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Blowfish Algorithm with Examples

Last Updated on August 30, 2023 by Mayank Dham

Information security has become critical in the age of digitization and data-driven landscapes. The Blowfish algorithm, a symmetric key block cipher, is a cryptographic institution. Blowfish, created in 1993 by Bruce Schneier, is a strong encryption method that ensures data confidentiality through a well-structured process. In this article, we will look inside the Blowfish algorithm with examples, going over each step in detail and providing a Java code example to show how it is done.

What is Blowfish Algorithm?

The Blowfish algorithm is a symmetric-key block cipher designed for secure data encryption and decryption. It was developed by Bruce Schneier in 1993 as an alternative to existing encryption algorithms, providing a strong level of security and efficiency. Blowfish operates on fixed-size blocks of data and uses a variable-length key to perform encryption and decryption operations.

Features of the Blowfish Algorithm

Here are some of the features of the Blowfish Algorithm.

1. Symmetric-Key Algorithm: Blowfish uses the same key for both encryption and decryption processes, making it a symmetric-key algorithm. This means that the party encrypting the data and the party decrypting it must possess the same secret key.

2. Block Cipher: Blowfish operates on fixed-size blocks of data. The standard block size is 64 bits, but it can work with smaller blocks as well. If the input data is not a multiple of the block size, padding is typically applied to the data before encryption.

3. Variable-Length Key: One of the unique features of Blowfish is its ability to accept variable-length encryption keys, making it adaptable to different security requirements. The key length can range from 32 to 448 bits, and it’s expanded during encryption to generate a series of subkeys.

4. Feistel Network Structure: Blowfish employs a Feistel network structure in which data is divided into two halves, subjected to a series of rounds of operations, and then recombined. This structure allows for efficient encryption and decryption processes.

5. F-Function: The F-function is a core component of the Blowfish algorithm. It involves a combination of XOR (exclusive OR), substitution, and permutation operations, which contribute to the algorithm’s strength and security.

6. Key Expansion: Before the actual encryption process, Blowfish generates a series of subkeys based on the provided key. These subkeys are used during the encryption and decryption rounds to introduce complexity and security.

7. Complexity and Security: Blowfish is designed to be highly secure against various cryptographic attacks. The complex F-function and key expansion process make it resistant to brute force and differential cryptanalysis.

How does The Blowfish Algorithm work?

The Blowfish algorithm is renowned for its robust encryption and relatively simple structure. To truly understand its inner workings, let’s dive into the encryption process step by step, shedding light on each intricate operation that contributes to its security.

1. Key Generation and Subkey Creation

The algorithm begins with a secret encryption key, which is used to generate a series of subkeys. Blowfish’s subkey generation involves a complex process that enhances security. Here’s how it works

Initialization of the P array and S boxes:
Blowfish uses a combination of pi (hexadecimal digits of π) and a series of S boxes (substitution boxes) to initialize its internal data structures.

Key Expansion:
The secret key is expanded using a key expansion routine. During this process, the key is used to modify the P array and S boxes. The subkeys derived from the key ensure that the encryption process remains secure and resistant to known attacks.

2. Data Encryption

Once the subkeys are generated, the algorithm proceeds with the encryption of the data block. The data block is divided into two 32 bit halves, L (left) and R (right). A series of rounds (typically 16) are performed on these halves to ensure strong encryption.

Feistel Network Rounds:
The algorithm employs a Feistel network structure, which involves applying a series of operations to the L and R halves in each round. These operations include XOR (exclusive OR) with the current subkey, applying the F function to R, and swapping L and R.

F function Operation
The F function takes the 32 bit R half and applies several steps
Subkey XOR The current subkey is XORed with R.
Substitution R is divided into four 8 bit quarters. Each quarter is used to index a specific S box, and the resulting values are combined.
Permutation The results from the S boxes are combined and transformed using the P array.

This step introduces confusion and diffusion, crucial components of cryptographic security.
Final Round After all rounds are executed, the resulting L and R halves are swapped one last time.

3. Data Decryption
The decryption process is essentially the reverse of encryption. The encrypted data block is divided into L and R halves, and the algorithm performs rounds in reverse order using the same subkeys

Feistel Network Rounds (Decryption)

Similar to encryption, rounds involve applying operations to L and R, but this time in reverse order using the corresponding subkey.

  1. F function Operation (Decryption) The F function is applied in reverse, with the subkey XOR and S box steps inverted. This reverse operation successfully decrypts the data block.
  2. Final Round (Decryption) After all decryption rounds, the decrypted L and R halves are combined to obtain the original data block.

