# Blowfish (cipher)

May 20, 2023

Blowfish is a symmetric-key block cipher algorithm designed in 1993 by Bruce Schneier, an American cryptographer, to provide a fast and secure alternative to existing encryption algorithms. It gained widespread adoption in various software applications due to its speed, simplicity, and robustness. Blowfish is particularly well-suited for hardware and software implementations that require a high level of security without consuming excessive system resources.

## Design and Operation

Blowfish is a symmetric encryption algorithm, meaning that it uses the same key for both encryption and decryption. It operates on 64-bit blocks of data and supports key sizes ranging from 32 to 448 bits, allowing for a high degree of flexibility and security. The algorithm is based on a Feistel network, a common structure used in block ciphers, which divides the input data into two halves and processes them through multiple rounds of substitution and permutation operations.

### Subkeys and S-boxes

Central to the Blowfish algorithm are its key-dependent subkeys and S-boxes (substitution boxes). The subkeys are a set of 18 32-bit values derived from the original encryption key, while the S-boxes consist of four arrays, each with 256 32-bit entries. During the key setup phase, the subkeys and S-boxes are initialized using a combination of the original key, bitwise exclusive or (XOR) operations, and the hexadecimal digits of the mathematical constant pi.

### Rounds and Feistel Function

Blowfish consists of 16 rounds of encryption or decryption, with each round involving the manipulation of the two halves of the input data block. The Feistel function, a critical component of the algorithm, takes one half of the data, divides it into four 8-bit segments, and uses these segments to index the S-boxes. The S-box outputs are combined using addition and XOR operations, resulting in a 32-bit value that is then XORed with the other half of the input data. The two halves are then swapped, and the process is repeated for the remaining rounds.

After the final round, the two halves of the data block are combined to form the encrypted or decrypted output. Due to the symmetric nature of the algorithm, the same process is used for both encryption and decryption, with the only difference being the order in which the subkeys are applied.

## Performance and Security

Blowfish is known for its speed and efficiency, particularly in software implementations. Its relatively simple structure and use of precomputed subkeys and S-boxes allow for rapid encryption and decryption of data. Furthermore, the algorithm’s flexibility in key sizes enables users to balance security and performance according to their specific needs.

The security of Blowfish has been extensively analyzed since its introduction, with no significant weaknesses or vulnerabilities discovered to date. The algorithm’s large key space and the complexity of its key-dependent S-boxes make it resistant to brute-force attacks and various cryptanalytic techniques. However, as with any encryption algorithm, the security of Blowfish depends on the proper implementation, key management, and overall system security.

## Applications and Adoption

Since its introduction, Blowfish has been widely adopted in a variety of software applications, including password management tools, file encryption software, and secure communication protocols. Notable examples of Blowfish’s use include the OpenBSD operating system, the OpenSSL cryptography library, and the Pretty Good Privacy (PGP) encryption program.

Blowfish’s success and popularity have also led to the development of several derivative algorithms, such as Twofish, Threefish, and Skipjack. Twofish, also designed by Bruce Schneier, is a 128-bit block cipher that builds upon Blowfish’s design principles and addresses some of its limitations, such as the fixed block size and the slow key setup process. Twofish was a finalist in the U.S. National Institute of Standards and Technology’s (NIST) Advanced Encryption Standard (AES) competition, which aimed to identify a successor to the widely used Data Encryption Standard (DES).