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About Abhinav Nath Gupta

Abhinav holds a Master degree in Computer Science and Engineering from the National Institute of Technology Karnataka. He has finished his graduation from Information Technology Department in the Anand Engineering College, Agra. During his studies he has been involved with a large number of projects ranging from Networking and Cryptography. He works as a software development engineer at a software development firm in bengaluru where he is mainly involved with projects based on Nodejs. He is interested in cryptography, data security, cryptocurrency and cloud computing, and published articles regarding these topics. He can be reached at [email protected].
  
Ceasar Cipher Java Example
Posted by: Abhinav Nath Gupta  in Core Java January 20th, 2020 0 692 Views


In this post, we feature a comprehensive Caesar Cipher Java Example.

1. What is Caeser Cipher?
The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the plaintext is shifted a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B would become C, and so on. The method is named after Julius Caesar, who apparently used it to communicate with his generals.

For Example,

Let plainText, text to be encrypted be, quick brown fox jumps over the lazy dog.

When we apply caesar cipher over the plain text, with a shift of 1 character, the ciphertext will be, rvjdl cspxo gpy kvnqt pwfs uif mbaz eph.

Let’s try to understand it character by character,

q with a shift of 1 becomes r.

u with a shift of 1 becomes v.

so on and so forth.

Once we finished encrypting all the alphabets we get to rvjdl cspxo gpy kvnqt pwfs uif mbaz eph . In this particular example, we have ignored the spaces, for encryption, between the words so they stayed the same.

Now let’s try to decrypt the encrypted text as rvjdl cspxo gpy kvnqt pwfs uif mbaz eph . We already know the shift used for encryption is 1. So the letters are shifted by 1 character in plain text to generate the cipherText. So In order to decrypt we need to shift each alphabet in cipherText by 1 in reverse order.

If we go character by character in the cipherText,

r will be shifted by 1 in reverse and become q.

v will be shifted by 1 in reverse and become u.

so on and so forth.

After reverse shifting all the characters in the cipherText, we will generate the original plain text, which will be, quick brown fox jumps over the lazy dog.

Ceasar Cipher Java - Single Shift
Caesar Cipher with a Single Shift
2. How to decrypt the cipher without the key?
There are multiple ways to decrypt a cipher without the key.

One approach would be to use all the possible instances of the keys and applying them on the cipher to generate the plainText. This is a sure shot, albeit, time-consuming process, to generate the plainText from the cipher.

Another technique is more mathematical in nature, widely used by cryptographers, and is called Cryptoanalysis. Cryptanalysis is the study of ciphertext, ciphers, and cryptosystems with the aim of understanding how they work and finding and improving techniques for defeating or weakening them.

3. Implementing Caeser Cipher Java Example
In this section, we will discuss the implementation of Caesar Cipher in java.

The code is shown below in CaesarCipher.java.

CaesarCipher.java
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class CaesarCipher {
    // Encrypts text using shift
    public static StringBuffer encrypt(String text, int shift) {
        StringBuffer result = new StringBuffer();
 
        for (int i = 0; i < text.length(); i++) {
            if (Character.isUpperCase(text.charAt(i))) {
                char ch = (char) (((int) text.charAt(i) +
                        shift - 65) % 26 + 65);
                result.append(ch);
            } else {
                char ch = (char) (((int) text.charAt(i) +
                        shift - 97) % 26 + 97);
                result.append(ch);
            }
        }
        return result;
    }
 
    // Decrypts cipher using shift
    public static StringBuffer decrypt(String cipher, int shift) {
        StringBuffer result = new StringBuffer();
 
        for (int i = 0; i < cipher.length(); i++) {
            if (Character.isUpperCase(cipher.charAt(i))) {
                char ch = (char) (((int) cipher.charAt(i) +
                        shift - 65) % 26 + 65);
                result.append(ch);
            } else {
                char ch = (char) (((int) cipher.charAt(i) +
                        shift - 97) % 26 + 97);
                result.append(ch);
            }
        }
        return result;
    }
 
    public static void main(String[] args) {
        String originalText = "ATTACKATONCE";
        int shiftCount = 1;
        System.out.println("Caesar Cipher Example");
        System.out.println("Encryption");
        System.out.println("Text  : " + originalText);
        System.out.println("Shift : " + shiftCount);
        String cipher = encrypt(originalText, shiftCount).toString();
        System.out.println("Encrypted Cipher: " + cipher);
        System.out.println("Decryption");
        System.out.println("Encrypted Cipher: " + cipher);
        System.out.println("Shift : " + shiftCount);
        String decryptedPlainText = decrypt(cipher, 26 - shiftCount).toString();
        System.out.println("Decrypted Plain Text  : " + decryptedPlainText);
    }
}