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To: Ethan Heilman <eth3rs@gmail.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
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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
>
> Lightning-dev mailing list
> Lightning-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
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