Let's Talk About OP_CAT and Its Potential

There's a new BIP proposal called OP_CAT. Draft's by Ethan Heilman and Armin Sabouri. It’s a simple change, just 13 lines, and it deals with concatenating inputs into one output. This opcode was actually in Bitcoin's early days but got disabled for safety reasons.

Yeah, after Bitcoin set a 520-byte limit for stack elements, some are saying we need to bring back some of those old opcodes. OP_CAT is straightforward and easy to grasp. Some folks think it might help make Bitcoin more quantum safe by tweaking EC signatures.

What’s the actual use case though? Like, where would OP_CAT really shine? Most stack items are hashes, signatures, or numbers. Concatenating them seems kinda pointless.

Nah, there are good examples. Check the 'Motivation' section of the draft. For one, you could open up a coin by signing any message. Like, sign 'Hello World' and use OP_CAT to mix your signature values. Go beyond just signing a transaction.

True, covenants are a big deal too. OP_CAT could enforce conditions on Bitcoin outputs. Plus, using it with BitVM could simplify complex verifications.

1. Signing messages: Input script: '<sigS>', Output script: '<sigR> OP_SWAP OP_CAT <pubkey> OP_CHECKSIG'. 2. Proof of Work for coins: Input script: '<message> <tailHash>', Output: '<target> OP_CAT OP_SWAP OP_SHA256 OP_EQUAL'.

Cool stuff. Could also be used for puzzles. Like, reveal a private key if you successfully solve it: Input script: '<sigS>', Output script: 'OP_TOALTSTACK <puzzle120> OP_DUP OP_FROMALTSTACK OP_CAT OP_SWAP OP_CHECKSIG'.

And here's a fun one: you can get a vanity address from someone else and reward them for it. Input: '<signature> <pubkey> <vanityHead>', Output: '<vanityTail> OP_CAT OP_SWAP OP_DUP OP_HASH160 OP_ROT OP_EQUALVERIFY OP_CHECKSIG'.

I see OP_CAT is officially in the BIPs repo now.

Hyped for the potential merkle proofs!

But wait, is OP_CAT really in the repo yet? Looks like just an open pull request.

For the merkle proof example, you can see there are 30 siblings in the proof, which indicates a massive tree with a ton of leaves. It's a bit complicated but interesting.

Yeah, all this is cool, but what do you plan to do with it in a Bitcoin script? Is it for validating block hashes or what? Also, how expensive is this gonna get in terms of witness stack?

Good point, costs could add up. For me, merkle proofs are key for a trustless peg-out system for a zk layer 2 on Bitcoin. It all depends on proving withdrawals while keeping costs manageable.

Looks like we're getting a clearer picture on Bitcoin's next soft fork. OP_CSFS, OP_CTV, and OP_CAT are the options devs support. I just think we should stick to features that genuinely enhance Bitcoin as a currency.

But not everyone sees it that way. There's no solid agreement on covenants. OP_CTV might be the favorite, but plenty of folks aren’t on board with it, just like OP_CAT.

What's the point of calling it money if it opens doors to weird non-money stuff? Like using Bitcoin for on-chain artifacts or nonsense? Isn’t that extending its usefulness too?

Yeah, the idea of programmable money definitely changes the game. It’s just tricky to balance useful features vs. the unnecessary ones.