Gaines, “Cryptanalysis”, 1939, Dover. ISBN 0486200973
 David Kahn, The Codebreakers  The Story of Secret Writing (ISBN 0684831309) (1967)
 Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1966. ISBN 0883856220
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