Cracking Codes with Python: A Beginner's Guide to Cryptography and Computer Programming

Description

In Cracking Codes with Python, you'll learn how to program in Python while making and breaking ciphers, which are used to encrypt secret messages. (No programming experience required!). After a quick crash course in programming, you'll make, test, and hack classic cipher programs. You'll begin with simple programs like the Caesar cipher and then work your way up to public key cryptography and the RSA cipher, which is used for modern secure data transmissions. Each program comes with the full code and a line-by-line explanation of how things work. By book's end, you'll walk away with a solid foundation in Python and same crafty programs under your belt. Learn how to: -Combine loops, variables, and flow control statements into real working programs -Use dictionary files to instantly detect whether text is English or nonsense -Create programs to test that the code you've written is working correctly -Write your own programming modules that you can import and use in other programs -Debug your programs and find common errors Cracking Codes with Python is a chance to pick up some Python skills while getting a peek into the intriguing world of cryptography--what more could an aspiring hacker want? Title Page Copyright Page Dedication About the Author About the Technical Reviewers Brief Contents Contents in Detail Acknowledgments Introduction Who Should Read This Book? What’s in This Book? How to Use This Book Downloading and Installing Python Downloading pyperclip.py Starting IDLE Summary Chapter 1: Making Paper Cryptography Tools What Is Cryptography? Codes vs. Ciphers The Caesar Cipher Why Double Encryption Doesn’t Work Summary Practice Questions Chapter 2: Programming in the Interactive Shell Some Simple Math Expressions Storing Values with Variables Summary Practice Questions Chapter 3: Strings and Writing Programs Working with Text Using String Values Printing Values with the print() Function Printing Escape Characters Quotes and Double Quotes Writing Programs in IDLE’s File Editor Source Code for the “Hello, World!” Program Checking Your Source Code with the Online Diff Tool Using IDLE to Access Your Program Later How the “Hello, World!” Program Works Summary Practice Questions Chapter 4: The Reverse Cipher Source Code for the Reverse Cipher Program Sample Run of the Reverse Cipher Program Setting Up Comments and Variables Finding the Length of a String Introducing the while Loop Improving the Program with an input() Prompt Summary Practice Questions Chapter 5: The Caesar Cipher Source Code for the Caesar Cipher Program Sample Run of the Caesar Cipher Program Importing Modules and Setting Up Variables Constants and Variables The for Loop Statement The if Statement The in and not in Operators The find() String Method Encrypting and Decrypting Symbols Displaying and Copying the Translated String Encrypting Other Symbols Summary Practice Questions Chapter 6: Hacking the Caesar Cipher with Brute-Force Source Code for the Caesar Cipher Hacker Program Sample Run of the Caesar Cipher Hacker Program Setting Up Variables Looping with the range() Function Decrypting the Message Using String Formatting to Display the Key and Decrypted Messages Summary Practice Question Chapter 7: Encrypting with the Transposition Cipher How the Transposition Cipher Works Source Code for the Transposition Cipher Encryption Program Sample Run of the Transposition Cipher Encryption Program Creating Your Own Functions with def Statements Passing the Key and Message As Arguments The List Data Type The Transposition Encryption Algorithm Augmented Assignment Operators Moving currentIndex Through the Message The join() String Method Return Values and return Statements The __name__ Variable Summary Practice Questions Chapter 8: Decrypting with the Transposition Cipher How to Decrypt with the Transposition Cipher on Paper Source Code for the Transposition Cipher Decryption Program Sample Run of the Transposition Cipher Decryption Program Importing Modules and Setting Up the main() Function Decrypting the Message with the Key Calling the main() Function Summary Practice Questions Chapter 9: Programming a Program to Test Your Program Source Code for the Transposition Cipher Tester Program Sample Run of the Transposition Cipher Tester Program Importing the Modules Creating Pseudorandom Numbers Creating a Random String Testing Each Message Checking Whether the Cipher Worked and Ending the Program Calling the main() Function Testing the Test Program Summary Practice Questions Chapter 10: Encrypting and Decrypting Files Plain Text Files Source Code for the Transposition File Cipher Program Sample Run of the Transposition File Cipher Program Working with Files Setting Up the main() Function Checking Whether a File Exists Using String Methods to Make User Input More Flexible Reading the Input File Measuring the Time It Took to Encrypt or Decrypt Writing the Output File Calling the main() Function Summary Practice Questions Chapter 11: Detecting English Programmatically How Can a Computer Understand English? Source Code for the Detect English Module Sample Run of the Detect English Module Instructions and Setting Up Constants The Dictionary Data Type Implementing the Dictionary File Counting the Number of English Words in message Removing Non-Letter Characters Detecting English Words Summary Practice Questions Chapter 