Which fields of an attachment structure in a document are part of the API, and which are subject to change?

The wiki only talks about “content_type”, “length”, “data” and “stub”.
But in real life, I also see “digest” and “revpos”.

The “digest” field in particular is very useful — I’m working on digital signatures for documents, and since attachments should be considered part of the document, they need to be signed along with it. If the _attachments dictionary contains a digest, then signing it effectively includes the attachment in the signature. But I don’t want to rely on this if the field might change format or go away in a future version of CouchDB.

—Jens