Hacking Secret Ciphers with Python

About This Book

Table of Contents

Chapter 1 - Making Paper Cryptography Tools

What is Cryptography?

Codes vs. Ciphers

Making a Paper Cipher Wheel

A Virtual Cipher Wheel

How to Encrypt with the Cipher Wheel

How to Decrypt with the Cipher Wheel

A Different Cipher Tool: The St. Cyr Slide

Practice Exercises, Chapter 1, Set A

Doing Cryptography without Paper Tools

Practice Exercises, Chapter 1, Set B

Double-Strength Encryption?.....................................................................................................................................

Programming a Computer to do Encryption

Chapter 2 - Installing Python

Downloading and Installing Python

Downloading pyperclip.py

Starting IDLE

The Featured Programs

Line Numbers and Spaces

Text Wrapping in This Book

Tracing the Program Online

Checking Your Typed Code with the Online Diff Tool

Copying and Pasting Text

More Info Links

Programming and Cryptography

Chapter 3 - The Interactive Shell

Some Simple Math Stuff

Integers and Floating Point Values

Expressions

Order of Operations

Evaluating Expressions

Errors are Okay!

Every Value has a Data Type

Storing Values in Variables with Assignment Statements

Overwriting Variables

Using More Than One Variable

Variable Names

Summary - But When Are We Going to Start Hacking?............................................................................................

Chapter 4 - Strings and Writing Programs

Strings

String Concatenation with the + Operator

String Replication with the * Operator

Printing Values with the print() Function

Escape Characters

Quotes and Double Quotes

Indexing

Negative Indexes

Slicing

Blank Slice Indexes....................................................................................................................................................

Writing Programs in IDLE’s File Editor

Hello World!

Source Code of Hello World

Saving Your Program

Running Your Program

Opening The Programs You’ve Saved

How the “Hello World” Program Works

Comments

Functions....................................................................................................................................................................

The print() function

The input() function

Ending the Program

Practice Exercises, Chapter 4, Set C

Summary

Chapter 5 - The Reverse Cipher

The Reverse Cipher....................................................................................................................................................

Source Code of the Reverse Cipher Program

Sample Run of the Reverse Cipher Program

Checking Your Source Code with the Online Diff Tool

How the Program Works............................................................................................................................................

The len() Function

Introducing the while Loop

The Boolean Data Type

Comparison Operators

Conditions

Blocks

The while Loop Statement

“Growing” a String

Tracing Through the Program, Step by Step

Using input() In Our Programs

Practice Exercises, Chapter 5, Section A

Summary

Chapter 6 - The Caesar Cipher

Implementing a Program

Source Code of the Caesar Cipher Program

Sample Run of the Caesar Cipher Program

Checking Your Source Code with the Online Diff Tool

Importing Modules with the import Statement

Constants....................................................................................................................................................................

The upper() and lower() String Methods

The for Loop Statement

A while Loop Equivalent of a for Loop

The if Statement

The else Statement

The elif Statement

The in and not in Operators

The find() String Method

Back to the Code

Displaying and Copying the Encrypted/Decrypted String

Encrypt Non-Letter Characters

Summary

Chapter 7 - Hacking the Caesar Cipher with the Brute-Force Technique

Hacking Ciphers

The Brute-Force Attack

Source Code of the Caesar Cipher Hacker Program

Sample Run of the Caesar Cipher Hacker Program

The range() Function

Back to the Code

String Formatting

Summary

Chapter 8 - Encrypting with the Transposition Cipher

Encrypting with the Transposition Cipher

A Transposition Cipher Encryption Program

Source Code of the Transposition Cipher Encryption Program

Sample Run of the Transposition Cipher Encryption Program

How the Program Works..........................................................................................................................................

Creating Your Own Functions with def Statements

The Program’s main() Function

Parameters

Variables in the Global and Local Scope

The global Statement

The List Data Type

Using the list() Function to Convert Range Objects to Lists

Reassigning the Items in Lists..................................................................................................................................

Reassigning Characters in Strings

Lists of Lists

Using len() and the in Operator with Lists

List Concatenation and Replication with the + and * Operators..............................................................................

Practice Exercises, Chapter 8, Set D

The Transposition Encryption Algorithm

Augmented Assignment Operators

Back to the Code

The join() String Method

Return Values and return Statements

Practice Exercises, Chapter 8, Set E

Back to the Code

The Special name Variable

Key Size and Message Length

Summary

Chapter 9 - Decrypting with the Transposition Cipher

Decrypting with the Transposition Cipher on Paper

A Transposition Cipher Decryption Program...........................................................................................................

