Caesar Cipher Program in C

Program for Caeser Cipher in C

In technical terms, the Caeser cipher, also termed a shift cipher, is an encryption technique based on the monoalphabetic cipher. The table for encryption and decryption by Caeser cipher is explained in the table below.

The formula of encryption is:
En (x) =(x + n) mod 26


The formula of decryption is:
Dn(x) = (x - n) mod 26

Where, 
En depicts encryption
Dn depicts decryption
x denotes the value of the letter
n denotes the shift or key-value


If any case (Dn) value becomes negative (-ve), in this case, we will add 26 in the negative value.

Time for an example:

Suppose you have to encrypt and decrypt the word NINJA.

The above table depicts the value x for each letter.

Caesar Cipher Encryption

You will traverse word by word, and for each letter, you will find its corresponding cipher letter. The letter N will become Q  and similarly, all letters will transform into their respective letters and the NINJA will become QLQMD. Use formula En (x) =(x + n) mod 26, here n(key)=3. 

Now let's see how to decrypt.

Caesar Cipher Decryption

Similarly, QLQMD can be decrypted to NINJA again using the formula Dn(x) = (xi - n) mod 26.

Implementation for Caesar Cipher

You can easily generate Caesar cipher for the given text; you can also choose a key and encrypt and decrypt your message accordingly. Now let's see the algorithm and C++ program for encryption and decryption of messages.

Algorithm

• Traverse the given text one character at a time.
• For every character in the text, transform it as per the rule( given key). Consider whether the text is encrypted or decrypted.
• Return the newly transformed string.
   

Code for Encryption

• C

C

#include <stdio.h>
#include <string.h>

int main() {
    // Declaring variables
    int i, key;
    char s[1000], c;

    // Taking Input
    printf("Enter a plaintext to encrypt:\n");
    fgets(s, sizeof(s), stdin);
    printf("Enter key:\n");
    scanf("%d", &key);

    int n = strlen(s);

    // Encrypting each character according to the given key
    for (i = 0; i < n; i++) {
        c = s[i];
        if (c >= 'a' && c <= 'z') {
            c = c + key;
            if (c > 'z') {
                c = c - 'z' + 'a' - 1;
            }
            s[i] = c;
        } else if (c >= 'A' && c <= 'Z') {
            c = c + key;
            if (c > 'Z') {
                c = c - 'Z' + 'A' - 1;
            }
            s[i] = c;
        }
    }

    // Output the cipher
    printf("Encrypted message: %s\n", s);

    return 0;
}

You can also try this code with Online C Compiler

Run Code

OUTPUT:

Code for Decryption

• C

C

#include <stdio.h>
#include <string.h>

int main() {
    // Declaring variables
    int i, key;
    char s[1000], c;

    // Taking ciphertext and key input
    printf("Enter encrypted text:\n");
    fgets(s, sizeof(s), stdin);
    printf("Enter key:\n");
    scanf("%d", &key);

    int n = strlen(s);

    // Decrypting each character according to the given key
    for (i = 0; i < n; i++) {
        c = s[i];
        if (c >= 'a' && c <= 'z') {
            c = c - key;
            if (c < 'a') {
                c = c + 'z' - 'a' + 1;
            }
            s[i] = c;
        } else if (c >= 'A' && c <= 'Z') {
            c = c - key;
            if (c < 'A') {
                c = c + 'Z' - 'A' + 1;
            }
            s[i] = c;
        }
    }

    // Output the original message
    printf("Decrypted message: %s\n", s);

    return 0;
}

You can also try this code with Online C Compiler

Run Code

OUTPUT:

Advantages Of Caeser Cipher

Caesar cipher has the following advantages.

1. It is simple and very easy to implement.
2. Only one short key is used in the entire process(the same key is used for encryption and decryption).
3. If the system does not use complex coding techniques, it is the method and easy too.
4. Only a few computing resources are required.

Disadvantages of Caesar Cipher 

1. The entire message can be decrypted by looking at the pattern of letters or trying brute force.
2. It is less secure.
3. It can easily be hacked since messages encrypted by this method can easily be decrypted.

Frequently Asked Questions

How do you program a Caesar cipher?

The first step is to traverse the given text one character at a time. Then for every character in the text, transform it as per the rule( given key). Consider whether the text is encrypted or decrypted. Finally, Return the newly transformed string.

Why is the Caesar cipher not a strong encryption method?

The Caesar cipher is not a strong encryption method due to its simplicity and vulnerability to brute-force attacks. It has only 26 possible keys, making it easily breakable by trying all possible shifts. It does not provide sufficient security for modern encryption requirements.

What is the logic of the Caesar cipher in C?

The logic of the Caesar cipher in C involves shifting each character in the plaintext message by a fixed number of positions (key) to obtain the corresponding character in the ciphertext.

What is code 3 in Caesar cipher?

In Caesar cipher, "code 3" refers to a shift of three positions in the alphabet. For example, A becomes D, B becomes E, and so on, shifting letters by three steps.

Conclusion

In this article, we discussed what Caeser cipher is. We discussed how a message is encrypted and decrypted using Caesar cipher. We also saw its implementation using code in C++. 

We hope that this blog has helped you enhance your knowledge regarding Caesar Cipher and if you would like to learn more, check out our articles, What is Cryptography, RSA algorithm, and Difference between Public Key and Private Key. Do upvote our blog to help other ninjas grow.