12: Hacking the Transposition Cipher Source Code of the Transposition Cipher Hacker Program Sample Run of the Transposition Cipher Hacker Program Importing the Modules Multiline Strings with Triple Quotes Displaying the Results of Hacking the Message Getting the Hacked Message Calling the main() Function Summary Practice Questions Chapter 13: A Modular Arithmetic Module for the Affine Cipher Modular Arithmetic The Modulo Operator Finding Factors to Calculate the Greatest Common Divisor Multiple Assignment Euclid’s Algorithm for Finding the GCD Understanding How the Multiplicative and Affine Ciphers Work Source Code for the Cryptomath Module Summary Practice Questions Chapter 14: Programming the Affine Cipher Source Code for the Affine Cipher Program Sample Run of the Affine Cipher Program Setting Up Modules, Constants, and the main() Function Calculating and Validating the Keys Writing the Encryption Function Writing the Decryption Function Generating Random Keys Calling the main() Function Summary Practice Questions Chapter 15: Hacking the Affine Cipher Source Code for the Affine Cipher Hacker Program Sample Run of the Affine Cipher Hacker Program Setting Up Modules, Constants, and the main() Function The Affine Cipher Hacking Function Calling the main() Function Summary Practice Questions Chapter 16: Programming the Simple Substitution Cipher How the Simple Substitution Cipher Works Source Code for the Simple Substitution Cipher Program Sample Run of the Simple Substitution Cipher Program Setting Up Modules, Constants, and the main() Function The sort() List Method Wrapper Functions The translateMessage() Function Generating a Random Key Calling the main() Function Summary Practice Questions Chapter 17: Hacking the Simple Substitution Cipher Using Word Patterns to Decrypt Overview of the Hacking Process The Word Pattern Modules Source Code for the Simple Substitution Hacking Program Sample Run of the Simple Substitution Hacking Program Setting Up Modules and Constants Finding Characters with Regular Expressions Setting Up the main() Function Displaying Hacking Results to the User Creating a Cipherletter Mapping The hackSimpleSub() Function Calling the main() Function Summary Practice Questions Chapter 18: Programming the Vigenère Cipher Using Multiple Letter Keys in the Vigenère Cipher Source Code for the Vigenère Cipher Program Sample Run of the Vigenère Cipher Program Setting Up Modules, Constants, and the main() Function Building Strings with the List-Append-Join Process Encrypting and Decrypting the Message Calling the main() Function Summary Practice Questions Chapter 19: Frequency Analysis Analyzing the Frequency of Letters in Text Matching Letter Frequencies Source Code for Matching Letter Frequencies Storing the Letters in ETAOIN Order Counting the Letters in a Message Getting the First Member of a Tuple Ordering the Letters in the Message by Frequency Calculating the Frequency Match Score of the Message Summary Practice Questions Chapter 20: Hacking the Vigenère Cipher Using a Dictionary Attack to Brute-Force the Vigenère Cipher Source Code for the Vigenère Dictionary Hacker Program Sample Run of the Vigenère Dictionary Hacker Program About the Vigenère Dictionary Hacker Program Using Kasiski Examination to Find the Key’s Length Source Code for the Vigenère Hacking Program Sample Run of the Vigenère Hacking Program Importing Modules and Setting Up the main() Function Finding Repeated Sequences Calculating the Factors of the Spacings Finding the Most Likely Key Lengths Getting Letters Encrypted with the Same Subkey Attempting Decryption with a Likely Key Length Returning the Hacked Message Calling the main() Function Modifying the Constants of the Hacking Program Summary Practice Questions Chapter 21: The One-Time Pad Cipher The Unbreakable One-Time Pad Cipher Summary Practice Questions Chapter 22: Finding and Generating Prime Numbers What Is a Prime Number? Source Code for the Prime Numbers Module Sample Run of the Prime Numbers Module How the Trial Division Algorithm Works Implementing the Trial Division Algorithm Test The Sieve of Eratosthenes Generating Prime Numbers with the Sieve of Eratosthenes The Rabin-Miller Primality Algorithm Finding Large Prime Numbers Generating Large Prime Numbers Summary Practice Questions Chapter 23: Generating Keys for the Public Key Cipher Public Key Cryptography The Problem with Authentication Steps for Generating Public and Private Keys Source Code for the Public Key Generation Program Sample Run of the Public Key Generation Program Creating the main() Function Generating Keys with the generateKey() Function Creating Key Files with the makeKeyFiles() Function Calling the main() Function Hybrid Cryptosystems Summary Practice Questions Chapter 24: Programming the Public Key Cipher How the Public Key Cipher Works Source Code for the Public Key Cipher Program Sample Run of the Public Key Cipher Program Setting Up the Program How the Program Determines Whether to Encrypt or Decrypt Converting Strings to Blocks with getBlocksFromText() Using getTextFromBlocks() to Decrypt Writing the encryptMessage() Function Writing the decryptMessage() Function Reading in the Public and Private Keys from Their Key Files Writing the Encryption to a File Decrypting from a File Calling the main() Function Summary Appendix Debugging Python Code How the Debugger Works Debugging the Reverse Cipher Program Setting Breakpoints Summary Index Resources