**Crack i n g C o d e s w i t h Python**: About The Technical Reviewers

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Brief Contents

Acknowledgments xix
Introduction xx
Chapter 1: Making Paper Cryptography Tools 1
Chapter 2: Programming in the Interactive Shell 11
Chapter 3: Strings and Writing Programs 21
Chapter 4: The Reverse Cipher 39
Chapter 5: The Caesar Cipher 53
Chapter 6: Hacking the Caesar Cipher with Brute-Force 69
Chapter 7: Encrypting with the Transposition Cipher 77
Chapter 8: Decrypting with the Transposition Cipher 99
Chapter 9: Programming a Program to Test Your Program 113
Chapter 10: Encrypting and Decrypting Files 127
Chapter 11: Detecting English Programmatically 141
Chapter 12: Hacking the Transposition Cipher 161
Chapter 13: A Modular Arithmetic Module for the Affine Cipher 171
Chapter 14: Programming the Affine Cipher 185
Chapter 15: Hacking the Affine Cipher 197
Chapter 16: Programming the Simple Substitution Cipher 207
Chapter 17: Hacking the Simple Substitution Cipher 221
Chapter 18: Programming the Vigenère Cipher 247
Chapter 19: Frequency Analysis 259
Chapter 20: Hacking the Vigenère Cipher 279
Chapter 21: The One-Time Pad Cipher 315
Chapter 22: Finding and Generating Prime
Numbers		 321
Chapter 23: Generating Keys for the Public Key Cipher 335
Chapter 24: Programming the Public Key Cipher 349
Appendix: Debugging Python Code 375
Index 381

