[WIP] Safe cancel

[tromp]

Cancel safely in face of play attacks.

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[pkariz]

Just want to mention that respend doesn't help if you reinstall the wallet before sending the next transaction because the new wallet has no clue about which utxos were marked for respend. That's also true if you use multiple devices since we don't have a shared wallet data

[tromp]

That's a separate issue. A restored wallet doesn't know what transactions are pending, and will count all their inputs towards its balance. This RFC doesn't deal with that general problem.

[pkariz]

So the safe-cancel should occur in these cases:

1. SRS: sender has signed the transaction and broadcasted it, but now wants to cancel it (reason being it was blocked by the receiver)
2. RSR: sender has signed the transaction and sent the slatepack to the receiver, but now wants to cancel it (cases described in the rfc)

should self-spend be a 1-1 or a 1-2 transaction? 1-1 consequences:

• everyone knows it's 99% a self-spend
• Bob is 99% this is Alice's self-spend so he knows her output
• less fees than a 1-2 transaction

1-2 consequences:

• nobody knows it's a self-spend, only Bob can speculate it is (could be a respend so he can't be sure)
• a bit higher fees than a 1-1 transaction

[tromp]

A cancel can be done on any pending transaction. That is, any transaction that the wallet has signed for, and that hasn't appeared in the mempool yet of the node queried by the wallet.

In the case of SRS, where sender has signed the transaction and broadcasted it, but it was blocked by the receiver, the status should change from "pending" to a new state "blocked by receiver". The user can then either inform the receiver to please unblock, or determine that the receiver is a deceiver, and refusing to further deal with them. In the latter case, they should choose to "unspend".

Unspends are always self spends, while respends can be any spend, or even a payjoin receive. Self spends would not stand out with the CoinSwap proposal at https://forum.grin.mw/t/mimblewimble-coinswap-proposal

[phyro]

I think it's best to plan this also for the scenario of full blocks. In this case, we'd need to have an option to define the cancel fee to define the priority of a cancel. I also wonder whether we should differentiate such a self-spend. Should self-spend outputs be labeled somehow in the wallet to distinguish from an actual receiving of money? I think this question is relevant for self-spends in both safe-cancel and the coinswap proposal.

[tromp]

What do you mean by a cancel fee? The fee of an unspend tx? This gets us into the replace-by-fee issue that Bitcoin faced. Should a higher (priority) fee replace a conflicting tx in the mempool? That is quite a difficult topic, which I don't want to spend too much time on until such a time when blocks filling up starts to feel like a remote possibility

I think a self-spend tx could show up in the wallet as a mark/label on an output, indicating that it might shift a little (change blinding factor and lose a fee in value) rather than a separate tx.

[phyro]

When you say mark/label on an output you mean literally on an output (using a new BP bit) or labeling in the wallet context?

[tromp]

I mean in the wallet.

[GeneFerneau]

After some work on atomic signatures, and a conversation with @phyro on keybase, came up with the idea of using a concurrent height_lock transaction spending the same input (set to some spender chosen threshold). Could be given as an option to the spender at the time of tx creation: "hey, would you like to create a height_lock tx to ensure you get your money back if the receiver pulls any shenanigans?" Then, when/if the receiver acts honestly, the height_lock tx is a double-spend, never to be accepted to the chain.

Thoughts?

[phyro]

I think it is an interesting idea that deserves to be written down for anyone else reading this discussion in the future. If the height_lock transaction had a 2x fees compared to the average accepted at that time, it might serve as a "contract expiration" transaction so it could be interpreted as the contract might expire after 60 blocks/~1 hour. I'm not sure how such transaction would behave in the mempool and whether they would accept a height locked transaction with the same input though.

[tromp]

AFAIK, neither Grin implementation has yet implemented a replace-by-fee policy, so extra fees on a refund tx (the idea of which dates back many years) don't matter much.

[phyro]

One possible issue is the following scenario. Suppose Alice sends Bob 100k Grin in RSR. Bob at step 3 withholds the transaction and says it doesn't work. Alice performs the safe-cancel and sees the tx on the chain. Now they create another transaction and when Bob gets to step 3, he reorgs the chain to remove the self-spend from Alice and include both of the transactions to double-spend Alice. This could be solved by using re-spend option, but users need to understand that unspend comes with the regular confirmation depending on the value.

[phyro]

What does transaction cancel mean it is done by Bob (the receiver) due to Alice (the sender) not finishing the steps? A scenario would be SRS flow where step3 is not done. There are two transaction cases I see:

1. Bob only contributed outputs to the tx
2. Bob contributed outputs and inputs to the tx (payjoin)

Handling 1.: The only way to really cancel the transaction from Bob side where he contributed only outputs is to create the same output in a self-spend and never spend it, but we really don't want this. So I assume we handle this by dropping the expected outputs that the wallet is tracking or similar. This is probably an implementation question, but what happens if you remove the expected outputs and the tx later appears on the chain?

Handling 2.: This is a more interesting scenario. When Bob cancels a payjoin transaction, does it mean that Bob double-spends the contributed inputs (at least one of them)?

