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Date: Mon, 23 Oct 2023 22:26:26 +1000
From: Anthony Towns <aj@erisian.com.au>
To: Rusty Russell <rusty@rustcorp.com.au>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Proposed BIP for OP_CAT
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On Mon, Oct 23, 2023 at 12:43:10PM +1030, Rusty Russell via bitcoin-dev wrote:
> 2. Was there a concrete rationale for maintaining 520 bytes?

Without a limit of 520 bytes, then you can construct a script:

    <p> CHECKSIGVERIFY
    {DUP CAT}x10
      (we know have a string that is the second witness repeated 1024 times
       on the stack; if it was 9 bytes, call it 9216B total)

    {DUP} x 990
      (we now have 1000 strings each of length 9216B bytes, for ~9.2MB total)

    SHA256SUM {CAT SHA256SUM}x999
      (we now have a single 32B field on the stack)
    <h> EQUAL
      (and can do a hardcoded check to make sure there weren't any
       shortcuts taken)

That raises the max memory to verify a single script from ~520kB (1000
stack elements by 520 bytes each) to ~10MB (1000 stack elements by
10kB each).

> 10k is the current script limit, can we get closer to that? :)

The 10k limit applies to scriptPubKey, scriptSig and segwit v0 scripts.
There's plenty of examples of larger tapscripts, eg:

    https://mempool.space/tx/0301e0480b374b32851a9462db29dc19fe830a7f7d7a88b81612b9d42099c0ae

    (3,938,182 bytes of script, non-standard due to being an oversized tx)

   https://mempool.space/tx/2d4ad78073f1187c689c693bde62094abe6992193795f838e8be0db898800434

    (360,543 bytes of script, standard, I believe)

Cheers,
aj