CAST5

CAST5, or CAST-5, is a symmetric-key block cipher designed by Carlisle Adams and Stafford Tavares and published in the mid-1990s. It is part of the CAST family of ciphers and is best known as a 64-bit block cipher with variable-length keys.

The algorithm operates on 64-bit data blocks and supports key lengths from 40 to 128 bits, in eight-bit steps. It uses a Feistel network with 16 rounds. In each round, a non-linear round function F combines subkeys derived from the user key with the data through a set of large S-boxes. The key scheduling process expands the input key into a collection of subkeys and into the inputs for the S-boxes, configuring F for every round.

CAST5 is designed to be reasonably fast in software and to offer strong diffusion and non-linearity through its S-box-based round function. It relies on four large S-boxes and a carefully arranged sequence of operations to produce the round results. The design emphasizes a balance between security and implementation efficiency, with emphasis on a compact, easy-to-audit structure.

Security and status: no practical cryptanalytic break is known for the full 16-round CAST5 when used with appropriate keys. There are known attacks on reduced-round variants, but these do not affect standard deployments. Today, CAST5 is considered secure for its era but is generally viewed as obsolete for new designs, having largely been superseded by the AES standard.

Applications and usage: CAST5 has been used in some cryptographic software and protocols, notably as a cipher option in OpenPGP. It also appeared in various legacy implementations of SSH and other systems, but its use has declined as AES has become the default choice for new deployments.