Add test vectors for `SIGHASH_UTXOS`

[cculianu]

In reference to: https://github.com/bitjson/cashtokens/blob/master/readme.md#sighash_utxos

Rationale:

• This is a complex change
• Breaking it out into a separate CHIP allows for separate testing
  • Why? implementation requires some significant changes to codebase, to how node signs, etc. In general signing doesn't take all the input coins as input to the sign functions.
• By breaking it out as a separate chip more focus can be placed on it and it can be studied and tested more carefully
• CashTokens is already a huge change, tacking this on as a "hey by the way" is rather significant and not ideal.

[A60AB5450353F40E]

Fyi in a version of Group I wanted to include prevout locking script in the hash for the ID, and realized it could lead to a huge amount of data being hashed when spending non-standard utxos so had to introduce a limit for that version of token genesis.

[A60AB5450353F40E]

This:

Someone with access to hashpower could prepare outputs with locking scripts of 10,000 bytes and then spend them all as group genesis inputs where every such input would require hashing 10,260 bytes per input.
Consensus limits transaction size to 1MB, and the smallest input is an anyone-can-spend with OP_1 as the signature and having the genesis declaration, which would be 55 bytes.
Smallest output would be an OP_RETURN output script, which would be 10 bytes.
From that we can calculate the maximum number of inputs in a transaction:
(1000000-10)/55 == 18181.
From there, we could calculate the total amount of hashed bytes: 10260*18181 == 178 MiB, which is close to comparable Script vectors such as OP_1 <520> OP_NUM2BIN OP_DUP OP_HASH256 OP_DROP OP_DUP... pattern.
To avoid any risk, we propose to mitigate this risk by prohibiting declaring a group genesis on inputs that have their len(lockingScript) greater than MAX_SCRIPT_ELEMENT_SIZE which is currently 520 bytes.
This would bring the upper bound on hashed bytes down to: 760*18181 == 13.2 MiB, making group genesis a very inefficient way to load node CPUs.

I think it applies the same to SIGHASH_UTXOS

[cculianu]

This is a great point! And precisely why i think it should be broken out into a separate CHIP{ so that all of this can be worked on and specified ...! :)

[cculianu]

@A60AB5450353F40E Here's another funny thing I discovered in the consensus rules -- nowhere in the rules can I see a limit on the size of an output script (in nonstandard txns). The MAX_SCRIPT_SIZE is only applied when spending an input -- and only at the point where you begin to evaluate the script in question! You can actually produce blocks with huge output scripts that exceed MAX_SCRIPT_SIZE. Not sure how this impacts this feature -- likely it doesn't at all, although maybe you can at least trick a node into hashing even more than 178 MiB somehow using that trick? (Of course the txn would fail to be spent anyway eventually when it gets to its input scripts that exceed MAX_SCRIPT_SIZE).

Just another curious thing that may or may not affect that calculation that leads to 178MiB .. but yeah. In principle -- that's a huge amount of data!

[bitjson]

Thanks for opening an issue @cculianu! I'd be ok with breaking out SIGHASH_UTXOS into its own CHIP if that would help. I do want to note: SIGHASH_UTXOS is security-critical for CashTokens, and they must be deployed at the same time. Every wallet I've reviewed (with the possible exception of Electron Cash, see several here) will be vulnerable to attacks where their data source(s) can:

• Burn tokens, and/or
• Trick the wallet into performing unintended actions in decentralized applications (e.g. Jedex), often allowing tokens to be directly stolen.

I think it would be a serious mistake to allow CashTokens to activate without SIGHASH_UTXOS, as many transactions involving tokens should enable it (for example, many of Libauth's test vectors for CashTokens include signatures with SIGHASH_UTXOS). I also don't really see how a separate CHIP would improve review (I'd hope that reviewers will apply just as much rigor to components of this spec as they would apply if each component got its own document 😅). So I'm inclined to leave it in the primary CHIP.

