Re: [bitcoin-dev] [Lightning-dev] OP_CAT was Re: Continuing the discussion about noinput / anyprevout

[ZmnSCPxj <ZmnSCPxj@protonmail.com>]

Good morning Jeremy,

> Awhile back, Ethan and I discussed having, rather than OP_CAT, an OP_SHA256STREAM that uses the streaming properties of a SHA256 hash function to allow concatenation of an unlimited amount of data, provided the only use is to hash it.
>
> You can then use it perhaps as follows:
>
> // start a new hash with item
> OP_SHA256STREAM  (-1) -> [state]
> // Add item to the hash in state
> OP_SHA256STREAM n [item] [state] -> [state]
> // Finalize
> OP_SHA256STREAM (-2) [state] -> [Hash]
>
> <-1> OP_SHA256STREAM <tag> <subnode 2> <subnode 3> <3> OP_SHA256STREAM <-2> OP_SHA256STREAM
>
> Or it coul
>

This seems a good idea.

Though it brings up the age-old tension between:

* Generically-useable components, but due to generalization are less efficient.
* Specific-use components, which are efficient, but which may end up not being useable in the future.

In particular, `OP_SHA256STREAM` would no longer be useable if SHA256 eventually is broken, while the `OP_CAT` will still be useable in the indefinite future.
In the future a new hash function can simply be defined and the same technique with `OP_CAT` would still be useable.


Regards,
ZmnSCPxj

> --
> @JeremyRubin
>
> On Thu, Oct 3, 2019 at 8:04 PM Ethan Heilman <eth3rs@gmail.com> wrote:
>
> > I hope you are having an great afternoon ZmnSCPxj,
> >
> > You make an excellent point!
> >
> > I had thought about doing the following to tag nodes
> >
> > || means OP_CAT
> >
> > `node = SHA256(type||SHA256(data))`
> > so a subnode would be
> > `subnode1 = SHA256(1||SHA256(subnode2||subnode3))`
> > and a leaf node would be
> > `leafnode = SHA256(0||SHA256(leafdata))`
> >
> > Yet, I like your idea better. Increasing the size of the two inputs to
> > OP_CAT to be 260 Bytes each where 520 Bytes is the maximum allowable
> > size of object on the stack seems sensible and also doesn't special
> > case the logic of OP_CAT.
> >
> > It would also increase performance. SHA256(tag||subnode2||subnode3)
> > requires 2 compression function calls whereas
> > SHA256(1||SHA256(subnode2||subnode3)) requires 2+1=3 compression
> > function calls (due to padding).
> >
> > >Or we could implement tagged SHA256 as a new opcode...
> >
> > I agree that tagged SHA256 as an op code that would certainty be
> > useful, but OP_CAT provides far more utility and is a simpler change.
> >
> > Thanks,
> > Ethan
> >
> > On Thu, Oct 3, 2019 at 7:42 PM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:
> > >
> > > 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 64
> > > >     Bytes or less.
> > >
> > > Why 64 bytes in particular?
> > >
> > > It seems obvious to me that this 64 bytes is most suited for building Merkle 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 the 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 leaves, 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 only 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 retained 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 coordinate).
> > > > > `OP_CHECKSIG` and friends then look at the public key to determine sighash 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 `SIGHASH`.
> > > > > However, we can then utilize the many many ways to hide public keys away 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 Taproot internal pubkey".
> > > > > -   `pubkey` may be 33-byte public key, in which case the `sighash` byte 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-spending 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, letting the `SIGHASH` flag be selected at time-of-signing, but only if the SCRIPT specifically is formed to do so.
> > > > > This is malleability-safe as the signature still commits to the `SIGHASH` it was created for.
> > > > > However, by default, public keys will not have an attached `SIGHASH` byte, 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
> > > > >
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> > > >
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> > >
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