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Contents in Detail

ACKNOWLEDGMENTS xix
INTRODUCTION xxi
Who Should Read This Book? xxii
What’s in This Book? xxiii
How to Use This Book. xxiv
Typing Source Code xxiv
Checking for Typos xxv
Coding Conventions in This Book xxv
Online Resources xxv
Downloading and Installing Python xxv
Windows Instructions. xxvi
macOS Instructions xxvi
Ubuntu Instructions xxvi
Downloading pyperclip.py xxvi
Starting IDLE xxvii
Summary xxvii
1
MAKING PAPER CRYPTOGRAPHY TOOLS 1
What Is Cryptography? 2
Codes vs. Ciphers 3
The Caesar Cipher 4
The Cipher Wheel 4
Encrypting with the Cipher Wheel 5
Decrypting with the Cipher Wheel 6
Encrypting and Decrypting with Arithmetic 7
Why Double Encryption Doesn’t Work 8
Summary 8
Practice Questions 9
2
PROGRAMMING IN THE INTERACTIVE SHELL 11
Some Simple Math Expressions 12
Integers and Floating-Point Values 13
Expressions 13
Order of Operations 14
Evaluating Expressions 14
Storing Values with Variables 15
Overwriting Variables 17
Variable Names 18
Summary 18
Practice Questions 19
3
STRINGS AND WRITING PROGRAMS 21
Working with Text Using String Values 22
String Concatenation with the + Operator 23
String Replication with the * Operator 24
Getting Characters from Strings Using Indexes 24
Printing Values with the print() Function 27
Printing Escape Characters 28
Quotes and Double Quotes 29
Writing Programs in IDLE’s File Editor 30
Source Code for the “Hello, World!” Program 31
Checking Your Source Code with the Online Diff Tool 31
Using IDLE to Access Your Program Later 32
Saving Your Program 32
Running Your Program 33
Opening the Programs You’ve Saved 34
How the “Hello, World!” Program Works 34
Comments 34
Printing Directions to the User 34
Taking a User’s Input 35
Ending the Program 35
Summary 36
Practice Questions 37
4
THE REVERSE CIPHER 39
Source Code for the Reverse Cipher Program 40
Sample Run of the Reverse Cipher Program 40
Setting Up Comments and Variables 41
Finding the Length of a String 41
Introducing the while Loop 42
The Boolean Data Type 43
Comparison Operators 43
Blocks 45
The while Loop Statement 46
“Growing” a String 47
Improving the Program with an input() Prompt 50
Summary 50
Practice Questions 51
5
THE CAESAR CIPHER 53
Source Code for the Caesar Cipher Program 54
Sample Run of the Caesar Cipher Program 55
Importing Modules and Setting Up Variables 56
Constants and Variables 57
The for Loop Statement 57
An Example for Loop 58
A while Loop Equivalent of a for Loop 59
The if Statement 59
An Example if Statement 60
The else Statement 60
The elif Statement 61
The in and not in Operators 61
The find() String Method 62
Encrypting and Decrypting Symbols 63
Handling Wraparound 64
Handling Symbols Outside of the Symbol Set 65
Displaying and Copying the Translated String 65
Encrypting Other Symbols 66
Summary 66
Practice Questions 67
6
HACKING THE CAESAR CIPHER WITH BRUTE-FORCE		 69
Source Code for the Caesar Cipher Hacker Program 70
Sample Run of the Caesar Cipher Hacker Program 71
Setting Up Variables 72
Looping with the range() Function 72
Decrypting the Message 73
Using String Formatting to Display the Key and Decrypted
Messages		 75
Summary 76
Practice Question 76
7
ENCRYPTING WITH THE TRANSPOSITION CIPHER		 77
How the Transposition Cipher Works 78
DECRYPTING WITH THE TRANSPOSITION CIPHER
99
How to Decrypt with the Transposition Cipher on Paper
100
Source Code for the Transposition Cipher Decryption Program
101
Sample Run of the Transposition Cipher Decryption Program
102
Importing Modules and Setting Up the main() Function
102
Decrypting the Message with the Key
103
The round(), math.ceil(), and math.floor() Functions
103
The decryptMessage() Function
104
Boolean Operators
106
Adjusting the column and row Variables
109
Calling the main() Function
110
Summary
110
Practice Questions
111
9
PROGRAMMING A PROGRAM TO TEST YOUR PROGRAM
113
Source Code for the Transposition Cipher Tester Program
114
Sample Run of the Transposition Cipher Tester Program
115
Importing the Modules
116
Creating Pseudorandom Numbers
116
Creating a Random String
118
Duplicating a String a Random Number of Times
118
List Variables Use References
119
Passing References
121
Using copy.deepcopy() to Duplicate a List
122
The random.shuffle() Function
122
Randomly Scrambling a String
123
Testing Each Message
123
Checking Whether the Cipher Worked and Ending the Program
124
Calling the main() Function
124
Testing the Test Program
125
Summary
125
Practice Questions
126
10
ENCRYPTING AND DECRYPTING FILES
127
Plain Text Files
128
Source Code for the Transposition File Cipher Program
128
Sample Run of the Transposition File Cipher Program
130
Working with Files
130
Opening Files
131
Writing to and Closing Files
131
Reading from a File
132
Setting Up the main() Function
132
Checking Whether a File Exists
133
The os.path.exists() Function
133
Checking Whether the Input File Exists with the os.path.exists() Function
134
Using String Methods to Make User Input More Flexible
134
The upper(), lower(), and title() String Methods
134
The startswith() and endswith() String Methods
135
Using These String Methods in the Program
