CRYPTOLOGY CHAPTER 3: CLASSICAL ENCRYPTION TECHNIQUES

Symmetric encryption

The most widely used type of encryption where the same key is used for both encryption and decryption.

Rotor machines

Sophisticated precomputer hardware devices that use substitution techniques.

Non-conventional encryption

A term incorrectly used to describe symmetric encryption, which actually uses the same key and is known as conventional encryption.

Key secrecy

The principal security problem in symmetric encryption, which involves maintaining the secrecy of the key.

Enciphering

The process of converting from plaintext to ciphertext.

Algorithm output

The output produced by an algorithm that depends on the specific secret key being used.

Kerckhoffs's principle

A principle stating that the algorithm should be public; only the key must be secret.

Brute-force attack

An attack method where, on average, half of all possible keys must be tried to achieve success.

Computationally secure encryption scheme

An encryption scheme where ciphertext is not impossible to decrypt, but it is computationally infeasible within the useful lifetime of the data.

Monoalphabetic ciphers

Ciphers that are easy to break because they reflect the frequency data of the original alphabet.

Hill cipher

A cipher that completely hides single letter frequencies.

One-time pad

A scheme that is unbreakable because it produces random output that bears no statistical relationship to the plaintext.

Utility of one-time pad

Limited utility and is useful primarily for low-bandwidth channels requiring high security.

Data Encryption Standard

Historically the most widely used cipher, but now superseded by AES.

Steganography

The practice of concealing the existence of a message, not just rendering it unintelligible.

Mapping techniques

Techniques that map plaintext elements (characters, bits) into ciphertext elements.

plaintext

An original intelligible message fed into the algorithm as input.

ciphertext

The coded message produced as output from the algorithm.

deciphering

Restoring the plaintext from the ciphertext.

cryptanalysis

Techniques used for deciphering a message without any knowledge of the enciphering details.

decryption algorithm

The process that takes the ciphertext and the secret key and produces the original plaintext.

conventional encryption

A system where both sender and receiver use the same key.

Cryptanalytic attacks

Attacks that exploit the characteristics of the algorithm to attempt to deduce a specific plaintext or to deduce the key being used.

Playfair cipher

The cipher that was used as the standard field system by the British Army in World War I and by the U.S. Army and other Allied forces during World War II.

ciphertext-only attack

The attack that is the easiest to defend against because the opponent has the least amount of information to work with.

Digrams

Common two-letter combinations in the English language.

polyalphabetic substitution cipher

An approach that uses different monoalphabetic substitutions as one proceeds through the plaintext message.

transposition cipher

A technique that is a mapping achieved by performing some sort of permutation on the plaintext letters.

hiding a secret message

A technique for embedding a secret message within a larger document or picture in such a way that others cannot discern the presence or contents of the hidden message.

computationally secure

An encryption scheme is said to be computationally secure if the cost of breaking the cipher exceeds the value of the encrypted information and the time required to break the cipher exceeds the useful lifetime of the information.

brute-force

The two types of attack on an encryption algorithm are cryptanalysis based on properties of the encryption algorithm, and brute-force which involves trying all possible keys.

the number of keys used

Cryptographic systems are characterized along three independent dimensions: The type of operations used for transforming plaintext to ciphertext; The way in which the plaintext is processed; and the number of keys used.

transposition

All encryption algorithms are based on two general principles: substitution and transposition.

Vigenère cipher

One of the simplest and best known polyalphabetic ciphers is Vigenère cipher. In this scheme, the set of related monoalphabetic substitution rules consists of the 26 Caesar ciphers with shifts of 0 through 25.

block, stream

A block cipher processes the input one block of elements at a time producing an output block for each input block whereas a stream cipher processes the input elements continuously producing output one element at a time.

unconditionally

An encryption scheme is unconditionally secure if the ciphertext generated by the scheme does not contain enough information to determine uniquely the corresponding plaintext, no matter how much ciphertext is available.

Caesar cipher

The earliest known and simplest use of a substitution cipher was called the Caesar cipher and involved replacing each letter of the alphabet with the letter standing three places further down the alphabet.

rail fence

The simplest transposition cipher is the rail fence technique in which the plaintext is written down as a sequence of diagonals and then read off as a sequence of rows.

Data Encryption Standard (DES)

The most widely used cipher ever is the Data Encryption Standard (DES).