Re: [bitcoin-dev] Should Graftroot be optional?

[Tim Ruffing <crypto@timruffing.de>]

I haven't read the original Graftroot thread, so maybe all of this has
b
een discussed already or is just wrong... Please correct me if this
is
the case.

On Thu, 2018-05-31 at 17:25 -0700, Pieter Wuille via bitcoin-dev wrote:
> The best argument for why Graftroot does not need to be optional I
> think was how Greg put it: "since the signer(s) could have signed an
> arbitrary transaction instead, being able to delegate is strictly
> less
> powerful.".

I'm trying to get a more abstract view of the problem. One issue with
Greg's argument is the following:

If g-script is a script (containing a public key) that allows for
Graftroot spending, then the following "flow" of coins is valid: 

  g-script --g-sig--> script1 ---tx2---> script2

Here, g-sig is a Graftroot signature on script1 and tx2 is a
transaction that fulfills script1 and sends to script2. In other words,
the only transaction involved here is tx2; it provides g-sig, script1,
and a solution for tx1, and it spends to script2.

Now Greg's argument (as I understand it) is that this can be already
done without Grafroot with two transactions, namely a normal
transaction tx1 that spends g-script normally and tx2 that spends tx1
to script1.

  g-script ---tx1---> script1 ---tx2---> script2.

So far, so good. A difference however is that g-sig *alone* can't be
committed to the chain but tx1 alone can be committed to the chain.
That means

  g-script --g-sig--> script1             (*)

is "incomplete" but

  g-script ---tx1---> script1             (**)

is a perfectly valid transaction that can be committed to the chain. So
I think Graftroot delegation is not "strictly less powerful" than just
using a normal transaction: Graftroot enables to delegate in a way such
that the delegation itself cannot be fixed in the chain. I think this
is not possible currently. (Okay, you can just pass around the secret
keys but has other problems obviously).

Does this have practical implications?
I don't see any but maybe this helps someone to identify an undesirable
implication.

One way to be on the safe side and probably make Greg's argument go
through is to just define the semantics such that (*) is allowed, i.e.,
call g-sig a "Graftroot transaction" and give it transaction semantics.
This provides a new perspective on Graftroot: Then Graftroot does not
introduce new semantics but (*) is just an optimized version of (**)
that uses fewer bytes and may be better for privacy. 

Interestingly Andrew's blind-sig example and Johnson's fix (g-sig signs
the outpoint) are just a special case. If g-sig has transaction
semantics, it must sign the outpoint (and other stuff).

Now you can say that this is not really useful: if g-sig is essentially
a full transaction that can committed to the blockchain, then it needs
to specify inputs, outputs etc. So all the optimizations are lost and
those were the reason we want to introduce Grafroot in the first place.

But one observation here is that g-sig only needs to be a full
transaction if it's used standalone as in (*). If we want to have 

  g-script --g-sig--> script1 ---tx2---> script2

(and this should be the common case) then just the bare signature  and
script1 suffices, as in the Graftroot proposal. In some sense, inputs
and outputs of the Graftroot transaction are just implicit in this
case.

Another way to look at this that instead of providing a transaction
with g-sig, script1, and a solution for script1, you can also choose to
provide a transaction with only g-sig and script1 (and don't solve
script1), which then just sends to script1.

I'm not saying that it's worth the hassle to add this possibility
without being aware of a problem that arises if we don't add it -- but
maybe my thoughts provide another perspective on the issue.

Best,
Tim