You’re right, but I think the commenter you’re replying to is also right.

The OP is using unreadable hex strings in a way that obscures what’s actually going on. If you turn those strings into functionally equivalent text, then the signatures are computed over:

    (serialized object, “This is a TreeRoot”)

    func Verify(key Key, sig []byte, obj VerifiableObjecter) error

This API is wrong, full stop. Do not use this design. Sure, it might catch one specific screwup, but it will not catch subtler errors like confusing a TreeRoot that the signer trusts with a TreeRoot that means something else entirely. And it requires canonical encodings, which serves no purpose here. And it forces the verifier to deserialize unverified data, which is a big mistake.

The right solution is to have the sender sign a message, where:

(a) At the time of verification, the message is just bytes, and

(b) The message is structured such that it contains all the information needed to interpret it correctly.

So the message might be a serialization of a union where one element is “I trust this TreeRoot” and another is “I revoke this key”, etc. and the verification API verifies bytes.

If you want to get fancy and make domain separation and forward-and-backward-compatibility easier, then build a mini deserializer into the verifier that deserializes tuples of bytes, or at most UUIDs or similar. So you could sign (UUID indicating protocol v1 message type Foo, serialization of a Foo). And you make that explicit to the caller. And the verifier (a) takes bytes as input and (b) does not even try to parse them into a tuple until after verifying the signature.

P.S. Any protocol that uses the OP’s design must be quite tortured. How exactly is there a sensible protocol where you receive a message, read enough of it to figure out what type (in the protobuf sense) it contains such that there is more than one possible choice, then verify the data of that type? Are they expecting that you have a message containing a oneof and you sign only the oneof instead of the entire message? Why?