135
Reading the Input File 136
Measuring the Time It Took to Encrypt or Decrypt 136
The time Module and time.time() Function 136
Using the time.time() Function in the Program 137
Writing the Output File 137
Calling the main() Function 138
Summary 138
Practice Questions 139
11
DETECTING ENGLISH PROGRAMMATICALLY
How Can a Computer Understand English? 142
Source Code for the Detect English Module 143
Sample Run of the Detect English Module 145
Instructions and Setting Up Constants 145
The Dictionary Data Type 146
The Difference Between Dictionaries and Lists 147
Adding or Changing Items in a Dictionary 147
Using the len() Function with Dictionaries 148
Using the in Operator with Dictionaries 148
Finding Items Is Faster with Dictionaries than with Lists 149
Using for Loops with Dictionaries 149
Implementing the Dictionary File 150
The split() Method 150
Splitting the Dictionary File into Individual Words 151
Returning the Dictionary Data 151
Counting the Number of English Words in message 152
Divide-by-Zero Errors 152
Counting the English Word Matches 153
The float(), int(), and str() Functions and Integer Division 154
Finding the Ratio of English Words in the Message 154
Removing Non-Letter Characters 155
The append() List Method 155
Creating a String of Letters 156
Detecting English Words 156
Using Default Arguments 157
Calculating Percentages 157
Summary 159
Practice Questions 160
12
HACKING THE TRANSPOSITION CIPHER
Source Code of the Transposition Cipher Hacker Program 162
Sample Run of the Transposition Cipher Hacker Program 163
Importing the Modules 164
Multiline Strings with Triple Quotes 164
Displaying the Results of Hacking the Message 165
Getting the Hacked Message 166
The strip() String Method 167
Applying the strip() String Method 168
Failing to Hack the Message 168
Calling the main() Function 169
Summary 169
Practice Questions 169
13 171
A MODULAR ARITHMETIC MODULE FOR THE AFFINE CIPHER
Modular Arithmetic. 172
The Modulo Operator. 173
Finding Factors to Calculate the Greatest Common Divisor 173
Multiple Assignment 175
Euclid’s Algorithm for Finding the GCD. 176
Understanding How the Multiplicative and Affine Ciphers Work 177
Choosing Valid Multiplicative Keys 178
Encrypting with the Affine Cipher 179
Decrypting with the Affine Cipher 179
Finding Modular Inverses. 181
The Integer Division Operator. 181
Source Code for the Cryptomath Module 182
Summary 183
Practice Questions 183
14 185
PROGRAMMING THE AFFINE CIPHER
Source Code for the Affine Cipher Program. 186
Sample Run of the Affine Cipher Program 188
Setting Up Modules, Constants, and the main() Function 188
Calculating and Validating the Keys 189
The Tuple Data Type 190
Checking for Weak Keys 190
How Many Keys Can the Affine Cipher Have? 191
Writing the Encryption Function 193
Writing the Decryption Function 194
Generating Random Keys 195
Calling the main() Function 196
Summary 196
Practice Questions 196
15 197
HACKING THE AFFINE CIPHER
Source Code for the Affine Cipher Hacker Program 198
Sample Run of the Affine Cipher Hacker Program 199
Setting Up Modules, Constants, and the main() Function 200
The Affine Cipher Hacking Function 201
The Exponent Operator 201
Calculating the Total Number of Possible Keys 201
The continue Statement 202
Using continue to Skip Code 203
Calling the main() Function 204
Summary 205
Practice Questions 16 205
Programming the Simple Substitution Cipher 207
How the Simple Substitution Cipher Works 208
Source Code for the Simple Substitution Cipher Program 209
Sample Run of the Simple Substitution Cipher Program 210
Setting Up Modules, Constants, and the main() Function 211
The sort() List Method 212
Wrapper Functions 213
The translateMessage() Function 215
The isupper() and islower() String Methods 216
Preserving Cases with isupper() 217
Generating a Random Key 218
Calling the main() Function 219
Summary 219
Practice Questions 219
17
Hacking the Simple Substitution Cipher 221
Using Word Patterns to Decrypt 222
Finding Word Patterns 222
Finding Potential Decryption Letters 223
Overview of the Hacking Process 225
The Word Pattern Modules 225
Source Code for the Simple Substitution Hacking Program 226
Sample Run of the Simple Substitution Hacking Program 229
Setting Up Modules and Constants 230
Finding Characters with Regular Expressions 230
Setting Up the main() Function 231
Displaying Hacking Results to the User 232
Creating a Cipherletter Mapping 232
Creating a Blank Mapping 232
Adding Letters to a Mapping 233
Intersecting Two Mappings 234
How the Letter-Mapping Helper Functions Work 235
Identifying Solved Letters in Mappings 238
Testing the removeSolvedLetterFromMapping() Function 240
The hackSimpleSub() Function 241
The replace() String Method 243
Decrypting the Message 243
Decrypting in the Interactive Shell 244
Calling the main() Function 245
Summary 246
Practice Questions 246
18
Programming the Vigenère Cipher 247
Using Multiple Letter Keys in the Vigenère Cipher 248
Longer Vigenère Keys Are More Secure 249
Choosing a Key That Prevents Dictionary Attacks 250
Source Code for the Vigenère Cipher Program 251
Sample Run of the Vigenère Cipher Program 252

