The purpose of a block cipher is to act like a random permutation, and indeed a common security definition is one in which the block cipher is taken to be indistinguishable from a random permutation (see Wikipedia). So then why not use a random permutation as a block cipher? That is, the secret...

The documentation for Ed25519-dalek states the following for Secretkey: The caller is responsible for ensuring that the bytes represent a masked secret key. https://docs.rs/ed25519-dalek/0.3.2/ed25519_dalek/struct.SecretKey.html What does a masked secret key mean in this context?