XSalsa20

XSalsa20 is a stream cipher that builds upon the Salsa20 cipher. Its primary innovation lies in its ability to handle nonces of arbitrary length, typically 192 bits, while still maintaining the security properties of Salsa20. This is achieved by using a key-derivation function, XoR, to generate a new 256-bit key from the original 256-bit key and the longer nonce. This derived key is then used with the standard Salsa20 algorithm.

The design of XSalsa20 addresses a common challenge in stream cipher usage: the requirement for unique nonces for each message encrypted with the same key. While Salsa20 uses a 64-bit nonce, which can be restrictive in some applications, XSalsa20's larger nonce space significantly reduces the probability of nonce reuse. This makes it more suitable for scenarios where generating truly unique 64-bit nonces might be difficult or error-prone, such as in distributed systems or over long periods.

The XoR function within XSalsa20 consists of two rounds of the Salsa20 core function, using the original key and a fixed IV, to expand the combination of the original key and the variable nonce into the 256-bit key used for the subsequent Salsa20 encryption. This process ensures that even with a potentially reused or predictable part of the nonce, the effective key for Salsa20 remains unique as long as the original secret key is not compromised. XSalsa20 is known for its speed and security, making it a viable option for high-performance cryptographic applications.