Salsa20 is a stream cipher submitted to eSTREAM by Daniel J. Bernstein. It is built on a pseudorandom function based on add-rotate-xor (ARX) operations — 32-bit addition, bitwise addition (XOR) and rotation operations. Salsa20 maps a 256-bit key, a 64-bit nonce, and a 64-bit stream position to a 512-bit block of the key stream (a version with a 128-bit key also exists). This gives Salsa20 the unusual advantage that the user can efficiently seek to any position in the key stream in constant time. It offers speeds of around 4–14 cycles per byte in software on modern x86 processors, and reasonable hardware performance. It is not patented, and Bernstein has written several public domain implementations optimized for common architectures. A related cipher, ChaCha, which has similar features but a different round function, was published by Bernstein in 2008.