Signatures object is not defined explicitly

I'm not quite sure how to organize this, but I think that the Signatures object could use a little more definition.

The Signatures object is referred to several times in the Federation API, but it isn't explicitly defined like other objects types are. There is a reference to the Signing JSON (section 3) in the appendix, but the actual schema isn't described until Section 3.2. It requires some scrolling to get down to the definition from the link given in the Federation API.

A sentence in Section 3.2 of the appendix mentions that the key for the field is "the name of the entity signing it." For device signatures, it is explicitly stated that the entity name is the User ID and the signing key identifier is the concatenation of the algorithm and device ID. (Cf. signatures request parameter in key claim endpoint.

On the other hand, for server signatures it doesn't seem to be explicitly stated that the key for a homeserver signature should be the server name. This can be inferred from the examples, but it could be made more explicit. I think it would be nice to have a type defined for Signatures something like this:

Signatures (generic)

This could be further specified for each signature type:

Server Signatures

Device Signatures

... and similarly for the Invite Signatures and Identity Server Signatures types.

(Now that I write this, it seems that the difficulty with this is being able to specify types for field names...)

Sorry for the rambling; let me know if I didn't get the problem across.

Federation API の Signatures オブジェクトに関するリファレンスから始め、次に Signing JSON の付録、特に Section 3.2 を読み、device と server の署名例を比較します。汎用的なエンティティ名のマッピングを明確にし、server、device、invite、identity-server の各バリアントを文書化します。参照されている API セクションがキー名と値のマップを定義し、読者が例から推測する必要がなければ完了です。