What is Blowfish encryption, and how does it work?

In the encryption process of the Blowfish algorithm, the first step involves breaking the plaintext into fixed-size blocks. Each block is 64 bits in length and is processed individually. This block-based approach is a common characteristic of block ciphers, where data is divided into manageable chunks for cryptographic operations.

Feistel Network

Blowfish employs a Feistel network structure, a design commonly used in block ciphers for its security and simplicity. In this structure, the 64-bit block is divided into two equal halves: the left half and the right half. The Feistel network consists of 16 rounds, during which the left and right halves undergo a series of modifications.

Round Function

The heart of the Feistel network is the round function, which is executed 16 times. This function incorporates the P-array subkeys and S-boxes. The right half of the data is XORed with the current round's P-array subkey, introducing key-dependent operations. The output then traverses a series of S-box substitutions, adding a non-linear component to the transformation. Finally, the result is XORed with the left half of the data, ensuring a mixing of the two halves.

Swap

After the completion of each round within the Feistel network, the left and right halves are swapped. This swapping process ensures that the modifications applied to one half in a given round are applied to the opposite half in the next round. This interchange prepares the data for the subsequent round of processing.

Final XOR

Upon the completion of all 16 rounds, a final swapping of the left and right halves occurs. Following this, the 64-bit block is subjected to a final XOR operation with the P-array subkeys. This last step introduces a final layer of key-dependent mixing, creating the encrypted output. The result is the ciphertext, providing confidentiality to the original plaintext based on the applied key schedule and the Feistel network structure.

Decryption

Decryption follows a similar process, but with the subkeys used in reverse order.

Key Advantages:

1. Speed: Very fast in software implementations.
2. Variable Key Length: Supports a wide range of key sizes.
3. No Patent Restrictions: Freely available for use.
4. No Successful Attacks: No effective cryptanalysis techniques have been found to date.

Disadvantages and Considerations

1. Small Block Size: The 64-bit block size is now considered small for modern security standards.
2. Vulnerable to Birthday Attacks: Susceptible to birthday attacks due to the small block size.
3. Not Recommended for New Applications: Not generally recommended for new applications due to the block size limitation.
4. Successor Algorithm: Twofish, designed by the same author, is a more secure alternative with a larger block size.

Current Use

While not the first choice for new applications, Blowfish continues to have relevance in certain legacy systems and software. Its usage extends to file encryption tools, password managers, and disk encryption software, where it provides a reliable encryption method. Despite being largely superseded by more modern algorithms, the adaptability and efficiency of Blowfish make it a viable option for specific use cases in preserving data confidentiality.

Conclusion

Blowfish is a symmetric-key block cipher designed by Bruce Schneier known for its simplicity, speed, and variable key length support ranging from 32 to 448 bits. Although not commonly used in new applications, it persists in legacy systems, file encryption tools, password managers, and disk encryption software.