For an even clearer understanding, let’s revisit the Java code example provided earlier. The code demonstrates how to use the Java Cryptography Architecture to implement the Blowfish algorithm for encryption and decryption. This example showcases the actual process described above in a concise and practical manner.

Code Implementation of Blowfish Algorithm in Java

A simple Java code snippet demonstrating the Blowfish algorithm for encryption and decryption is provided below.

import javax.crypto.*;

public class BlowfishExample {
    public static void main(String[] args) throws Exception {
        String keyString = "mySecretKey";
        String data = "Hello, Blowfish!";

        SecretKeySpec secretKey = new SecretKeySpec(keyString.getBytes(), "Blowfish");
        Cipher cipher = Cipher.getInstance("Blowfish");

        cipher.init(Cipher.ENCRYPT_MODE, secretKey);
        byte[] encryptedData = cipher.doFinal(data.getBytes());
        System.out.println("Encrypted " + new String(encryptedData));

        cipher.init(Cipher.DECRYPT_MODE, secretKey);
        byte[] decryptedData = cipher.doFinal(encryptedData);
        System.out.println("Decrypted " + new String(decryptedData));


Encrypted: ����Y�����^�ݞ
Decrypted: Hello, Blowfish!

Here is an detailed explanation of the above Java code.

Key Initialization:
The variable keyString holds the secret encryption key as a string, which is "mySecretKey". This key is used to create a SecretKeySpec object named secretKey using the Blowfish algorithm.

Cipher Initialization:
The Cipher object named cipher is initialized to work in encryption mode (Cipher.ENCRYPT_MODE) using the secretKey for encryption and decryption.

The cipher object is now set up for encryption. The doFinal method is called on the byte representation of the input string "Hello, Blowfish!". The resulting encrypted data is stored in the encryptedData byte array.

Output Encrypted Data:
The encrypted data is printed to the console using System.out.println("Encrypted: " + new String(encryptedData));.

The cipher object is re-initialized, this time for decryption mode (Cipher.DECRYPT_MODE). The same secretKey is used.

Decryption Process:
The doFinal method is called on the encryptedData byte array. This decrypts the encrypted data using the same key, and the decrypted data is stored in the decryptedData byte array.

Output Decrypted Data:
The decrypted data, obtained by converting the decryptedData byte array to a string, is printed to the console using System.out.println("Decrypted: " + new String(decryptedData));

The Blowfish algorithm’s security lies not only in its use of the Feistel network structure and the F function but also in its intricate subkey generation process. By meticulously expanding the original key into a series of subkeys and performing numerous rounds of operations, Blowfish ensures that the encrypted data remains secure and resistant to various attacks. Understanding the detailed encryption process allows developers and security professionals to appreciate the depth of thought and expertise behind this renowned cryptographic method.

Frequently Asked Questions (FAQs)

Here are some of the frequently asked questions about the blowfish algorithm with examples.

Q1. What is the Blowfish algorithm?
The Blowfish algorithm is a symmetric-key block cipher designed for data encryption and decryption. It was developed by Bruce Schneier in 1993 and is known for its variable-length key, efficient Feistel network structure, and complex F-function. Blowfish operates on fixed-size blocks of data and is widely used for securing sensitive information.

Q2. How does the Blowfish algorithm work?
Blowfish operates by dividing input data into fixed-size blocks, typically 64 bits. It employs a series of rounds that involve complex operations such as the F-function, XOR, substitution, and permutation. The algorithm uses a variable-length key to generate subkeys, which are used in the rounds to ensure security and confidentiality.

Q3. What is the advantage of the variable-length key in Blowfish?
The variable-length key in Blowfish allows users to choose key lengths ranging from 32 to 448 bits, providing adaptability to different security requirements. This flexibility enables stronger encryption and resistance to brute force attacks.

Q4. What is the Feistel network structure in Blowfish?
The Feistel network is a fundamental structure in many block ciphers, including Blowfish. It involves dividing the data block into two halves, applying a series of rounds of operations to these halves, and then recombining them. This structure enables efficient encryption and decryption processes while enhancing security.

Q5. Is Blowfish still considered secure for modern applications?
While Blowfish has historically been considered secure and has withstood various cryptographic attacks, modern encryption standards like AES (Advanced Encryption Standard) have gained more prominence due to their higher levels of security and performance. Blowfish’s shorter block size and potential vulnerabilities to certain attacks have led to its decreased usage in favor of more robust algorithms.

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