[cm/prv/] with Ed25519.
32-byte Ed25519 private key is used, as described in EdDSA RFC.
In many libraries it is called "seed".
=> EdDSA

    prv-ed25519-blake2b {
        {field . {map} len=~}
        {field a {str} =ed25519-blake2b}
        {field v {bin} len=32}
    }