Implementing a Simple Encryption and Decryption Tool in C

In today’s article, we will implement a simple encryption and decryption tool using the C programming language. This tool is based on a very basic character substitution algorithm, allowing users to input a plaintext, encrypt it using a defined method, and then decrypt it back through another process.

Tool Overview

The functionalities we aim to implement are as follows:

1. The user inputs a piece of text.
2. The text is encrypted using a simple method (for example, a shift value).
3. The user can choose to input the encrypted text to decrypt it back to the original text.

Code Structure

Our program needs to include several basic parts:

• Loading user input
• Encryption function
• Decryption function
• Main function to control the flow

Next, we will build this tool step by step.

Code Demonstration

The following code demonstrates the complete implementation of the tool:

#include <stdio.h>
#include <string.h>
#define MAX_LENGTH 256 // Set maximum length constant
// Encryption function, using a simple character shift method
void encrypt(char *input, char *output, int shift) {
    for (int i = 0; input[i] != '\0'; i++) {
        output[i] = input[i] + shift; // Shift each character
    }
    output[strlen(input)] = '\0'; // Add string terminator
}
// Decryption function, restoring characters using the same method
void decrypt(char *input, char *output, int shift) {
    for (int i = 0; input[i] != '\0'; i++) {
        output[i] = input[i] - shift; // Restore each shifted character
    }
    output[strlen(input)] = '\0'; // Add string terminator
}
int main() {
    char text[MAX_LENGTH];     // To store user input plaintext or ciphertext
    char encrypted[MAX_LENGTH]; // To store the encrypted text
    char decrypted[MAX_LENGTH]; // To store the restored plaintext
    int shift;                 // Define the shift value
    printf("Please enter the text to encrypt: ");
    fgets(text, MAX_LENGTH, stdin);   // Read text from standard input
    printf("Please enter the shift value (positive integer): ");
    scanf("%d", &shift); 
    // Call the encryption function
    encrypt(text, encrypted, shift);
    printf("Encrypted text: %s\n", encrypted);
    // Call the decryption function
    decrypt(encrypted, decrypted, shift);
    printf("Decrypted text: %s\n", decrypted);
    return 0;
}

Program Explanation

1. <span>encrypt</span> function: This function takes three parameters: the string to be encrypted, the output result, and the shift value. It iterates through each character and adjusts the ASCII code according to the shift value, generating a new string as the “ciphertext”.

2. <span>decrypt</span> function: Corresponding to <span>encrypt</span>, here we perform the reverse operation on the input content, obtaining the original text by subtracting the shift value used during character encoding.

3. Main program <span>main</span>:

• First, it retrieves user input, including the information to be encrypted and the specified shift value (e.g., how many positions).
• It calls the two functions mentioned above to process the content and outputs the results.

Usage Instructions

1. Compile and run the program.
2. Follow the prompts to input the text to be encrypted and the shift value, separated by a space.
3. After the program completes, it will display the corresponding outputs, including the encrypted and decrypted texts.

I hope this tutorial helps you understand the basic programming logic in C! If you enjoy such projects, you can continue to delve into topics related to programming security and more.