Metaformat

Every Commonplace leaf has a leaf header, which is a JSON file documenting

1. The Commonplace format version
2. The public key of the user who signs the document
3. The signature of the left content, unless the data is in-band, in which case the signature is omitted
4. The type of the data contained in the leaf

The name of the leaf is the signature of the header (or of header+content when they are in-band). The signer is identified by the public key in the header.

Alternatives: the name could be a public key/name pair, and the server returns proof that the payload is correct (e.g. a signed binding of name and key to the hash of the data). This means that the name doesn't itself reference the header, just the content. This has major advantages, such as allowing the hash algorithm to be updated later.