book

Cracking Codes with Python

Name: Cracking Codes with Python
Author: Al Sweigart
ISBN: 9781593278229

by Al Sweigart

January 2018

Intermediate to advanced

416 pages

10h 32m

English

No Starch Press

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Cover Page
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 BookDownloading and Installing PythonDownloading pyperclip.pyStarting IDLESummary

Chapter 1: Making Paper Cryptography Tools
What Is Cryptography?Codes vs. CiphersThe Caesar CipherWhy Double Encryption Doesn’t WorkSummaryPractice Questions
Chapter 2: Programming in the Interactive Shell
Some Simple Math ExpressionsStoring Values with VariablesSummaryPractice Questions
Chapter 3: Strings and Writing Programs
Working with Text Using String ValuesPrinting Values with the print() FunctionPrinting Escape CharactersQuotes and Double QuotesWriting Programs in IDLE’s File EditorSource Code for the “Hello, World!” ProgramChecking Your Source Code with the Online Diff ToolUsing IDLE to Access Your Program LaterHow the “Hello, World!” Program WorksSummaryPractice Questions
Chapter 4: The Reverse Cipher
Source Code for the Reverse Cipher ProgramSample Run of the Reverse Cipher ProgramSetting Up Comments and VariablesFinding the Length of a StringIntroducing the while LoopImproving the Program with an input() PromptSummaryPractice Questions
Chapter 5: The Caesar Cipher
Source Code for the Caesar Cipher ProgramSample Run of the Caesar Cipher ProgramImporting Modules and Setting Up VariablesConstants and VariablesThe for Loop StatementThe if StatementThe in and not in OperatorsThe find() String MethodEncrypting and Decrypting SymbolsDisplaying and Copying the Translated StringEncrypting Other SymbolsSummaryPractice Questions
Chapter 6: Hacking the Caesar Cipher with Brute-Force
Source Code for the Caesar Cipher Hacker ProgramSample Run of the Caesar Cipher Hacker ProgramSetting Up VariablesLooping with the range() FunctionDecrypting the MessageUsing String Formatting to Display the Key and Decrypted MessagesSummaryPractice Question
Chapter 7: Encrypting with the Transposition Cipher
How the Transposition Cipher WorksSource Code for the Transposition Cipher Encryption ProgramSample Run of the Transposition Cipher Encryption ProgramCreating Your Own Functions with def StatementsPassing the Key and Message As ArgumentsThe List Data TypeThe Transposition Encryption AlgorithmAugmented Assignment OperatorsMoving currentIndex Through the MessageThe join() String MethodReturn Values and return StatementsThe __name__ VariableSummaryPractice Questions
Chapter 8: Decrypting with the Transposition Cipher
How to Decrypt with the Transposition Cipher on PaperSource Code for the Transposition Cipher Decryption ProgramSample Run of the Transposition Cipher Decryption ProgramImporting Modules and Setting Up the main() FunctionDecrypting the Message with the KeyCalling the main() FunctionSummaryPractice Questions
Chapter 9: Programming a Program to Test Your Program
Source Code for the Transposition Cipher Tester ProgramSample Run of the Transposition Cipher Tester ProgramImporting the ModulesCreating Pseudorandom NumbersCreating a Random StringTesting Each MessageChecking Whether the Cipher Worked and Ending the ProgramCalling the main() FunctionTesting the Test ProgramSummaryPractice Questions
Chapter 10: Encrypting and Decrypting Files
Plain Text FilesSource Code for the Transposition File Cipher ProgramSample Run of the Transposition File Cipher ProgramWorking with FilesSetting Up the main() FunctionChecking Whether a File ExistsUsing String Methods to Make User Input More FlexibleReading the Input FileMeasuring the Time It Took to Encrypt or DecryptWriting the Output FileCalling the main() FunctionSummaryPractice Questions
Chapter 11: Detecting English Programmatically
How Can a Computer Understand English?Source Code for the Detect English ModuleSample Run of the Detect English ModuleInstructions and Setting Up ConstantsThe Dictionary Data TypeImplementing the Dictionary FileCounting the Number of English Words in messageRemoving Non-Letter CharactersDetecting English WordsSummaryPractice Questions
Chapter 12: Hacking the Transposition Cipher
Source Code of the Transposition Cipher Hacker ProgramSample Run of the Transposition Cipher Hacker ProgramImporting the ModulesMultiline Strings with Triple QuotesDisplaying the Results of Hacking the MessageGetting the Hacked MessageCalling the main() FunctionSummaryPractice Questions
Chapter 13: A Modular Arithmetic Module for the Affine Cipher
Modular ArithmeticThe Modulo OperatorFinding Factors to Calculate the Greatest Common DivisorMultiple AssignmentEuclid’s Algorithm for Finding the GCDUnderstanding How the Multiplicative and Affine Ciphers WorkSource Code for the Cryptomath ModuleSummaryPractice Questions
Chapter 14: Programming the Affine Cipher
Source Code for the Affine Cipher ProgramSample Run of the Affine Cipher ProgramSetting Up Modules, Constants, and the main() FunctionCalculating and Validating the KeysWriting the Encryption FunctionWriting the Decryption FunctionGenerating Random KeysCalling the main() FunctionSummaryPractice Questions
