Date: Tue, 16 Mar 2021 08:36:09 +0000
To: Jeremy <jlrubin@mit.edu>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Delegated signatures in Bitcoin within existing
	rules, no fork required
Precedence: list

Good morning Jeremy,

Thank you.

Assuming only keys, an easier way of delegating would be simply to give a c=
opy of the privkey outright to the delegatee.

However, an advantage of this technique you described is that the delegator=
 can impose additional restrictions that are programmable via any SCRIPT, a=
n ability that merely handing over the privkey cannot do.
Thus the technique has an ability that mere handover cannot achieve.

If the delegatee is a known single entity, and S is simply the delegatee ke=
y plus some additional restrictions, it may be possible to sign with `SIGHA=
SH_ALL` a transaction that spends A and D, and outputs to a singlesig of th=
e delegatee key.
This would avoid the use of `SIGHASH_NONE`, for a mild improvement in priva=
cy.
The output would still allow the delegatee to dispose of the funds by its u=
nilateral decision subject to the fulfillment of the script S (at the cost =
of yet another transaction).
On the other hand, if S is unusual enough, the enhanced privacy may be moot=
 (the S already marks the transaction as unusual), so this variation has li=
ttle value.

In terms of offchain technology, if the delegator remains online, the deleg=
atee may present a witness satisfying S to the delegator, and ask the deleg=
ator to provide an alternate transaction that spends A directly without spe=
nding D and outputs to whatever the delegatee wants.
The delegator cannot refuse since the delegatee can always use the `SIGHASH=
_NONE` signature and spend to whatever it decides provided it can present a=
 witness satisfying S.
This is basically a typical "close transaction" for layer 2 technology.
On the other hand, one generalized use-case for delegation would be if the =
delegator suspects it may not be online or able to sign with the delegator =
key, so this variation has reduced value as well.

Regards,
ZmnSCPxj