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To: vjudeu@gazeta.pl,
Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Pay to signature hash as a covenant
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------- Original Message -------
On Tuesday, May 3rd, 2022 at 02:37, vjudeu via bitcoin-dev <bitcoin-dev@lis=
ts.linuxfoundation.org> wrote:
> Typical P2PK looks like that: "<signature> <pubkey> OP_CHECKSIG". In a ty=
pical scenario, we have "<signature>" in out input and "<pubkey> OP_CHECKSI=
G" in our output. I wonder if it is possible to use covenants right here an=
d right now, with no consensus changes, just by requiring a specific signat=
ure. To start with, I am trying to play with P2PK and legacy signatures, bu=
t it may turn out, that doing such things with Schnorr signatures will be m=
ore flexible and will allow more use cases.
>
>
> The simplest "pay to signature" script I can think of is: "<signature> OP=
_SWAP OP_CHECKSIG". Then, any user can provide just a "<pubkey>" in some in=
put, as a part of a public key recovery. The problem with such scheme is th=
at it is insecure. Another problem is that we should handle it carefully, b=
ecause signatures are removed from outputs. However, we could replace it wi=
th some signature hash, then it will be untouched, for example: "OP_TOALTST=
ACK OP_DUP OP_HASH160 <signatureHash> OP_EQUALVERIFY OP_FROMALTSTACK OP_CHE=
CKSIG".
>
Doesn't this suffer from the standard "circular reference" problem for cove=
nants? To pay to a utxo U1, whose scriptpubkey is a (p2wsh wrapped, say) sc=
ript of: sig, op_checksig, I must create that sig, using my chosen public k=
ey, and a message which is a (signature style) hashing of a tx TX1, where a=
n/the input to TX1 is U1, and the txid of U1 'hashes over' that script, whi=
ch includes the sig we're trying to create. You can't make a hash of data w=
hich includes that hash (unless the hash fn is broken ofc).
I don't think that's affected by the later discussion here or in Zmn's resp=
onse right?
Also a side detail which you might find useful in these ponderings: pubkey =
recovery is, as you know, possible in ECDSA but is not possible in BIP340 s=
chnorr (which has key prefixing, i.e. the pubkey is included in the message=
hash binding, i.e. the e in s =3D k + ex), but is in the original Schnorr =
where only R(=3DkG), m are included in e. Easy to see why: from R, s and e(=
=3DH(R,m) .. easy to calculate) you can get P =3D (sG - R)* e^-1.
But in BIP340 Schnorr where (R, s) is published and e (=3DH(P, R, m)) is no=
t, you cannot reconstruct e and so can only calculate e*P, which by DL assu=
mption does not reveal P. Another way to put it is: if you made up a random=
R, s you wouldn't be able to find the right 'P' to put into e=3DH(P, R, m)=
so that the same P came out, and so that the sig actually verifies.
Cheers,
waxwing/AdamISZ
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