[tromp]

In case 1, I don't see any need for Bob to cancel the tx, since no funds of his are at stake. As to implementation, it should probably remain to show as unconfirmed output.

In case 2, the funds in his input are still considered part of his balance, and again he is not at risk of losing funds. The only risk is this output getting doublespent when Bob cancels with a respend and reused the input in another tx, which Bob can avoid by doing an unspend instead. The difference with cancelling a sending tx is that Bob can delay the cancellation of a payjoin receive until he's running out of other inputs to use.

[phyro]

I think we could make it a 2-step process. The main difference between respend/unspend is the speed at which they execute (fees are much less important imo). Both do the exact same thing. They first mark a transaction for cancellation and at some point in the future, a transaction we create consumes an input of this transaction. We can thus simplify these to:

1. Have cancel_pool which is a pool of transactions marked for cancellation
2. The next transaction we create where we are the sender simply iterates over cancel_pool, takes the first input we contributed and adds it to the transaction

This means that safe-cancel can be just ./grin-wallet cancel --mark-respend which would put it in the cancel_pool and then have ./grin-wallet self-spend command which creates a transaction to self and since every transaction where we're the sender consumes the cancel_pool it would cancel them.

Separating marking from the respend itself might also be a better option for services consuming this API because you don't need a separate transaction for every cancelled transaction. You can mark multiple transaction for respending and then create a self-spend to consume all of them at once.

Thus a fast respend is just a mark + self-spend e.g. ./grin-wallet cancel --mark-respend txid && ./grin-wallet self-spend 0 where 0 means that we want to send 0 amount (along with the mandatory inputs picked from the cancel_pool).

Edit: Hmm, while this would allow to batch cancel transactions in a single transaction, it might be a bit more complex handling a failure of this respend e.g. one of the cancel transactions actually lands on the chain before it is cancelled.

[pkariz]

Cancel pools are a nice idea, but when you run self-spend you would leak some of your tx graph connections if you self-spend all marked txs in a single self-spend and the implementation is, as you noted, a little complicated when one of the canceled txs lands on the chain. I'm not sure I'm convinced respend is necessary at this point, here are the cons I see:

• it complicates cancel explanation (unspend is very simple to explain if we go with contract/sign/sign flow explanation since it's just unsign and you need to pay for signature removal, respend requires some tech understanding which 99% of the users won't have)
• it complicates implementation (you mark tx for respend, maybe you also want to allow unmarking, ux becomes more complicated)
• when a user does unspend or respend he agrees that this tx should be canceled and respend doesn't guarantee you that since it waits for the user to create another tx (or multiple if you want to leak less data and respend each canceled tx separately). Before the user creates new tx he might buy a new phone and the tx is again in a pending state and not canceled (but the user doesn't see it in the wallet, so it's dangerous and would need to self-spend everything, that's very non-intuitive to non-tech users). With unspend the user needs to have the wallet installed only for the next minute or two until tx is mined.

respend is of course useful in some cases, but imo it only really makes a meaningful difference if a single user does many respends and exchanges are candidates for doing that. However exchanges have an easy way to deal with this, they can just do unspend and reduce user's amount in their system. So maybe it might be best to just implement grin-wallet cancel with optional --unsafe flag to avoid creating a self-spend tx and if it later turns out that the community wants to have an option to respend it can be added later. Note that grin-wallet cancel should work also on step1 txs since they're already in db, even if there are no inputs commited yet (it should basically do the "unsafe" variant which is safe in this case)

[phyro]

Thanks for the comments and you have some good points, especially that it may complicate the handling when one of the transactions appears on the chain (though I'm not yet sure it does). However, observing this self-spend transaction and handling possible cases is unavoidable, even in the case of unspend because like respend it doesn't guarantee the transaction will not be race-conditioned on the chain. So the wallet would need to observe which of the two double-spend transactions actually landed on the chain to clean up the input/output state (drop the outputs created in the transaction that didn't arrive and unlock the inputs that have not been spent). What would --unsafe do in this case? mark the transaction for spending the output in the next transaction? and since you want each cancel to be done separately then we'd have a queue/sequence of cancel attempts, correct?

[pkariz]

even in the case of unspend because like respend it doesn't guarantee the transaction will not be race-conditioned on the chain

yes, it would show as "cancelling", but in case of unspend you know that this cancelling is short-lived. It can turn into "completed" though yes if the original tx gets mined.

What would --unsafe do in this case?

only set status in db as "canceled" and unlock any locked inputs, same as cancel does today

and since you want each cancel to be done separately then we'd have a queue/sequence of cancel attempts, correct?

for each tx you cancel new "canceling tx" is created. I'm not sure if the user should see these "cancel-txs" or not, probably not.

[Anynomouss]

for each tx you cancel new "canceling tx" is created. I'm not sure if the user should see these "cancel-txs" or not, probably not.

The best would be to show the original transaction, labelled as 'cancelled safely', or something similar. So basically you combine the safe cancelling transaction with the original, at least in the way that it is presented to the user.