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Ac knowledgments

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I ntrodu c tion

“I couldn’t help but overhear, probably because I was eavesdropping.” —Anonymous

If you could travel back to the early 1990s with this book, the contents of Chapter 23 that implement part of the RSA cipher would be illegal to export out of the United States.

Because messages encrypted with RSA are impossible to hack, the export of encryption software like RSA

was deemed a matter of national security and required State Department approval. In fact, strong cryptography was regulated at the same level as tanks, missiles, and flamethrowers.

In 1990, Daniel J. Bernstein, a student at the University of California, Berkeley, wanted to publish an academic paper that featured source code of his Snuffle encryption system. The US government informed him that he would need to become a licensed arms dealer before he could post his source code on the internet. The government also told him that it would deny him an export license if he applied for one because his technology was too secure.

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The Electronic Frontier Foundation, a young digital civil liberties organization, represented Bernstein in Bernstein v. United States. For the first time ever, the courts ruled that written software code was speech protected by the First Amendment and that the export control laws on encryption violated Bernstein’s First Amendment rights.

Now, strong cryptography is at the foundation of a large part of the global economy, safeguarding businesses and e-commerce sites used by millions of internet shoppers every day. The intelligence community’s predictions that encryption software would become a grave national security threat were unfounded.

But as recently as the 1990s, spreading this knowledge freely (as this book does) would have landed you in prison for arms trafficking. For a more detailed history of the legal battle for freedom of cryptography, read Steven Levy’s book Crypto: How the Code Rebels Beat the Government, Saving Privacy in the Digital Age (Penguin, 2001).

Who Should Read This Book?

Many books teach beginners how to write secret messages using ciphers. A couple of books teach beginners how to hack ciphers. But no books teach beginners how to program computers to hack ciphers. This book fills that gap.

This book is for those who are curious about encryption, hacking, or cryptography. The ciphers in this book (except for the public key cipher in Chapters 23 and 24) are all centuries old, but any laptop has the computational power to hack them. No modern organizations or individuals use these ciphers anymore, but by learning them, you’ll learn the foundations cryptography was built on and how hackers can break weak encryption.

NOTE

N ote The ciphers you’ll learn in this book are fun to play with, but they don’t provide true security. Don’t use any of the encryption programs in this book to secure your actual files. As a general rule, you shouldn’t trust the ciphers that you create. Real-world ciphers are subject to years of analysis by professional cryptographers before being put into use.

This book is also for people who have never programmed before. It teaches basic programming concepts using the Python programming language, which is one of the best languages for beginners. It has a gentle learning curve that novices of all ages can master, yet it’s also a powerful language used by professional software developers. Python runs on Windows, macOS, Linux, and even the Raspberry Pi, and it’s free to download and use. (See “Downloading and Installing Python” on page xxv for instructions.)

In this book, I’ll use the term hacker often. The word has two definitions. A hacker can be 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’re not limited by that system’s original rules and can modify it in creative ways.

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A hacker can also be a criminal who breaks into computer systems, violates people’s privacy, and causes damage. This book uses the term in the first sense. Hackers are cool. Criminals are just people who think they’re being clever by breaking stuff.

