Twofish

Twofish is a symmetric key block cipher and one of the finalists in the Advanced Encryption Standard (AES) contest, intended to become a replacement for the older and less secure Data Encryption Standard (DES). Twofish is known for its speed and versatility.

Twofish operates on 128-bit blocks of data, providing a good balance between efficiency and security. One of the notable features of Twofish is its flexibility in key sizes. It can support key sizes of 128 bits, 192 bits, or 256 bits, making it adaptable to various security requirements.

Twofish uses a Feistel network, a common method in block cipher design. This structure involves dividing the data block into two halves and then running several rounds of encryption operations on them.

Twofish applies a series of 16 rounds of encryption, regardless of the key size. Each round consists of key mixing, a substitution step using a structure known as the S-box, and a permutation. Twofish's key schedule involves precomputing a large number of subkeys before the actual data encryption or decryption. This precomputation contributes to the cipher's efficiency.

Twofish uses complex, key-dependent S-boxes (substitution boxes), which are tables used to transform data in a non-linear way. These S-boxes contribute to Twofish's resistance to cryptographic attacks.

Twofish is known for its speed, especially in software implementations. It performs well on a wide range of hardware, from low-end devices to high-performance servers. As of now, Twofish is considered secure, with no significant vulnerabilities identified. It has withstood considerable scrutiny from the cryptographic community.

While Twofish was a strong contender in the AES contest, it was not selected as the final standard. The AES title went to the Rijndael algorithm, which was deemed to be more efficient in a wider range of hardware environments.

Like its predecessor Blowfish, Twofish is unencumbered by patents and freely available for anyone to use without licensing or royalty fees.