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

Ethan Heilman eth3rs at gmail.com
Thu Oct 3 15:05:52 UTC 2019

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.

On Tue, Oct 1, 2019 at 10:04 PM ZmnSCPxj via bitcoin-dev
<bitcoin-dev at 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:
> 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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