What’s in This Book?

The first few chapters introduce basic Python and cryptography concepts. Thereafter, chapters generally alternate between explaining a program for a cipher and then explaining a program that hacks that cipher. Each chapter also includes practice questions to help you review what you’ve learned.

  • • Chapter 1: Making Paper Cryptography Tools covers some simple paper tools, showing how encryption was done before computers.
  • • Chapter 2: Programming in the Interactive Shell explains how to use Python’s interactive shell to play around with code one line at a time.
  • • Chapter 3: Strings and Writing Programs covers writing full programs and introduces the string data type used in all programs in this book.
  • • Chapter 4: The Reverse Cipher explains how to write a simple program for your first cipher.
  • • Chapter 5: The Caesar Cipher covers a basic cipher first invented thousands of years ago.
  • • Chapter 6: Hacking the Caesar Cipher with Brute-Force explains the brute-force hacking technique and how to use it to decrypt messages without the encryption key.
  • • Chapter 7: Encrypting with the Transposition Cipher introduces the transposition cipher and a program that encrypts messages with it.
  • • Chapter 8: Decrypting with the Transposition Cipher covers the second half of the transposition cipher: being able to decrypt messages with a key.
  • • Chapter 9: Programming a Program to Test Your Program introduces the programming technique of testing programs with other programs.
  • • Chapter 10: Encrypting and Decrypting Files explains how to write programs that read files from and write files to the hard drive.
  • • Chapter 11: Detecting English Programmatically describes how to make the computer detect English sentences.
  • • Chapter 12: Hacking the Transposition Cipher combines the concepts from previous chapters to hack the transposition cipher.
  • • Chapter 13: A Modular Arithmetic Module for the Affine Cipher explains the math concepts behind the affine cipher.
  • • Chapter 14: Programming the Affine Cipher covers writing an affine cipher encryption program.
  • • Chapter 15: Hacking the Affine Cipher explains how to write a program to hack the affine cipher.

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  • • Chapter 16: Programming the Simple Substitution Cipher covers writing a simple substitution cipher encryption program.
  • • Chapter 17: Hacking the Simple Substitution Cipher explains how to write a program to hack the simple substitution cipher.
  • • Chapter 18: Programming the Vigenère Cipher explains a program for the Vigenère cipher, a more complex substitution cipher.
  • • Chapter 19: Frequency Analysis explores the structure of English words and how to use it to hack the Vigenère cipher.
  • • Chapter 20: Hacking the Vigenère Cipher covers a program for hacking the Vigenère cipher.
  • • Chapter 21: The One-Time Pad Cipher explains the one-time pad cipher and why it’s mathematically impossible to hack.
  • • Chapter 22: Finding and Generating Prime Numbers covers how to write a program that quickly determines whether a number is prime.
  • • Chapter 23: Generating Keys for the Public Key Cipher describes public key cryptography and how to write a program that generates public and private keys.
  • • Chapter 24: Programming the Public Key Cipher explains how to write a program for a public key cipher, which you can’t hack using a mere laptop.
  • • The appendix, Debugging Python Code, shows you how to use IDLE’s debugger to find and fix bugs in your programs.
Setting Up Modules, Constants, and the main() Function 252
Building Strings with the List-Append-Join Process 253
Encrypting and Decrypting the Message 255
Calling the main() Function 257
Summary 257
Practice Questions 258
19 FREQUENCY ANALYSIS 259
Analyzing the Frequency of Letters in Text 260
Matching Letter Frequencies 262
Calculating the Frequency Match Score for the Simple Substitution Cipher 262
Calculating the Frequency Match Score for the Transposition Cipher 263
Using Frequency Analysis on the Vigenère Cipher 264
Source Code for Matching Letter Frequencies 265
Storing the Letters in ETAOIN Order 266
Counting the Letters in a Message 267
Getting the First Member of a Tuple 268