> On 20 Mar 2019, at 4:07 PM, ZmnSCPxj via bitcoin-dev > <bitcoin-...@lists.linuxfoundation.org> wrote: > > Hi aj, > > Re-reading again, I think perhaps I was massively confused by this: > >> - alternatively, we could require every script to have a valid signature >> that commits to the input. In that case, you could do eltoo with a >> script like either: >> >> <A> CHECKSIGVERIFY <B> CHECKSIG >> or <P> CHECKSIGVERIFY <Q> CHECKSIG >> >> >> where A is Alice's key and B is Bob's key, P is muSig(A,B) and Q is >> a key they both know the private key for. In the first case, Alice >> would give Bob a NOINPUT sig for the tx, and when Bob wanted to publish >> Bob would just do a SIGHASH_ALL sig with his own key. In the second, >> Alice and Bob would share partial NOINPUT sigs of the tx with P, and >> finish that when they wanted to publish. > > Do you mean that *either* of the above two scripts is OK, *or* do you mean > they are alternatives within a single MAST or `OP_IF`? >
It means either. If you use <A> CHECKSIGVERIFY <B> CHECKSIG style, A and B will exchange the NOINPUT sig, and they will add the required non-NOINPUT sig when needed. If you use <muSig(A,B)> CHECKVERIFY <Q> CHECKSIG, A and B will co-sign the muSig(A,B) with NOINPUT. They will also share the private key of Q, so they could produce a non-NOINPUT sig when needed. The first style is slightly easier as it doesn’t need muSig. But with 3 or more parties, the second style is more efficient. However, if you use watchtower, you have to use the second style. That means you need to share the private key for Q with the watchtower, That also means the watchtower will have the ability to reply the NOINPU muSig. But it is still strictly better than anyone-can-replay.
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