Constant CRYPTO_KEY_TYPES 

pub const CRYPTO_KEY_TYPES: &'static [&'static str];

The opaque key types bynk.crypto exports. A value of one holds a non-extractable platform key, so it is never stored, persisted, logged or sent (bynk.types.key_at_boundary), as a Stream is not. The checker finds them by the unit’s reserved span file, so a user type of the same name is not mistaken for one.