Also, having multiple documents makes implementation harder for end-readers (in my experience), and deciding where to chop up the spec isn't obvious. E.g. if SIGHASH_UTXOS belongs in its own document, do the Token Inspection Operations? What about CashAddress Token Support or Fungible Token Supply Definitions? I'd really prefer to keep everything in a single document.

---

@A60AB5450353F40E on hashing: note that this is the purpose of transaction "standardness" – miners can safely include standard transactions in blocks knowing that standard transactions will not require excessive resource usage during validation. As with other nonstandard transactions, the worst-case validation performance you're describing affects only the miner choosing to include the transaction: if they pack their block full of expensive-to-validate transactions, their block will not be accepted fast enough by the network, and it will become stale if another miner finds a competing block that is accepted more quickly by other miners. This is the same reason that several other attacks are not currently practical.

If we were to relax non-P2SH transaction standardness, we'd need to address this and several other issues, but for now, there's no additional value to adding other consensus limits around bytes hashed by SIGHASH_UTXOS; the existing non-P2SH standardness limit is already far more restrictive.

[cculianu]

@bitjson: @A60AB5450353F40E and I both fixed the accidental burn issue. Are you aware that signing a token input now requires additional data in the signature hash? It is impossible for unupgraded software to burn tokens, or for unupgraded software to be tricked or for tokens to be stolen.

Are you aware of the sighash modification that @A60AB5450353F40E and I came up with? It just requires the token data to be committed in the signature hash, basically.

If you are not familiar with it, please familiarize yourself with it.

if you are aware of it, then please enlighten me as to how one can steal tokens or how unupgraded software can burn tokens inadvertently?

[bitjson]

Yes! The spec includes a Signing Serialization of Tokens section now. (Fixed in #11. Thank you again for realizing that we don't need to reserve a SIGHASH bit for tokens in general.)

SIGHASH_UTXOS addresses a different set of wallet vulnerabilities arising when transactions with multiple inputs can require multiple signing attempts (the issue I was talking about here).

This was also addressed by BTC BIP341/Taproot's SIGHASH_DEFAULT, rationale here (and one exploit is described here), but the issue is even more acute for BCH because CashTokens enables significantly more powerful contracts: contract logic (and tokens) can be split across multiple UTXOs to accomplish much more interesting validation (e.g. Jedex).

So not only can wallets be mislead into signing multiple variations of the same transaction to burn fees or tokens, wallets can also be tricked into interacting with unexpected UTXOs, performing unexpected actions in decentralized applications. In practice, that means that the attacker can actually trick the user into giving value to the attacker (the wallet service backend, another entity in a multisig wallet, a hardware wallet's compromised host computer, etc.). E.g. a user's wallet thinks they are buying some tokens on a DEX, but in reality they are providing the backend with all the signatures necessary to reconstruct a transaction that "sells" their whole token balance to the attacker at price near 0 (effectively stealing those tokens).

Attacks can be prevented if the signing wallet/device decodes and inspects the original transaction behind every input to confirm that a UTXO of the expected shape is actually being spent (e.g. the signer performs full SPV or neutrino validation). However, just looking at our existing landscape of BCH wallets, Electron Cash is the only wallet that could implement the additional validation without major upgrades to its internal database – I'm not even aware of any actively-maintained mobile BCH wallets that could implement this validation without major re-architecting.

And that's not to mention hardware wallets: if we want hardware wallets to ever be able to safely interact with the decentralized applications made possible by CashTokens, the hardware wallets would need to either 1) implement that same validation (requiring possibly MBs of transaction data – remember this alone is why BTC added SIGHASH_DEFAULT in the Taproot upgrade), or 2) commit to UTXO contents in the signing serialization (requires no additional data, even among mobile BCH wallets that rely on trusted backends).

TLDR: we can either:

1. Force all wallets/signing devices to validate more data from previous transactions to safely interact with the decentralized applications made possible by CashTokens (this would typically require architectural changes and significantly more data/memory/storage), or
2. Allow wallets to commit to the precise contents of UTXOs (SIGHASH_UTXOS).