Chapter 15: Hacking the Affine Cipher
Source Code for the Affine Cipher Hacker ProgramSample Run of the Affine Cipher Hacker ProgramSetting Up Modules, Constants, and the main() FunctionThe Affine Cipher Hacking FunctionCalling the main() FunctionSummaryPractice Questions
Chapter 16: Programming the Simple Substitution Cipher
How the Simple Substitution Cipher WorksSource Code for the Simple Substitution Cipher ProgramSample Run of the Simple Substitution Cipher ProgramSetting Up Modules, Constants, and the main() FunctionThe sort() List MethodWrapper FunctionsThe translateMessage() FunctionGenerating a Random KeyCalling the main() FunctionSummaryPractice Questions
Chapter 17: Hacking the Simple Substitution Cipher
Using Word Patterns to DecryptOverview of the Hacking ProcessThe Word Pattern ModulesSource Code for the Simple Substitution Hacking ProgramSample Run of the Simple Substitution Hacking ProgramSetting Up Modules and ConstantsFinding Characters with Regular ExpressionsSetting Up the main() FunctionDisplaying Hacking Results to the UserCreating a Cipherletter MappingThe hackSimpleSub() FunctionCalling the main() FunctionSummaryPractice Questions
Chapter 18: Programming the Vigenère Cipher
Using Multiple Letter Keys in the Vigenère CipherSource Code for the Vigenère Cipher ProgramSample Run of the Vigenère Cipher ProgramSetting Up Modules, Constants, and the main() FunctionBuilding Strings with the List-Append-Join ProcessEncrypting and Decrypting the MessageCalling the main() FunctionSummaryPractice Questions
Chapter 19: Frequency Analysis
Analyzing the Frequency of Letters in TextMatching Letter FrequenciesSource Code for Matching Letter FrequenciesStoring the Letters in ETAOIN OrderCounting the Letters in a MessageGetting the First Member of a TupleOrdering the Letters in the Message by FrequencyCalculating the Frequency Match Score of the MessageSummaryPractice Questions
Chapter 20: Hacking the Vigenère Cipher
Using a Dictionary Attack to Brute-Force the Vigenère CipherSource Code for the Vigenère Dictionary Hacker ProgramSample Run of the Vigenère Dictionary Hacker ProgramAbout the Vigenère Dictionary Hacker ProgramUsing Kasiski Examination to Find the Key’s LengthSource Code for the Vigenère Hacking ProgramSample Run of the Vigenère Hacking ProgramImporting Modules and Setting Up the main() FunctionFinding Repeated SequencesCalculating the Factors of the SpacingsFinding the Most Likely Key LengthsGetting Letters Encrypted with the Same SubkeyAttempting Decryption with a Likely Key LengthReturning the Hacked MessageCalling the main() FunctionModifying the Constants of the Hacking ProgramSummaryPractice Questions
Chapter 21: The One-Time Pad Cipher
The Unbreakable One-Time Pad CipherSummaryPractice Questions
Chapter 22: Finding and Generating Prime Numbers
What Is a Prime Number?Source Code for the Prime Numbers ModuleSample Run of the Prime Numbers ModuleHow the Trial Division Algorithm WorksImplementing the Trial Division Algorithm TestThe Sieve of EratosthenesGenerating Prime Numbers with the Sieve of EratosthenesThe Rabin-Miller Primality AlgorithmFinding Large Prime NumbersGenerating Large Prime NumbersSummaryPractice Questions
Chapter 23: Generating Keys for the Public Key Cipher
Public Key CryptographyThe Problem with AuthenticationSteps for Generating Public and Private KeysSource Code for the Public Key Generation ProgramSample Run of the Public Key Generation ProgramCreating the main() FunctionGenerating Keys with the generateKey() FunctionCreating Key Files with the makeKeyFiles() FunctionCalling the main() FunctionHybrid CryptosystemsSummaryPractice Questions
Chapter 24: Programming the Public Key Cipher
How the Public Key Cipher WorksSource Code for the Public Key Cipher ProgramSample Run of the Public Key Cipher ProgramSetting Up the ProgramHow the Program Determines Whether to Encrypt or DecryptConverting Strings to Blocks with getBlocksFromText()Using getTextFromBlocks() to DecryptWriting the encryptMessage() FunctionWriting the decryptMessage() FunctionReading in the Public and Private Keys from Their Key FilesWriting the Encryption to a FileDecrypting from a FileCalling the main() FunctionSummary
Appendix Debugging Python Code
How the Debugger WorksDebugging the Reverse Cipher ProgramSetting BreakpointsSummary
Index

Content preview from Cracking Codes with Python

10ENCRYPTING AND DECRYPTING FILES

“Why do security police grab people and torture them? To get their information. And hard disks put up no resistance to torture. You need to give the hard disk a way to resist. That’s cryptography.”—Patrick Ball, Human Rights Data Analysis Group

In previous chapters, our programs have only worked on small messages that we type directly into the source code as string values. The cipher program we’ll make in this chapter will allow you to encrypt and decrypt entire files, which can be millions of characters in size.

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Cracking Codes with Python

by Al Sweigart

10ENCRYPTING AND DECRYPTING FILES

Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.