Blowfish

Blowfish is a symmetric-key block cipher designed by Bruce Schneier and released in 1993 as a fast, free alternative to existing ciphers. It operates on 64-bit data blocks and supports variable key lengths from 32 to 448 bits. Blowfish is structured as a Feistel network with 16 rounds, using a key‑dependent P-array and four S-boxes to mix key material and data.

The algorithm uses a key schedule that initializes an 18-entry P-array of 32-bit subkeys and four 256-entry S-boxes. These structures are derived from the user-supplied key and then transformed through encryption of all-zero data to produce the subkeys used during encryption and decryption. In operation, the left and right halves of the data are processed through 16 rounds of the Feistel network, with the F-function combining S-box lookups and arithmetic, followed by a final swap and key-dependent output.

Security and usage context: No practical attacks against the full 16-round Blowfish have been demonstrated. The 64-bit block size makes it less ideal for very large data streams or modern high‑volume protocols, where 128-bit blocks are preferred. Blowfish is not part of contemporary security standards and has largely been superseded by AES, but it remains available in many software libraries as a legacy option. The algorithm and its reference implementation are freely available and not patented.

Other uses of the term: Blowfish should not be confused with the blowfish, a common name for certain pufferfish species in the biological sense.