Subject: Re: [bitcoin-dev] [Pre-BIP] Motivating Address type for OP_RETURN

From: "David A. Harding" <[email-masked]>
Date: Sat, 24 Apr 2021 11:59:00 -1000

On Sat, Apr 24, 2021 at 01:05:25PM -0700, Jeremy wrote:
> I meant the type itself is too wide, not the length of the value. As in
> Script can represent things we know nothing about. 

I guess I still don't understand your concern, then.  If script can
represent things we know nothing about, then script commitments such as
P2SH, P2WSH, and P2TR also represent things we know nothing about.  All
you know is what container format they used.  For P2PK, bare multisig,
OP_RETURN, and other direct uses of scriptPubKey, that container format
is "bare" (or whatever you want to call it).

> Btw: According to... Oh wait... You?
> https://bitcoin.stackexchange.com/questions/35878/is-there-a-maximum-size-of-a-scriptsig-scriptpubkey
> the max size is 10k bytes.

I'm not sure what I knew at the time I wrote that answer, but the 10,000
byte limit is only applied when EvalScript is run, which only happens
when the output is being spent.  I've appended to this email a
demonstration of creating a 11,000 byte OP_RETURN on regtest (I tried
999,000 bytes but ran into problems with bash's maximum command line
length limit).  I've updated the answer to hopefully make it more
correct.

> Is it possible/easy to, say, using bech32m make an inappropriate message in
> the address? You'd have to write the message, then see what it decodes to
> without checking, and then re encode? I guess this is worse than hex?

If someone wants to abuse bech32m, I suspect they'll do it the same way
people have abused base58check[1], by using the address format's
alphabet directly.  E.g., you compose your message using only
the characters qpzry9x8gf2tvdw0s3jn54khce6mua7l and then append
the appropriate checksum.

[1] https://en.bitcoin.it/wiki/P2SH%C2%B2#The_problem:_storing_data_in_hashes

> But it seems this is a general thing... If you wanted an inappropriate
> message you could therefore just use bech32m addressed outputs.

Yes, and people have done that with base58check.  IsStandard OP_RETURN
attempts to minimize that abuse by being cheaper in two ways:

1. More data allowed in scriptSig, e.g. 80 byte payload (81 actually, I
   think) for OP_RETURN versus 40 bytes for a BIP141 payload.
   Maximizing payload size better amortizes the overhead cost of the
   containing transaction and the output's nValue field.

2. Exemption from the dust limit.  If you use a currently defined
   address type, the nValue needs to pay at least a few thousand nBTC
   (few hundred satoshis), about $0.15 USD minimum at $50k USD/BTC.  For
   OP_RETURN, the nValue can be 0, so there's no additional cost beyond
   normal transaction relay fees.

Although someone creating an OP_RETURN up to ~1 MB with miner support
can bypass the dust limit, the efficiency advantage remains no matter
what.

> One of the nice things is that the current psbt interface uses a blind
> union type whereby the entires in an array are either [address, amount] or
> ["data", hex]. Having an address type would allow more uniform handling,
> which is convenient for strongly typed RPC bindings (e.g. rust bitcoin uses
> a hashmap of address to amount so without a patch you can't create op
> returns).

I don't particularly care how the data in PSBTs are structured.  My mild
opposition was to adding code to the wallet that exposes everyday users
to OP_RETURN addresses.

> I would much prefer to not have to do this in a custom way, as opposed
> to a way which is defined in a standard manner across all software
> (after all, that's the point of standards).

I'm currently +0.1 on the idea of an address format of OP_RETURN, but I
want to make sure this isn't underwhelmingly motivated or will lead to a
resurgence of block chain graffiti.

-Dave

## Creating an 11,000 byte OP_RETURN

$ bitcoind -daemon -regtest -acceptnonstdtxn
Bitcoin Core starting

$ bitcoin-cli -regtest -generate 101
{
  "address": "bcrt1qh9uka5z040vx2rc3ltz3tpwmq4y2mt0eufux9r",
  "blocks": [
[...]
}

$ bitcoin-cli -regtest send '[{"data": "'$( dd if=/dev/zero bs=1000 count=11 | xxd -g0 -p | tr -d '\n' )'"}]'
11+0 records in
11+0 records out
11000 bytes (11 kB, 11 KiB) copied, 0.000161428 s, 68.1 MB/s
{
  "txid": "ef3d396c7d21914a2c308031c9ba1857694fc33df71f5a349b409ab3406dab51",
  "complete": true
}

$ bitcoin-cli -regtest getrawmempool
[
  "ef3d396c7d21914a2c308031c9ba1857694fc33df71f5a349b409ab3406dab51"
]

$ bitcoin-cli -regtest -generate 1
{
  "address": "bcrt1qlzjd90tkfkr09m867zxhte9rqd3t03wc5py5zh",
  "blocks": [
    "2986e9588c5bd26a629020b1ce8014d1f4ac9ac19106d216d3abb3a314c5604b"
  ]
}

$bitcoin-cli -regtest getblock 2986e9588c5bd26a629020b1ce8014d1f4ac9ac19106d216d3abb3a314c5604b 2 | jq .tx[1].txid
"ef3d396c7d21914a2c308031c9ba1857694fc33df71f5a349b409ab3406dab51"