Hacking Secret Ciphers with Python

1 Hacking Secret Ciphers with Python By Al Sweigart Copyright 2013 by Al Sweigart Some Rights Reserved. Hacking Secret Ciphers with Python is licensed under a Creative Commons Attribution-Noncommercial-Share Alike United States License. You are free: To Share to copy, distribute, display, and perform the work To Remix to make derivative works Under the following conditions: Attribution You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). (Visibly include the title and author's name in any excerpts of this work.) Noncommercial You may not use this work for commercial purposes. Share Alike If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one.

2 This summary is located here: Your fair use and other rights are in no way affected by the above. There is a human-readable summary of the Legal Code (the full license), located here: Book Version 3 Special thanks to Ari Lacenski. I can t thank her enough. Without her efforts there d be typos literally on every page. Thanks to Jason Kibbe. Cover lock photo by walknboston Romeo & Juliet and other public domain texts from Project Gutenberg. Various image resources from Wikipedia. Wrinkled paper texture by Pink Sherbet Photography Computer User icon by Katzenbaer. If you've downloaded this book from a torrent, it s probably out of date. Go to to download the latest version. ISBN 978-1482614374 1st Edition Nedroid Picture Diary by Anthony Clark, Movies and TV shows always make Hacking look exciting with furious typing and meaningless ones and zeros flying across the screen.

3 They make Hacking look like something that you have to be super smart to learn. They make Hacking look like magic. It s not magic. It s based on computers, and everything computers do have logical principles behind them which can be learned and understood. Even when you don t understand or when the computer does something frustrating or mysterious, there is always, always, always a reason why. And it s not hard to learn. This book assumes you know nothing about cryptography or programming, and helps you learn, step by step, how to write programs that can hack encrypted messages. Good luck and have fun! 100% of the profits from this book are donated to the Electronic Frontier Foundation, the Creative Commons, and the Tor Project. Dedicated to Aaron Swartz, 1986 2013 Aaron was part of an army of citizens that believes democracy only works when the citizenry are informed, when we know about our rights and our obligations.

4 An army that believes we must make justice and knowledge available to all not just the well born or those that have grabbed the reins of power so that we may govern ourselves more wisely. When I see our army, I see Aaron Swartz and my heart is broken. We have truly lost one of our better angels. - ABOUT THIS BOOK There are many books that teach beginners how to write Secret messages using Ciphers . There are a couple books that teach beginners how to hack Ciphers . As far as I can tell, there are no books to teach beginners how to write programs to hack Ciphers . This book fills that gap. This book is for complete beginners who do not know anything about encryption, Hacking , or cryptography. The Ciphers in this book (except for the RSA cipher in the last chapter) are all centuries old, and modern computers now have the computational power to hack their encrypted messages.

5 No modern organization or individuals use these Ciphers anymore. As such, there s no reasonable context in which you could get into legal trouble for the information in this book. This book is for complete beginners who have never programmed before. This book teaches basic programming concepts with the Python programming language. Python is the best language for beginners to learn programming: it is simple and readable yet also a powerful programming language used by professional software developers. The Python software can be downloaded for free from and runs on Linux, Windows, OS X, and the Raspberry Pi. There are two definitions of hacker . A hacker is a person who studies a system (such as the rules of a cipher or a piece of software) to understand it so well that they are not limited by the original rules of that system and can creatively modify it to work in new ways.

6 Hacker is also used to mean criminals who break into computer systems, violate people s privacy, and cause damage. This book uses hacker in the first sense. Hackers are cool. Criminals are just people who think they re being clever by breaking stuff. Personally, my day job as a software developer pays me way more for less work than writing a virus or doing an Internet scam would. On a side note, don t use any of the encryption programs in this book for your actual files. They re fun to play with but they don t provide true security. And in general, you shouldn t trust the Ciphers that you yourself make. As legendary cryptographer Bruce Schneier put it, Anyone, from the most clueless amateur to the best cryptographer, can create an algorithm that he himself can t break.

7 It s not even hard. What is hard is creating an algorithm that no one else can break, even after years of analysis. And the only way to prove that is to subject the algorithm to years of analysis by the best cryptographers around. This book is released under a Creative Commons license and is free to copy and distribute (as long as you don t charge money for it). The book can be downloaded for free from its website at If you ever have questions about how these programs work, feel free to email me at TABLE OF CONTENTS About This Book .. 6 Table of Contents .. 7 Chapter 1 - Making Paper Cryptography Tools .. 1 What is Cryptography? .. 2 Codes vs. Ciphers .. 2 Making a Paper Cipher Wheel .. 3 A Virtual Cipher Wheel .. 7 How to Encrypt with the Cipher Wheel.

8 8 How to Decrypt with the Cipher Wheel .. 9 A Different Cipher Tool: The St. Cyr Slide .. 10 Practice Exercises, Chapter 1, Set A .. 11 Doing Cryptography without Paper Tools .. 11 Practice Exercises, Chapter 1, Set B .. 13 Double-Strength Encryption?.. 13 Programming a Computer to do Encryption .. 14 Chapter 2 - Installing Python .. 16 Downloading and Installing Python .. 17 Downloading .. 18 Starting IDLE .. 18 The Featured Programs .. 19 Line Numbers and Spaces .. 20 Text Wrapping in This Book .. 20 Tracing the Program Online .. 21 Checking Your Typed Code with the Online Diff Tool .. 21 Copying and Pasting Text .. 21 More Info Links .. 22 Programming and Cryptography .. 22 Chapter 3 - The Interactive Shell .. 26 Some Simple Math Stuff .. 26 Integers and Floating Point Values.

9 27 Expressions .. 27 Order of Operations .. 28 Evaluating Expressions .. 29 Errors are Okay! .. 29 Practice Exercises, Chapter 3, Set A .. 30 Every Value has a Data Type .. 30 Storing Values in Variables with Assignment Statements .. 30 Overwriting Variables .. 32 Using More Than One Variable .. 33 Variable Names .. 34 Practice Exercises, Chapter 3, Set B .. 35 Summary - But When Are We Going to Start Hacking ?.. 35 Chapter 4 - Strings and Writing Programs .. 36 Strings .. 36 String Concatenation with the + Operator .. 38 String Replication with the * Operator .. 39 Printing Values with the print() Function .. 39 Escape Characters .. 40 Quotes and Double Quotes .. 41 Practice Exercises, Chapter 4, Set A .. 42 Indexing .. 42 Negative Indexes .. 44 Slicing.

10 44 Blank Slice 45 Practice Exercises, Chapter 4, Set B .. 46 Writing Programs in IDLE s File Editor .. 46 Hello World! .. 47 Source Code of Hello World .. 47 Saving Your Program .. 48 Running Your Program .. 49 Opening The Programs You ve Saved .. 50 How the Hello World Program Works .. 50 Comments .. 50 51 The print() function .. 51 The input() function .. 51 Ending the Program .. 52 Practice Exercises, Chapter 4, Set C .. 52 Summary .. 53 Chapter 5 - The Reverse Cipher .. 54 The Reverse 54 Source Code of the Reverse Cipher Program .. 55 Sample Run of the Reverse Cipher Program .. 55 Checking Your Source Code with the Online Diff Tool .. 56 How the Program 56 The len() Function .. 57 Introducing the while Loop .. 58 The Boolean Data Type .. 59 Comparison Operators.