Twofish is a block cipher developed by Bruce Schneier and Counterpane Labs, published in 1998 and remains unbroken to this day and in the foreseeable future.

Most cryptographers believe twofish to be much more secure than  the rijndael/AES algorithm - which is the commonly used algorithm today and recommended by the NSA (Make of that what you will...).

Twofish's distinctive features are the use of pre-computed key-dependent S-boxes, and a relatively complex key schedule. One half of an n-bit key is used as the actual encryption key and the other half of the n-bit key is used to modify the encryption algorithm (key-dependent S-boxes). Twofish borrows some elements from other designs; for example, the pseudo-Hadamard transform from the SAFER family of ciphers. Twofish has a Feistel structure like DES. Twofish also employs a Maximum Distance Separable matrix.