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.

**PART ONE SYMMETRIC CIPHERS
**

**2 C****LASSICAL**** E****NCRYPTION**** T****ECHNIQUES**

**2.1
****Symmetric Cipher
Model **

Cryptography

Cryptanalysis and Brute-Force Attack

**2.2
****Substitution
Techniques **

Caesar Cipher

Monoalphabetic Ciphers

Playfair Cipher

Hill Cipher

Polyalphabetic Ciphers

One-Time Pad

**2.3
****Transposition
Techniques **

**2.4
****Rotor Machines **

**2.5
****Steganography **

*“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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Cryptography and Network Security Principles and Practice : One Symmetric Ciphers : Classical Encryption Techniques : Classical Encryption Techniques |

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