Introduction

Cryptography comes from the two Greek words meaning “secret writing” and is the art and science of concealing meaning. Cryptanalysis is the breaking of codes. Basically, what we have is

Def: A cryptosystem is a 5-tuple (E, D, M, K, C), where M is the set of plaintexts, K is the set of keys, C is the set of ciphertexts, E: M K C is the set of enciphering functions, and D: C K M is the set of deciphering functions

Cryptography is a collection of mathematical techniques for protecting information. We will not be studying those mathematical techniques in depth. We will be learning about cryptography and how it impacts a system administrator. The process of using cryptography to scramble a message is called encryption. The process of unscrambling the message by use of the appropriate key is called decryption. A key is necessary to unlock a message.

Cryptography is used to prevent information from being accessed by an unauthorized recipient. In theory, one a piece of information is encrypted, the encrypted data can be accidentally disclosed or intercepted by a third party without compromising the security of the information, provided that the key necessary to decrypt the information is not disclosed and that the method of encryption will resist attempts to decrypt the message without the key.

Basic Methods

Cryptosystems are based on two techniques for hiding information: substitution and transposition.

Substitution Cipher

Substitution is based on the principle of replacing each letter in the message with another letter. The Caesar cipher is an example of a substitution cipher. Let’s say that you replace each “a” in the message with a “d”, each “b” with an “e”, and so on. This would be called a 3-shift Caesar cipher because you are taking the third letter after the letter in the message as the letter to substitute. e.g. We have ABCDEFGHIJKLMNOPQRSTUVWXYZ

Let’s do a 4-shift cipher for G O T I G E R S

For our cipher text we would have

Note: Substitution ciphers are susceptible to statistical attacks.

Transposition Cipher

A transposition cipher is based on scrambling the characters that are in the message. One transposition system involves writing a message into a table row by row and then reading it out column by column. Double transposition ciphers involved using two such transformations. This is really a permutation function. An example of a transposition cipher is the rail fence cipher. e.g. Plain text: H E L L O W O R L D

H L W L E O O D L R

Cipher text: H L W L E O O D L R

Note: Transposition ciphers are susceptible to frequency of occurrence to discover the transposition.

Cryptography Concepts

There are two additional important cryptography concepts, namely confusion and diffusion

Confusion An encrypting algorithm should take the information from the plaintext and transform it so that the interceptor cannot readily recognize the message. The interceptor should not be able to predict what will happen to the ciphertext by changing one character in the plaintext. We call this characteristic confusion. An algorithm providing good confusion has a complex function relationship between the plaintext/key pair and the ciphertext.

Diffusion The cipher should also spread the information from the plaintext over the entire ciphertext so that changes in the plaintext affect many parts of the ciphertext. This principle is called diffusion, the characteristic of distributing the information from single plaintext letters over the entire ciphertext. Good diffusion means that the interceptor needs access to much of the ciphertext to be able to infer the algorithm.