Example: Caesar Cipher Encryption and Decryption in C

// Caesar Cipher Encryption and Decryption
#include <stdio.h>
#include <string.h>
#include <ctype.h> // For isalpha, isupper, islower

// Function to encrypt a message using Caesar Cipher
void encryptCaesar(char text[], int key) {
    int i;
    for (i = 0; text[i] != '\0'; ++i) {
        char ch = text[i];

        // Encrypt uppercase letters
        if (isupper(ch)) {
            ch = (char)(((ch - 'A' + key) % 26) + 'A');
        }
        // Encrypt lowercase letters
        else if (islower(ch)) {
            ch = (char)(((ch - 'a' + key) % 26) + 'a');
        }
        // Non-alphabetic characters remain unchanged
        text[i] = ch;
    }
}

// Function to decrypt a message using Caesar Cipher
void decryptCaesar(char text[], int key) {
    int i;
    for (i = 0; text[i] != '\0'; ++i) {
        char ch = text[i];

        // Decrypt uppercase letters
        if (isupper(ch)) {
            ch = (char)(((ch - 'A' - key + 26) % 26) + 'A'); // Add 26 to handle negative results of (ch - 'A' - key)
        }
        // Decrypt lowercase letters
        else if (islower(ch)) {
            ch = (char)(((ch - 'a' - key + 26) % 26) + 'a'); // Add 26 for similar reasons
        }
        text[i] = ch;
    }
}

int main() {
    // Step 1: Declare variables for message and key
    char message[100];
    int key;

    printf("Enter a message to encrypt: ");
    fgets(message, sizeof(message), stdin);
    message[strcspn(message, "\n")] = 0; // Remove trailing newline

    printf("Enter key (an integer): ");
    scanf("%d", &key);
    key = key % 26; // Ensure key is within 0-25 range for alphabet wrap-around

    // Step 2: Encrypt the message
    char encryptedMessage[100];
    strcpy(encryptedMessage, message); // Copy original message to encrypt
    encryptCaesar(encryptedMessage, key);
    printf("Encrypted message: %s\n", encryptedMessage);

    // Step 3: Decrypt the message
    char decryptedMessage[100];
    strcpy(decryptedMessage, encryptedMessage); // Copy encrypted message to decrypt
    decryptCaesar(decryptedMessage, key);
    printf("Decrypted message: %s\n", decryptedMessage);

    return 0;
}