Simple File Encryptor (C)

A minimal command-line utility written in C that performs file encryption and decryption using a Caesar cipher (shift of 3).

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Overview

This project demonstrates a basic implementation of file-based encryption.

• Encrypts text using a Caesar cipher (+3 shift)
• Decrypts text using a Caesar cipher (-3 shift)
• Preserves uppercase and lowercase letters
• Leaves non-alphabet characters unchanged

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Project Structure

.
├── encrypt.c
├── encrypt.exe
├── data.txt
├── secret.txt
├── output.txt

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Compilation

gcc encrypt.c -o encrypt

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▶️ Usage

./encrypt <mode> <inputfile> <outputfile>

Arguments

Argument	Description
mode	E for encryption, D for decryption
inputfile	Path to the input file
outputfile	Path to the output file

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Examples

Encrypt

./encrypt E data.txt secret.txt

Decrypt

./encrypt D secret.txt output.txt

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Example

Input (data.txt)

Hello! My name is Johan.

Encrypted (secret.txt)

Khoor! Pb qdph lv Mrkdq.

Decrypted (output.txt)

Hello! My name is Johan.

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How It Works

The program reads the file character-by-character and applies transformations:

• Uppercase letters:

  • Encryption: (ch - 'A' + 3) % 26 + 'A'
  • Decryption: (ch - 'A' - 3 + 26) % 26 + 'A'

• Lowercase letters:

  • Encryption: (ch - 'a' + 3) % 26 + 'a'
  • Decryption: (ch - 'a' - 3 + 26) % 26 + 'a'

• Other characters are unchanged

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Disclaimer

This project uses a Caesar cipher, which is not secure and is intended for educational purposes only.

Do not use this for protecting sensitive information.

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Improvements

• Add custom shift value
• Support binary files
• Add password-based encryption
• Implement stronger algorithms (AES, XOR)
• Improve error handling
• Build a GUI

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Author

Johan

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