[bitcoin-dev] Safety of committing only to transaction outputs

Johnson Lau jl2012 at xbt.hk
Sat May 25 07:53:34 UTC 2019

> On 25 May 2019, at 4:59 AM, Jeremy <jlrubin at mit.edu> wrote:
> Hi Johnson,
> As noted on the other thread, witness replay-ability can be helped by salting the taproot key or the taproot leaf script at the last stage of a congestion control tree.

The salt will be published when it is first spent. Salting won’t help if the address is reused.

> I also think that chaperone signatures should be opt-in; there are cases where we may not want them. OP_COSHV is compatible with an additional checksig operation.
> There are also other mechanisms that can improve the safety. Proposed below:
> OP_CHECKINPUTSHASHVERIFY -- allow checking that the hash of the inputs is a particular value. The top-level of a congestion control tree can check that the inputs match the desired inputs for that spend, and default to requiring N of N otherwise. This is replay proof! This is useful for other applications too.

It is circular dependent: the script has to commit to the txid, and the txid is a function of script

> OP_CHECKFEEVERIFY -- allowing an explicit commitment to the exact amount of fee limits replay to txns which were funded with the exact amount of the prior. If there's a mismatch, an alternative branch can be used. This is a generally useful mechanism, but means that transactions using it must have all inputs/outputs set.

This restricts replayability to input with same value, but is still replay-able, just like ANYPREVOUT committing to the input value

> Best,
> Jeremy
> --
> @JeremyRubin <https://twitter.com/JeremyRubin> <https://twitter.com/JeremyRubin>
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