Is there a permanent fix for OP_CHECKMULTISIG issues?

I just started digging into multi-signatures and found out that the OP code has a bug that doesn’t get much attention since there's a workaround using a dummy variable as a temporary fix. So far, I’ve been trying to wrap my head around the whole multi-sig thing, focusing on a 2 of 3 setup, which is what I studied. For a standard 2 of 3, we have this locking script and its corresponding unlocking script, where the two signatures need to match with any of the public keys, assuming I didn’t pick specific numbers. I’ve learned that the issue caused by OP_CHECKMULTISIG could be fixed permanently, but they opted for a dummy variable (the 0 in this case) as a quick fix. I’m curious about a couple of things: 1. Why is that the case? 2. Can anyone explain how the bug could’ve been resolved if it were possible? The scripts I posted are basically the same and combine scriptsig, scriptpubkey, and opcode. I’m just trying to understand the execution order and how values are pushed onto the stack. 3. Which execution order is correct between the two? I broke this down into two parts for easier reading. Each part has questions just in case you missed something. I’m totally open to corrections since I still see myself as learning. Sorry for any mistakes, and please do your own research if any of this is new to you. I can’t guarantee everything I wrote is spot on.

You're kinda right. Ever since Blackhatcoiner's last quiz I noticed similar things concerning OP codes. For the "OP_CHECKMULTISIG" Bug the issue usually comes up  when verifying signatures. And I think this is mainly because the opcode expects a specific stack layout but doesn't properly handle the errors that comes along with it. However a temporary solution like you said which I usually use is to make use of a dummy variable (0) to ensure correct stack alignment. A more kinda advanced approach  would involve modifying "OP_CHECKMULTISIG" to properly handle errors and stack layout like improving the way the stacks are managed, However, introducing changes to OP_CHECKMULTISIG risks disrupting contracts and transactions that already exists. Also, for you code I think; Should solve the problems. Like I said before, 0 here is a dummy variable and is pushed into the stack. Next, the OP code pushes the Signatures (`<signature1>` and `<signature2>`), alongside the Public keys (`<pubkey1>`, `<pubkey2>`, and `<pubkey3>`) then (`2` and `3`) specify the required signature count and public key count after which OP_CHECKMULTISIG` verifies the signatures. I believe this is wrong because the public keys were pushed before (2) which is supposed to be the signature count. Locking script: Unlocking script And this should be the combined script The biggest issues I've faced in OP codes is always stack error. Op codes depend so much on their data stack.

There are already great answers on the internet so I'll just point you to them: 1. Here's a nicely written reply by 'Nick ODell' that briefly explains (compared to the length of the OP) how and why. Link: https://bitcoin.stackexchange.com/a/40673 2. Here's a not-so-old Bitcointalk thread that asked the same question with replies explaining how troublesome to implement a fix since it requires a hard fork. Link: https://bitcointalk.org/index.php?topic=5271566.0 Additionally, for number 2, TapRoot essentially "solved" it (if used for multi signature scripts) since it use OP_CHECKSIGADD which doesn't have that bug. Ref: https://github.com/bitcoin/bips/blob/master/bip-0342.mediawiki#design

Yeah. I am aware that the application of the dummy is a temporary solution, but I was a little curious to know if there are other alternatives to this. I think I was able to find out something from the GitHub link that nc50lc shared, and here it is.. Thanks for pointing this out. Didn't notice at first. I really find these references very helpful. The order of execution was well explained from the first link, while the discussion from the second link was quite interesting to read through, though the last reply didn't agree much to the idea of calling it a bug, which he gave some reasonable explanations to that.

Considering how multisig usually corresponds to a script-hash output script, this script is wrong because the entire redeem script (ie. 3 <pubkey1>...) has to be pushed to the stack as raw bytes not like this. Because the output script will have to first create a duplicate of it (the top stack element), then first hash it to see if it corresponds to the hash inside the script-hash (P2SH or P2WSH) and then evaluate it as a script. So what you posted has to become: And the redeem script itself is: Also notice that m(=2) comes before n(=3) in an m-of-n (2of3) multi sig. The reason is because stack works from top to bottom meaning OP_CHECKMULTISIG first pops the top stack element (ie. 3) and knows it has to read n number of public keys. Then it reads n number of pubkeys to reach m (ie. 2) and after popping that and reading it the interpreter knows it has to read 2 signatures. If it were the way you posted, after OP_CHECKMULTISIG pops the top stack element and sees 2 (signatures) it has no way of knowing how many public keys it has to read.

So,, it's obvious that 3 verifies the number of n which must be at the top of the stack, while 2 verifies the number of m that must come after the pubkeys assuming we are using the 2of3 Multi-sig?. Now assuming we are reading from the top (pubkeys), we also have to confirm for their corresponding signature at that instance?, (Like read for pubkey3 and confirm with signature2. if it doesn't Correspond, we read again for pubkey3 and confirm with signature1)?. And again, since no pubkey can share same signature, when a pubkey finally matches a signature, it's seems those two matches(assuming pubkey2 and signature2) will be omitted, and won't be used in the next verification attempt while verifying the last m and corresponding n?.

Yeah, you mostly correct. But the Pubkeys doesn't have to be matched with the signature in a sequence. Instead the script interpreter would match any of the signature with any of the Pubkeys Once a signature corresponds with any of the Pubkeys it's considered valid and no longer available for further checking and moves to the next signature until this is satisfied 2-of-3 and if this isn't meant i.e if a signature couldn't correspond with a Pubkey, the entire script fails and the transaction becomes invalid.

I actually asked that question initially just to confirm if the reverse process can occur. But I don't think we can just freely match any of the signature without using a sequential process. From what has been discussed so far, it's certain that the Pubkeys are loaded at the top of the stack, while the signatures proceeds it. From the first link shared by nc50lc, this was suppose to be the actual matching order from a sequential sense.. You see what I was trying to say. But if were to use any signature and any pubkey, then we might have not followed this sequential process that the quoted text suggested above. The reason for my question earlier was actually to confirm if a reverse process of the quoted text can occur. But there are about 3 available Pubkeys to match with, so I don't think it will fail automatically since there are two more pubkeys to go. So, if we follow the actual sequential process from the quoted text, I think the whole process might end up resulting to 1, which proves to be valid. Except we've successfully tested a signature with the 3 Pubkeys and there isn't any match. But Incase I am missing something here which I don't think so, please clearify me.

Hmm you right I guess I missed it on how I understood it. The signature has to be matched with the Pubkeys in sequence I guess it's like a 2*3 matrix. About it failing, I was Talking a scenario where the Sig couldn't match any Pubkeys. Yeah if one isn't matching it goes to the next until all are exhausted And if non corresponds it fails. I will check this out. Later maybe with different arrangements of the Pubkeys and Sig.