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PART ONE SYMMETRIC CIPHERS
2 CLASSICAL ENCRYPTION TECHNIQUES
2.1 Symmetric Cipher Model
Cryptanalysis and Brute-Force Attack
2.2 Substitution Techniques
2.3 Transposition Techniques
2.4 Rotor Machines
“I am fairly familiar with all the forms of secret writings, and am myself the author of a trifling monograph upon the subject, in which I analyze one hundred and sixty separate ciphers,” said Holmes.
—The Adventure of the Dancing Men, Sir Arthur Conan Doyle
◆ Symmetric encryption is a form of cryptosystem in which encryption and decryption are performed using the same key. It is also known as conven-tional encryption.
◆ Symmetric encryption transforms plaintext into ciphertext using a secret key and an encryption algorithm. Using the same key and a decryption algorithm, the plaintext is recovered from the ciphertext.
◆ 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.
◆ Traditional (precomputer) symmetric ciphers use substitution and/or transposition techniques. Substitution techniques map plaintext elements (characters, bits) into ciphertext elements. Transposition techniques sys-tematically transpose the positions of plaintext elements.
◆ Rotor machines are sophisticated precomputer hardware devices that use substitution techniques.
◆ Steganography is a technique for hiding a secret message within a larger one in such a way that others cannot discern the presence or contents of the hidden message.
Symmetric encryption, also referred to as conventional encryption or single-key encryption, was the only type of encryption in use prior to the development of public-key encryption in the 1970s. It remains by far the most widely used of the two types of encryption. Part One examines a number of symmetric ciphers. In this chapter, we begin with a look at a general model for the symmetric encryption process; this will enable us to understand the context within which the algorithms are used. Next, we examine a variety of algorithms in use before the computer era. Finally, we look briefly at a different approach known as steganography. Chapters 3 and 5 examine the two most widely used symmetric cipher: DES and AES.
Before beginning, we define some terms. An original message is known as the plaintext, while the coded message is called the ciphertext. The process of converting from plaintext to ciphertext is known as enciphering or encryption; restoring the plain-text from the ciphertext is deciphering or decryption. The many schemes used for encryption constitute the area of study known as cryptography. Such a scheme is known as a cryptographic system or a cipher. Techniques used for deciphering a message without any knowledge of the enciphering details fall into the area of cryptanalysis. Cryptanalysis is what the layperson calls “breaking the code.” The areas of cryptography and cryptanalysis together are called cryptology.
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