Hacking Secret Ciphers with Python

436 Pages·2013·6.61 MB·English

by  Al Sweigart

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
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Book Version 1
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” http://www.flickr.com/photos/walkn/3859852351/ Romeo
& Juliet and other public domain texts from Project Gutenberg. Various image resources from Wikipedia. Wrinkled
paper texture by Pink Sherbet Photography http://www.flickr.com/photos/pinksherbet/2978651767/ Computer User
icon by Katzenbaer.
If you've downloaded this book from a torrent, it’s probably out of date. Go
to http://inventwithpython.com/hacking to download the latest version.

ISBN 978-1482614374
1st Edition
Nedroid Picture Diary by Anthony Clark, http://nedroid.com

Movies and TV shows always make hacking look exciting with furious typing and meaningless
ones and zeros flying across the screen. 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. 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.”

- C.M.
A T B
BOUT HIS OOK
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. 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 http://python.org 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. “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. 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
http://inventwithpython.com/hacking. If you ever have questions about how these programs work,
feel free to email me at [email protected].
T C
ABLE OF ONTENTS
About This Book ....................................................................................................................... 6
Table of Contents ..................................................................................................................... 7
Chapter 1 - Making Paper Cryptography Tools ...................................................................... 1
What is Cryptography? ................................................................................................................................................ 2
Codes vs. Ciphers ........................................................................................................................................................ 3
Making a Paper Cipher Wheel ..................................................................................................................................... 4
A Virtual Cipher Wheel ............................................................................................................................................... 7
How to Encrypt with the Cipher Wheel ....................................................................................................................... 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 pyperclip.py ......................................................................................................................................... 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 ............................................................................................................................ 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 ........................................................................................................................................................ 43
Slicing ........................................................................................................................................................................ 44
Blank Slice Indexes.................................................................................................................................................... 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 ................................................................................................................................................ 47
Running Your Program .............................................................................................................................................. 48
Opening The Programs You’ve Saved ....................................................................................................................... 49
How the “Hello World” Program Works ................................................................................................................... 50
Comments .................................................................................................................................................................. 50
Functions.................................................................................................................................................................... 50
The print() function ............................................................................................................................................. 51
The input() function ............................................................................................................................................. 51
Ending the Program ................................................................................................................................................... 52
Practice Exercises, Chapter 4, Set C .......................................................................................................................... 52
Summary .................................................................................................................................................................... 52
Chapter 5 - The Reverse Cipher ............................................................................................ 53
The Reverse Cipher.................................................................................................................................................... 53
Source Code of the Reverse Cipher Program ............................................................................................................. 54
Sample Run of the Reverse Cipher Program .............................................................................................................. 54
Checking Your Source Code with the Online Diff Tool ............................................................................................ 55
How the Program Works............................................................................................................................................ 55
The len() Function ................................................................................................................................................. 56
Introducing the while Loop ..................................................................................................................................... 57
The Boolean Data Type ............................................................................................................................................. 58
Comparison Operators ............................................................................................................................................... 58
Conditions .................................................................................................................................................................. 61
Blocks ........................................................................................................................................................................ 61
The while Loop Statement ...................................................................................................................................... 62
“Growing” a String .................................................................................................................................................... 62
Tracing Through the Program, Step by Step .............................................................................................................. 65
Using input() In Our Programs ............................................................................................................................. 67
Practice Exercises, Chapter 5, Section A ................................................................................................................... 67
Summary .................................................................................................................................................................... 68
Chapter 6 - The Caesar Cipher .............................................................................................. 69
Implementing a Program ............................................................................................................................................ 69
Source Code of the Caesar Cipher Program ............................................................................................................... 70
Sample Run of the Caesar Cipher Program ................................................................................................................ 71
Checking Your Source Code with the Online Diff Tool ............................................................................................ 72
Practice Exercises, Chapter 6, Set A .......................................................................................................................... 72
How the Program Works............................................................................................................................................ 72
Importing Modules with the import Statement ....................................................................................................... 72
Constants.................................................................................................................................................................... 73
The upper() and lower() String Methods ........................................................................................................ 74
The for Loop Statement ........................................................................................................................................... 75
A while Loop Equivalent of a for Loop ................................................................................................................ 76
Practice Exercises, Chapter 6, Set B .......................................................................................................................... 77
The if Statement ...................................................................................................................................................... 77
The else Statement .................................................................................................................................................. 78
The elif Statement .................................................................................................................................................. 78
The in and not in Operators ................................................................................................................................. 79
The find() String Method ...................................................................................................................................... 80
Practice Exercises, Chapter 6, Set C .......................................................................................................................... 81
Back to the Code ........................................................................................................................................................ 81
Displaying and Copying the Encrypted/Decrypted String ......................................................................................... 83
Encrypt Non-Letter Characters .................................................................................................................................. 84
Summary .................................................................................................................................................................... 85
Chapter 7 - Hacking the Caesar Cipher with the Brute-Force Technique .......................... 87
Hacking Ciphers ........................................................................................................................................................ 87
The Brute-Force Attack ............................................................................................................................................. 88
Source Code of the Caesar Cipher Hacker Program .................................................................................................. 88
Sample Run of the Caesar Cipher Hacker Program ................................................................................................... 89
How the Program Works............................................................................................................................................ 90
The range() Function ............................................................................................................................................ 90
Back to the Code ........................................................................................................................................................ 92
String Formatting ....................................................................................................................................................... 93
Practice Exercises, Chapter 7, Set A .......................................................................................................................... 94
Summary .................................................................................................................................................................... 94
Chapter 8 - Encrypting with the Transposition Cipher ........................................................ 95
Encrypting with the Transposition Cipher ................................................................................................................. 95
Practice Exercises, Chapter 8, Set A .......................................................................................................................... 97
A Transposition Cipher Encryption Program ............................................................................................................. 97
Source Code of the Transposition Cipher Encryption Program ................................................................................. 97
Sample Run of the Transposition Cipher Encryption Program .................................................................................. 98
How the Program Works............................................................................................................................................ 99
Creating Your Own Functions with def Statements ................................................................................................. 99
The Program’s main() Function ........................................................................................................................... 100
Parameters ................................................................................................................................................................ 101
Variables in the Global and Local Scope ................................................................................................................. 103