Maybe I'm misunderstanding the article but I'm fairly sure the magic number is not transmitted.

It's used exactly as you say: a shared context used as input for the signature that is not transmitted.

▲ amluto 9 hours ago | parent | next [-]

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”)

and the verifier calls the API:

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

(I assume they meant Object not Objector.)

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?

▲ lokar 11 hours ago | parent | prev [-]

No, I'm pretty sure they are saying you need to transmit it

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nightpool 10 hours ago | parent | next [-]

No, they propose just concatenating it with the data received from the network

> it makes a concatenation of the domain separator (@0x92880d38b74de9fb) and the serialization of the object, and then feeds the byte stream into the signing primitive. Similarly, verification of an object verifies this same reconstructed concatenation against the supplied signature.

> Note that the domain separator does not appear in the eventual serialization (which would waste bytes), since both signer and receiver agree on it via this shared protocol specification. Encrypt, HMAC, and hash work the same way

	▲	tennysont 10 hours ago \| parent \| next [-]
		You are, of course, right. And this distinction is important for this chain of comments. Though, in fairness, that is /kind of/ like transmitting it---in the sense that it impacts the message that is returned. It's more akin to sending a checksum of the magic number, rather than the magic number itself. But conceptually, that is just an optimization. The desire is for the client to ensure the server is using the same magic number, we just so happen to be able to overload the signature to encode this data without increasing the message size.
	▲	lokar 10 hours ago \| parent \| prev [-]
		Oh, it's just in the hash input. So if you don't use the right ID when you check the hash, it fails.

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jcalvinowens 10 hours ago | parent | prev | next [-]

I think not:

> Note that the domain separator does not appear in the eventual serialization (which would waste bytes), since both signer and receiver agree on it via this shared protocol specification.

But saying it's about wasting bytes is a little confusing, as you observe that isn't really the point.

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jeffrallen 10 hours ago | parent | prev [-]

It is definitely not transmitted.

Domain separation happens in the input to the hash function, not on the wire. Because what arrives off the wire is UNTRUSTED input.