Key schedule
A key schedule is the algorithm that derives round keys or subkeys from a cipher's supplied key.
Definition
A key schedule transforms the supplied key into the subkeys used by individual rounds or stages of a cipher. Depending on the design, it may rotate, substitute, mix, or add constants to key material.
Purpose and security
Round keys should differ in ways that resist related-key and structural attacks. A schedule is part of the cipher specification, not a password-strengthening function, and its subkeys are not the position-by-position output called a keystream.
No. A schedule supplies subkeys to cipher rounds; a keystream is combined sequentially with message units.
No. It deterministically rearranges and derives material from the original key; it does not add independent secret information.