Date: Thu, 03 Oct 2019 23:42:25 +0000
To: Ethan Heilman <eth3rs@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
Reply-To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: ZmnSCPxj via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>,
	"lightning-dev@lists.linuxfoundation.org"
	<lightning-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] [Lightning-dev] OP_CAT was Re: Continuing the
	discussion about noinput / anyprevout
Precedence: list

Good morning Ethan,


> To avoid derailing the NO_INPUT conversation, I have changed the
> subject to OP_CAT.
>
> Responding to:
> """
>
> -   `SIGHASH` flags attached to signatures are a misdesign, sadly
>     retained from the original BitCoin 0.1.0 Alpha for Windows design, on
>     par with:
>     [..]
>
> -   `OP_CAT` and `OP_MULT` and `OP_ADD` and friends
>     [..]
>     """
>
>     OP_CAT is an extremely valuable op code. I understand why it was
>     removed as the situation at the time with scripts was dire. However
>     most of the protocols I've wanted to build on Bitcoin run into the
>     limitation that stack values can not be concatenated. For instance
>     TumbleBit would have far smaller transaction sizes if OP_CAT was
>     supported in Bitcoin. If it happens to me as a researcher it is
>     probably holding other people back as well. If I could wave a magic
>     wand and turn on one of the disabled op codes it would be OP_CAT. Of
>     course with the change that size of each concatenated value must be 6=
4
>     Bytes or less.

Why 64 bytes in particular?

It seems obvious to me that this 64 bytes is most suited for building Merkl=
e trees, being the size of two SHA256 hashes.

However we have had issues with the use of Merkle trees in Bitcoin blocks.
Specifically, it is difficult to determine if a hash on a Merkle node is th=
e hash of a Merkle subnode, or a leaf transaction.
My understanding is that this is the reason for now requiring transactions =
to be at least 80 bytes.

The obvious fix would be to prepend the type of the hashed object, i.e. add=
 at least one byte to determine this type.
Taproot for example uses tagged hash functions, with a different tag for le=
aves, and tagged hashes are just prepend-this-32-byte-constant-twice-before=
-you-SHA256.

This seems to indicate that to check merkle tree proofs, an `OP_CAT` with o=
nly 64 bytes max output size would not be sufficient.

Or we could implement tagged SHA256 as a new opcode...

Regards,
ZmnSCPxj


>
>     On Tue, Oct 1, 2019 at 10:04 PM ZmnSCPxj via bitcoin-dev
>     bitcoin-dev@lists.linuxfoundation.org wrote:
>
>
> > Good morning lists,
> > Let me propose the below radical idea:
> >
> > -   `SIGHASH` flags attached to signatures are a misdesign, sadly retai=
ned from the original BitCoin 0.1.0 Alpha for Windows design, on par with:
> >     -   1 RETURN
> >     -   higher-`nSequence` replacement
> >     -   DER-encoded pubkeys
> >     -   unrestricted `scriptPubKey`
> >     -   Payee-security-paid-by-payer (i.e. lack of P2SH)
> >     -   `OP_CAT` and `OP_MULT` and `OP_ADD` and friends
> >     -   transaction malleability
> >     -   probably many more
> >
> > So let me propose the more radical excision, starting with SegWit v1:
> >
> > -   Remove `SIGHASH` from signatures.
> > -   Put `SIGHASH` on public keys.
> >
> > Public keys are now encoded as either 33-bytes (implicit `SIGHASH_ALL`)=
 or 34-bytes (`SIGHASH` byte, followed by pubkey type, followed by pubkey c=
oordinate).
> > `OP_CHECKSIG` and friends then look at the public key to determine sigh=
ash algorithm rather than the signature.
> > As we expect public keys to be indirectly committed to on every output =
`scriptPubKey`, this is automatically output tagging to allow particular `S=
IGHASH`.
> > However, we can then utilize the many many ways to hide public keys awa=
y until they are needed, exemplified in MAST-inside-Taproot.
> > I propose also the addition of the opcode:
> >
> >     <sighash> <pubkey> OP_SETPUBKEYSIGHASH
> >
> >
> > -   `sighash` must be one byte.
> > -   `pubkey` may be the special byte `0x1`, meaning "just use the Tapro=
ot internal pubkey".
> > -   `pubkey` may be 33-byte public key, in which case the `sighash` byt=
e is just prepended to it.
> > -   `pubkey` may be 34-byte public key with sighash, in which case the =
first byte is replaced with `sighash` byte.
> > -   If `sighash` is `0x00` then the result is a 33-byte public key (the=
 sighash byte is removed) i.e. `SIGHASH_ALL` implicit.
> >
> > This retains the old feature where the sighash is selected at time-of-s=
pending rather than time-of-payment.
> > This is done by using the script:
> >
> >     <pubkey> OP_SETPUBKEYSIGHASH OP_CHECKSIG
> >
> >
> > Then the sighash can be put in the witness stack after the signature, l=
etting the `SIGHASH` flag be selected at time-of-signing, but only if the S=
CRIPT specifically is formed to do so.
> > This is malleability-safe as the signature still commits to the `SIGHAS=
H` it was created for.
> > However, by default, public keys will not have an attached `SIGHASH` by=
te, implying `SIGHASH_ALL` (and disallowing-by-default non-`SIGHASH_ALL`).
> > This removes the problems with `SIGHASH_NONE` `SIGHASH_SINGLE`, as they=
 are allowed only if the output specifically says they are allowed.
> > Would this not be a superior solution?
> > Regards,
> > ZmnSCPxj
> >
> > bitcoin-dev mailing list
> > bitcoin-dev@lists.linuxfoundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
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