[bitcoin-dev] CHECKSEQUENCEVERIFY - We need more usecases to motivate the change

[s7r]

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Hi aj,

On 10/4/2015 11:35 AM, Anthony Towns via bitcoin-dev wrote:
> On Sat, Oct 03, 2015 at 04:30:56PM +0200, Peter Todd via
> bitcoin-dev wrote:
>> So we need to make the case for two main things: 1) We have
>> applications that need a relative (instead of absolute CLTV) 2)
>> Additionally to RCLTV, we need to implement this via nSequence
> 
>> However I don't think we've done a good job showing why we need
>> to implement this feature via nSequence. BIP68 describes the new
>> nSequence semantics, and gives the rational for them as being a 
>> "Consensus-enforced tx replacement" mechanism, with a
>> bidirectional payment channel as an example of this in action.
>> However, the bidirectional payment channel concept itself can be
>> easily implemented with CLTV alone.
> 
> Do you mean "with RCLTV alone" here?
> 
> RCLTV/OP_CSV is used in lightning commitment transactions to
> enforce a delay between publishing the commitment transaction, and
> spending the output -- that delay is needed so that the
> counterparty has time to prove the commitment was revoked and claim
> the outputs as a penalty.
> 

I partially understand - can you please provide a simple Alice and Bob
example here with the exact scenario? Thanks. Why is there a need to
'delay between publishing the commitment transaction and spending the
output'? If the absolute CLTV script reached its maturity it means
something went wrong (e.g. counterparty cheated or got hit by a bus)
so what is with the delay time needed for proving that the commitment
was revoked? I assume an absolute CLTV script reaching its maturity
(nLockTime) is the proof itself that the commitment was revoked - but
maybe I'm missing something obvious, sorry if this is the case.

> Using absolute CLTV instead would mean that once the effective
> delay a commitment transaction has decreases over time -- initially
> it will be longer than desirable, causing unwanted delays in
> claiming funds when no cheating is going on; but over time it will
> become too short, which means there is not enough time to prove
> cheating (and the channel has to be closed prematurely). You can
> trade those things off and pick something that works, but it's
> always going to be bad.
> 
I agree, I see the logic here. Absolute CLTV is not necessary inferior
to RCLTV - there are use cases and use cases. For example, you can
avoid unnecessary waiting until the nLockTime expires if you use
absolute CLTV in combination with P2SH (2/2). Again, it always depends
on the use case - it might be a good solution, it might not be such a
good solution, but even absolute CLTV alone clearly fixes a lot of
things and takes smart contracts to the next level.

>> There is a small drawback in that the initial transaction could
>> be delayed, reducing the overall time the channel exists, but the
>> protocol already assumes that transactions can be reliably
>> confirmed within a day - significantly less than the proposed 30
>> days duration of the channel.
> 
> Compared to using a CLTV with 30 days duration, With RCLTV a
> channel could be available for years (ie 20x longer), but in the
> case of problems funds could be reclaimed within hours or days (ie
> 30x faster).
> 
Indeed. I for one _need_ CLTV / RCLTV in my day to day use cases, it
would be neat to have both, but if I can only have (for the time
being) absolute CLTV so be it - it's still a lot better.

> But that's all about RCLTV vs CLTV, not about RCLTV vs
> nSequence/OP_CSV. ie, it needs BIP 112 (OP_CSV) but not necessarily
> BIP 68 (nSequence relative locktime), if they could be
> disentangled.
> 
> You could do all that with "<n> OP_CHECK_HEIGHT_DELTA_VERIFY" that
> ignores nSequence, and directly compares the height of the current 
> block versus the input tx's block (or the diff of their
> timestamps?) though, I think?
> 
> I think the disadvantage is that (a) you have to look into the
> input transaction's block height when processing the script; and
> (b) you don't have an easy lookup to check whether the transaction
> can be included in the next block.
> 
> You could maybe avoid (b) by using locktime though. Have "<n> 
> OP_CHECK_RELATIVE_LOCKTIME_VERIFY" compare the transactions
> locktime against the input's block height or time; if the locktime
> is 0 or too low, the transaction is invalid. (So if nLockTime is in
> blockheight, you can only spend inputs with blockheight based
> OP_CRLTV tests; and if it's in blocktime, you can only spend inputs
> with blocktime based OP_CRLTV. "n" does need to encode whether it's
> time/block height though).
> 
> That way, when you see a txn:
> 
> - run the script. if you see <n> RCLTV, then + if the tx's locktime
> isn't set, it's invalid; drop it + if the input txn is unconfirmed,
> it's invalid; try again later + workout "locktime - n" if that's >=
> the input tx's block height/time, it's good; keep it in mempool,
> forward it, etc
> 
> - if you're mining, include the tx when locktime hits, just like
> you would any other valid tx with a locktime
> 
> I think the use cases for BIP68 (nSequence) are of the form:
> 
> 1) published input; here's a signed tx that spends it to you,
> usable after a delay. might as well just use absolute locktime
> here, though.
> 
> 2) here's an unpublished input, you can build your own transaction
> to spend it, just not immediately after it's published. BIP112 is 
> required, and OP_RCLTV as defined above works fine, just include
> it in the published input's script.
> 
> 3) here's an unpublished input, and a signed transaction spending
> it, that you can use to spend it after a delay. BIP68 is enough;
> but OP_RCLTV in the second transaction works here. however without 
> normalised tx ids, the input could be malleated before
> publication, so maybe this use case isn't actually important
> anyway.
> 
> So I think OP_CRLTV alone works fine for them too...
> 
> (Does that make sense, or am I missing something obvious?)
> 
> Cheers, aj
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