Source Code of the Transposition Cipher Decryption Program

The math.ceil(), math.floor() and round() Functions

The and and or Boolean Operators

Truth Tables

The and and or Operators are Shortcuts

Order of Operations for Boolean Operators

Back to the Code

Summary

Chapter 10 - Programming a Program to Test Our Program

Source Code of the Transposition Cipher Tester Program

Sample Run of the Transposition Cipher Tester Program

Pseudorandom Numbers and the random.seed() Function

The random.randint() Function

References

The copy.deepcopy() Functions

The random.shuffle() Function

Randomly Scrambling a String

Back to the Code

The sys.exit() Function

Testing Our Test Program

Summary

Chapter 11 - Encrypting and Decrypting Files

Plain Text Files

Source Code of the Transposition File Cipher Program

Sample Run of the Transposition File Cipher Program

Reading From Files

Writing To Files

The os.path.exists() Function

The startswith() and endswith() String Methods

The title() String Method

The time Module and time.time() Function

Back to the Code

Summary

Chapter 12 - Detecting English Programmatically

How Can a Computer Understand English?

Practice Exercises, Chapter 12, Section A

The Detect English Module

Source Code for the Detect English Module

Dictionaries and the Dictionary Data Type

Adding or Changing Items in a Dictionary

Using the len() Function with Dictionaries

Using the in Operator with Dictionaries

Using for Loops with Dictionaries

The Difference Between Dictionaries and Lists

Finding Items is Faster with Dictionaries Than Lists

The split() Method

The None Value

Back to the Code

“Divide by Zero” Errors

The float(), int(), and str() Functions and Integer Division

Back to the Code

The append() List Method

Default Arguments

Calculating Percentage

Summary

Chapter 13 - Hacking the Transposition Cipher

Source Code of the Transposition Cipher Hacker Program

Sample Run of the Transposition Breaker Program

Multi-line Strings with Triple Quotes

Back to the Code

The strip() String Method

Summary

Chapter 14 - Modular Arithmetic with the Multiplicative and Affine Ciphers

Oh No Math!

Math Oh Yeah!

Modular Arithmetic (aka Clock Arithmetic)

The % Mod Operator

GCD: Greatest Common Divisor (aka Greatest Common Factor)

Visualize Factors and GCD with Cuisenaire Rods

Multiple Assignment

Swapping Values with the Multiple Assignment Trick

Euclid’s Algorithm for Finding the GCD of Two Numbers.....................................................................................

“Relatively Prime”

The Multiplicative Cipher

Multiplicative Cipher + Caesar Cipher = The Affine Cipher

The First Affine Key Problem..................................................................................................................................

Decrypting with the Affine Cipher...........................................................................................................................

Finding Modular Inverses

The // Integer Division Operator

Source Code of the cryptomath Module

Summary

Chapter 15 - The Affine Cipher

Source Code of the Affine Cipher Program

Sample Run of the Affine Cipher Program

Splitting One Key into Two Keys

The Tuple Data Type

Input Validation on the Keys

The Affine Cipher Encryption Function

The Affine Cipher Decryption Function

Generating Random Keys

The Second Affine Key Problem: How Many Keys Can the Affine Cipher Have?

Summary

Chapter 16 - Hacking the Affine Cipher

Source Code of the Affine Cipher Hacker Program

Sample Run of the Affine Cipher Hacker Program

The Affine Cipher Hacking Function

The ** Exponent Operator

The continue Statement

Summary

Chapter 17 - The Simple Substitution Cipher

The Simple Substitution Cipher with Paper and Pencil

Source Code of the Simple Substitution Cipher

Sample Run of the Simple Substitution Cipher Program

The Program’s main() Function

The sort() List Method

Wrapper Functions

The Program’s translateMessage() Function

The isupper() and islower() String Methods

Generating a Random Key

Encrypting Spaces and Punctuation

Summary

Chapter 18 - Hacking the Simple Substitution Cipher

Computing Word Patterns

Getting a List of Candidates for a Cipherword

Source Code of the Word Pattern Module

Sample Run of the Word Pattern Module

The pprint.pprint() and pprint.pformat() Functions

Building Strings in Python with Lists

Calculating the Word Pattern

The Word Pattern Program’s main() Function

Hacking the Simple Substitution Cipher

Source Code of the Simple Substitution Hacking Program......................................................................................

