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Subject: [bitcoin-dev] Pay to signature hash as a covenant
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Typical P2PK looks like that: "<signature> <pubkey> OP_CHECKSIG". In a typi=
cal scenario, we have "<signature>" in out input and "<pubkey> OP_CHECKSIG"=
 in our output. I wonder if it is possible to use covenants right here and =
right now, with no consensus changes, just by requiring a specific signatur=
e. To start with, I am trying to play with P2PK and legacy signatures, but =
it may turn out, that doing such things with Schnorr signatures will be mor=
e flexible and will allow more use cases.

The simplest "pay to signature" script I can think of is: "<signature> OP_S=
WAP OP_CHECKSIG". Then, any user can provide just a "<pubkey>" in some inpu=
t, as a part of a public key recovery. The problem with such scheme is that=
 it is insecure. Another problem is that we should handle it carefully, bec=
ause signatures are removed from outputs. However, we could replace it with=
 some signature hash, then it will be untouched, for example: "OP_TOALTSTAC=
K OP_DUP OP_HASH160 <signatureHash> OP_EQUALVERIFY OP_FROMALTSTACK OP_CHECK=
SIG".

And then, signatures are more flexible than public keys, because we can use=
 many different sighashes to decide, what kind of transaction is allowed an=
d what should be rejected. Then, if we could use the right signature with c=
orrect sighashes, it could be possible to disable key recovery and require =
some specific public key, then that scheme could be safely used again. I st=
ill have no idea, how to complete that puzzle, but it seems to be possible =
to use that trick, to restrict destination address. Maybe I should wrap suc=
h things in some kind of multisig or somehow combine it with OP_CHECKSIGADD=
, any ideas?