Cipher Program

Overview

This program is a simple cipher tool capable of performing block and stream cipher encryption and decryption on input files using a specified key.

Compilation

To compile the program, run the following command in the directory containing the program files. This will generate a binary named cipher in the current directory.

make

Usage

The cipher program requires five command-line arguments in the following order:

1. Cipher Function Type: 'B' for Block cipher or 'S' for Stream cipher.
2. Input File Name: Name of the file containing plaintext (for encryption) or ciphertext (for decryption).
3. Output File Name: Name of the file where the encrypted or decrypted output will be stored.
4. Key File Name: Name of the file containing the symmetric key. Key size should be 128 bits (16 bytes) for Block Encryption. For Stream Encryption, the key can be of any length.
5. Mode of Operation: 'E' for encryption or 'D' for decryption.

Command Structure

./cipher [B/S] [Input File] [Output File] [Key File] [E/D]

Example Commands

• Encrypt plaintext.txt using a block cipher with key.txt:

  ./cipher B plaintext.txt encrypted.txt key.txt E
  

• Decrypt encrypted.txt using a stream cipher with key.txt:

  ./cipher S encrypted.txt decrypted.txt key.txt D
  

Program Behavior

• Argument Checking: Validates the correct number of arguments and the cipher function type and operation mode.
• File Handling: Reads input and key files, checks their existence.
• Block Cipher:
  • Encrypts/decrypts in 128-bit blocks.
  • Uses padding (0x81) for blocks smaller than 128 bits.
  • Applies XOR operation and a byte-swap algorithm.
• Stream Cipher:
  • Performs an XOR operation of the input data with the key, looping over the key if necessary.

Error Handling

The program includes basic error handling for invalid arguments and file read/write errors. Error messages are displayed in case of any issues.