The Blowfish cipher is a symmetric-key block cipher designed by Bruce Schneier in 1993 as a fast, secure alternative to existing encryption algorithms like DES. It operates on 64-bit blocks of plaintext and uses variable-length keys ranging from 32 to 448 bits. Blowfish employs a Feistel network structure consisting of 16 rounds of substitution and permutation. Each round mixes the plaintext with key-dependent subkeys and uses four S-boxes to introduce nonlinearity, enhancing resistance against linear and differential cryptanalysis.
Encryption begins by splitting the 64-bit plaintext block into two 32-bit halves. The left half is XORed with a subkey and then passed through the F-function, which combines the S-box outputs. The result is XORed with the right half, and the halves are swapped. This process repeats for all 16 rounds, followed by a final swap and XOR with additional subkeys to produce the ciphertext. Decryption follows the same process in reverse, using the same key schedule, making Blowfish symmetric and straightforward to implement.
One of the strengths of Blowfish lies in its key setup, which precomputes 18 32-bit subkeys and four 256-entry S-boxes derived from the key. This makes the encryption process fast, though key setup can be relatively slow. The cipher is highly flexible, allowing users to choose key lengths appropriate for their security requirements. Despite being strong and efficient, it is largely replaced today by more modern algorithms like AES, especially in new applications requiring 128-bit block sizes.
For example, to encrypt the 64-bit plaintext block represented in hexadecimal as 0123456789ABCDEF using a key such as SECRETKEY, Blowfish would process it through its 16 Feistel rounds, producing ciphertext like 6F1ED002AB559585. Decryption simply reverses the process using the same key, restoring the original block. This example illustrates how Blowfish combines substitution, permutation, and key-dependent transformations to provide a secure encryption mechanism.
Overall, the Blowfish cipher represents a significant milestone in modern cryptography. Its combination of variable-length keys, strong resistance to cryptanalytic attacks, and efficient encryption/decryption operations made it a popular choice in the 1990s and early 2000s. Although superseded by algorithms with larger block sizes in high-security applications, Blowfish remains widely used in legacy systems, file encryption tools, and embedded devices where speed and security balance is essential. Understanding Blowfish is crucial for appreciating the evolution of block cipher design and the practical challenges in implementing secure symmetric-key algorithms.