Hacking the Simple Substitution Cipher (in Theory)

Explore the Hacking Functions with the Interactive Shell

Import All the Things

A Brief Intro to Regular Expressions and the sub() Regex Method

The Hacking Program’s main() Function

Partially Hacking the Cipher

Blank Cipherletter Mappings

Adding Letters to a Cipherletter Mapping

Intersecting Two Letter Mappings

Removing Solved Letters from the Letter Mapping

Hacking the Simple Substitution Cipher

Creating a Key from a Letter Mapping

Couldn’t We Just Encrypt the Spaces Too?

Summary

Chapter 19 - The Vigenère Cipher

Le Chiffre Indéchiffrable

Multiple “Keys” in the Vigenère Key

Source Code of Vigenère Cipher Program

Sample Run of the Vigenère Cipher Program

Summary

Chapter 20 - Frequency Analysis

The Code for Matching Letter Frequencies

The Most Common Letters, “ETAOIN”

The Program’s getLettersCount() Function

The Program’s getItemAtIndexZero() Function

The Program’s getFrequencyOrder() Function

The sort() Method’s key and reverse Keyword Arguments

Passing Functions as Values

Converting Dictionaries to Lists with the keys(), values(), items() Dictionary Methods

Sorting the Items from a Dictionary

The Program’s englishFreqMatchScore() Function

Summary

Chapter 21 - Hacking the Vigenère Cipher

The Dictionary Attack

Source Code for a Vigenère Dictionary Attack Program

Sample Run of the Vigenère Dictionary Hacker Program

The readlines() File Object Method

The Babbage Attack & Kasiski Examination

Kasiski Examination, Step 1 – Find Repeat Sequences’ Spacings

Kasiski Examination, Step 2 – Get Factors of Spacings

Get Every Nth Letters from a String

Frequency Analysis

Brute-Force through the Possible Keys

Source Code for the Vigenère Hacking Program

Sample Run of the Vigenère Hacking Program

Finding Repeated Sequences

Calculating Factors

Removing Duplicates with the set() Function

The Kasiski Examination Algorithm........................................................................................................................

The extend() List Method

The end Keyword Argument for print()

The itertools.product() Function

The break Statement

Modifying the Constants of the Hacking Program

Summary

Chapter 22 - The One-Time Pad Cipher

The Unbreakable One-Time Pad Cipher

Why the One-Time Pad is Unbreakable

Beware Pseudorandomness

Beware the Two-Time Pad

The Two-Time Pad is the Vigenère Cipher

Summary

Chapter 23 - Finding Prime Numbers

Prime Numbers

Composite Numbers.................................................................................................................................................

Source Code for The Prime Sieve Module

How to Calculate if a Number is Prime

The Sieve of Eratosthenes

The primeSieve() Function...............................................................................................................................

Detecting Prime Numbers

Source Code for the Rabin-Miller Module

Sample Run of the Rabin Miller Module

The Rabin-Miller Primality Algorithm

The New and Improved isPrime() Function

Summary

Chapter 24 - Public Key Cryptography and the RSA Cipher

Public Key Cryptography.........................................................................................................................................

The Dangers of “Textbook” RSA

A Note About Authentication

The Man-In-The-Middle Attack

Generating Public and Private Keys.........................................................................................................................

Source Code for the RSA Key Generation Program

Sample Run of the RSA Key Generation Program

How the Key Generation Program Works

The Program’s generateKey() Function

RSA Key File Format

Hybrid Cryptosystems

Source Code for the RSA Cipher Program

Sample Run of the RSA Cipher Program

Digital Signatures

How the RSA Cipher Program Works

ASCII: Using Numbers to Represent Characters

The chr() and ord() Functions

Blocks

Converting Strings to Blocks with getBlocksFromText()

The encode() String Method and the Bytes Data Type

The bytes() Function and decode() Bytes Method

Back to the Code

The min() and max() Functions

The insert() List Method

The Mathematics of RSA Encrypting and Decrypting

The pow() Function

Reading in the Public & Private Keys from their Key Files

The Full RSA Encryption Process

The Full RSA Decryption Process

Why Can’t We Hack the RSA Cipher

Summary

About the Author

Hacking Secret Ciphers with Python

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