Introduction to Cryptography with Coding Theory, 3rd Edition

B.3 Examples for Chapter 2

Example 1

A shift cipher was used to obtain the ciphertext kddkmu. Decrypt it by trying all possibilities.

> allshifts("kddkmu")
                        "kddkmu"
                        "leelnv"
                        "mffmow"
                        "nggnpx"
                        "ohhoqy"
                        "piiprz"
                        "qjjqsa"
                        "rkkrtb"
                        "sllsuc"
                        "tmmtvd"
                        "unnuwe"
                        "voovxf"
                        "wppwyg"
                        "xqqxzh"
                        "yrryai"
                        "zsszbj"
                        "attack"
                        "buubdl"
                        "cvvcem"
                        "dwwdfn"
                        "exxego"
                        "fyyfhp"
                        "gzzgiq"
                        "haahjr"
                        "ibbiks"
                        "jccjlt"

As you can see, attack is the only word that occurs on this list, so that was the plaintext.

Example 2

Encrypt the plaintext message cleopatra using the affine function $7x+8\text{:}$

> affinecrypt("cleopatra", 7, 8)

                           "whkcjilxi"

Example 3

The ciphertext mzdvezc was encrypted using the affine function $5x+12\text{.}$ Decrypt it.

SOLUTION

First, solve $y\equiv 5x+12(\text{mod ...}$