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 character
char encryptChar(char ch, int key) {
    if (isalpha(ch)) {
        char base = isupper(ch) ? 'A' : 'a';
        return (char)(((ch - base + key) % 26) + base);
    }
    return ch; // Return non-alphabetic characters as is
}

// Function to decrypt a character
char decryptChar(char ch, int key) {
    if (isalpha(ch)) {
        char base = isupper(ch) ? 'A' : 'a';
        // Add 26 to handle negative results from (ch - base - key)
        return (char)(((ch - base - key + 26) % 26) + base);
    }
    return ch; // Return non-alphabetic characters as is
}

int main() {
    char message[100];
    int key;

    // Step 1: Get input message from the user
    printf("Enter a message: ");
    fgets(message, sizeof(message), stdin);
    // Remove trailing newline character if present
    message[strcspn(message, "\n")] = 0;

    // Step 2: Get the encryption/decryption key
    printf("Enter the key (1-25): ");
    scanf("%d", &key);

    // Step 3: Normalize key to be within 0-25 range
    key = key % 26;
    if (key < 0) { // Handle negative keys to make them positive shifts
        key += 26;
    }

    // Step 4: Encrypt the message
    char encryptedMessage[100];
    strcpy(encryptedMessage, message); // Copy original message to encrypt
    for (int i = 0; encryptedMessage[i] != '\0'; i++) {
        encryptedMessage[i] = encryptChar(encryptedMessage[i], key);
    }
    printf("Encrypted message: %s\n", encryptedMessage);

    // Step 5: Decrypt the message
    char decryptedMessage[100];
    strcpy(decryptedMessage, encryptedMessage); // Copy encrypted message to decrypt
    for (int i = 0; decryptedMessage[i] != '\0'; i++) {
        decryptedMessage[i] = decryptChar(decryptedMessage[i], key);
    }
    printf("Decrypted message: %s\n", decryptedMessage);

    return 0;
}