Investigate use of binary protocol instead of JSON for data transfer

A disadvantage of JSON is that in order to verify the metadata, it has to be parsed first. It would be better to use a binary protocol that lets us treat the signed field as raw bytes until after we verify it. Then, do a second round of parsing on the trusted data. For example:

message SignedRootMetadata {
  repeated Signature signature = 1;
  bytes signed = 2;   
}

message Signature {
    bytes keyid = 1;
    bytes sig = 2;
}

Followed by

let signed_root = parse_signed_root(bytes_from_network)?;
let root = if verify(signed_root) {
    parse_root(signed_root.signed)?
} else {
    return Err("oh dear")
}
// do stuff with root

Protobufs were used in the example here, but they may not be deterministic, so something like ASN.1 / DER would need to be used.

This is motivated by my dislike of parsers being allowed to work on any more untrusted data than necessary.

No files, tests, or entry points are named. Start by locating the current JSON metadata parsing and signature-verification flow, then assess a deterministic binary format such as ASN.1/DER against the stated two-stage parsing goal. Done would require a decided approach and an agreed implementation scope.