Option 1 would likely either stifle adoption of CashTokens in existing wallets (since proper support becomes a much more significant upgrade) or encourage existing wallets to cut corners and ship implementations that are vulnerable to subtle, hard-to-understand exploits (and any successful exploits could be reasonably blamed on "the CashTokens CHIP not addressing a security issue").

Option 2 conclusively solves all related vulnerabilities and makes adding CashTokens support to wallets significantly easier (without increasing data/memory/storage requirements).

---

Thanks for prompting this discussion, and sorry I haven't had a chance to write a more thorough explanation! I'll try to clean this up and get a more detailed rationale section into the CHIP.

[cculianu]

I mean this just proves that SIGHASH_UTXOS needs to be its own chip. It's not essential for the basic use-case of CashTokens, which would be an SLP 2.0 work-alike.

It's perhaps essential for what you had in mind using CashTokens -- which is to enable some really advanced smart contract features.

As such, it really should be its own chip, and it needs to be fleshed out more and thought about separately, I think. We can make it for May 2023 with it, it just should be broken out for clarity.

[cculianu]

I mean look at the very least SIGHASH_UTXOs needs to be specified well -- I merely suggest breaking it out into a separate CHIP for organizational purposes. You can even say both CHIPs are required to go together else we won't get all the sexy features... that's fine. We can promise to implement the two together...

If you don't want to do a separate CHIP .. then can you at least break down SIGHASH_UTXOs more to specify it fully along with test vectors or something like that?

You can make it a sub-CHIP of this big master chip maybe?

[cculianu]

ok @bitjson and @A60AB5450353F40E thanks for explaining to me why SIGHASH_UTXOS is critical. Shall I rename this issue to "specify SIGHASH_UTOXS more completely" or something more abstract? It seems @bitjson doesn't want to beak it out into a separate CHIP.. which is fine. I still think it needs more meat in terms of the spec document...

[A60AB5450353F40E]

@bitjson just to add something from my chat with Calin to the rationale, is this right:

Say you're being offered a CoinJoin TX that swaps some tokens and are supposed to sign with sighash_all. Without fetching non-local data, you only know: what you're spending (your 1 utxo) and what you're getting (transaction outputs), but you have no idea what's behind other inputs - you only "see" outpoints (txid/index). Now normally with p2pkh it's all fine, those will need signatures of their own.However, for advanced contracts introspection allows us to have outputs that will allow them to delegate signing to other outputs, so by signing just 1 output of yours, you could be allowing other otputs you own to be spent alongside it - without realizing you're also authorizing those (unless you looked up each input's prevout) by signing just 1 of yours. You could be fooled into giving away more than you think you're giving by signing that 1.

yeah it's not critical for cashtokens - but it seems like really important for some applications that will be built using cashtokens/introspection

wallets that don't sign using sighash_utxo wouldn't need to change their architecture, but they also couldn't be fooled into producing a signature that would spend some other coins they own because... they'd be unable to produce a signature with sighash_utxo

if you have sighash_utxo you can write safe contracts - those that require the bit on the sibling input, so even if unaware wallets wouldn't bail, they just couldn't spend the dependent utxos

[imaginaryusername]

I haven't fully formed my opinion around this yet, but it seems like the one identified nonstandard DoS vector is shared with introspection. It may not be immediately urgent, but next upgrade cycle we should aim to align standard and nonstandard output formats, which will eliminate a whole host of possible vectors including very large outputs.

Wallets and indexers can be recommended to keep the implementations loose, and keep upgrade paths open - and given the status quo on other bitcoin-like chains, it's unlikely they'll specifically tighten their implementations for BCH anyway.

[A60AB5450353F40E]

Yeah I reached the same conclusion after our brief chat - we should specify sane limits using consensus specification, with the ultimate goal of being aligned. SIGHASH_UTXO would not have that 200-ish MiB edge case if consensus was limiting script size to 1650.