Cracking Codes with Python An Introduction to Building and Breaking Ciphers

Learn how to program in Python while making and breaking ciphers—algorithms used to create and send secret messages! 

After a crash course in Python programming basics, you’ll learn to make, test, and hack programs that encrypt text with classical ciphers like the transposition cipher and Vigenère cipher. You’ll begin with simple programs for the reverse and Caesar ciphers and then work your way up to public key cryptography, the type of encryption used to secure today’s online transactions, including digital signatures, email, and Bitcoin.

Each program includes the full code and a line-by-line explanation of how things work. By the end of the book, you’ll have learned how to code in Python and you’ll have the clever programs to prove it!

You’ll also learn how to:

- Combine loops, variables, and flow control statements into real working programs
- Use dictionary files to instantly detect whether decrypted messages are valid English or gibberish
- Create test programs to make sure that your code encrypts and decrypts correctly
- Code (and hack!) a working example of the affine cipher, which uses modular arithmetic to encrypt a message
- Break ciphers with techniques such as brute-force and frequency analysis

There’s no better way to learn to code than to play with real programs. Cracking Codes with Python makes the learning fun!

Al Sweigart is a professional software developer who teaches programming to kids and adults. He is the author of Automate the Boring Stuff with Python, Invent Your Own Computer Games with Python, and Scratch Programming Playground, also from No Starch Press. His programming tutorials can be found at inventwithpython.com.

Introduction

Chapter 1: Making Paper Cryptography Tools
Chapter 2: Installing Python
Chapter 3: The Interactive Shell
Chapter 4: Strings and Writing Programs
Chapter 5: The Reverse Cipher
Chapter 6: The Caesar Cipher
Chapter 7: Hacking the Caesar Cipher with the Brute-Force Technique
Chapter 8: Encrypting with the Transposition Cipher
Chapter 9: Decrypting with the Transposition Cipher
Chapter 10: Programming a Program to Test Our Program
Chapter 11: Encrypting and Decrypting Files
Chapter 12: Detecting English Programmatically
Chapter 13: Hacking the Transposition Cipher
Chapter 14: Making a Modular Arithmetic Module for the Multiplicative and Affine Ciphers
Chapter 15: Programming the Affine Cipher
Chapter 16: Hacking the Affine Cipher
Chapter 17: The Simple Substitution Cipher
Chapter 18: Hacking the Simple Substitution Cipher
Chapter 19: The Vigenere Cipher
Chapter 20: Frequency Analysis
Chapter 21: Hacking the Viginere Cipher
Chapter 22: The One-Time Pad Cipher
Chapter 23: Finding Prime Numbers
Chapter 24: Generating Keys for the RSA Cipher
Chapter 25: Public Key Cryptography and Programming the RSA Cipher

Appendix: Debugger