Subject: Re: [bitcoin-dev] [Lightning-dev] BIP sighash_noinput

From: Christian Decker <[email-masked]>
Date: Fri, 13 Jul 2018 13:07:48 +0200

fred savage via bitcoin-dev <[email-masked]> writes:
> the issues with sighash_noinput is this
>
>   1.  you cannot prevent address-reuse. because bitcoin is a PUSH
>   payment. meaning other people can send funds to one address without
>   the owner of the key approval/refusal. thus luke cannot control
>   address reuse if many people start spamming him donations.

This point is pretty much moot since these are scripts, that are used in
very specialized contexts, and should not be shown to any
end-user. Sure, if you go through the blockchain looking for these
addresses, and send the exact same value to it, and create a matching
script then you could end up exposing those funds to this, however
that'd be very silly of you, and you'd have jumped through a lot of
hoops to lose money :-)

>   2.  for average users who would just 'autopilot' LN and only see the
> GUI. they will have no clue what transaction types and technicals are
> happening under the hood. also with LN being not validated by the
> community. a user creating a channel could tweak their own LN node to
> make their counterparty sign a sighash-noinput as a term/condition of
> the channel this is also a risk for the under the hood raw tx risks
> where a tx can be signed but then allow the out's to alter value(using
> a different opcode). .. you know the premiss of allowing a counterpart
> to alter the outs value to vary so that they can control the broadcast
> fee at the time of broadcast to cover being acceptd onchain.. which
> can be abused by a counter party just editing it so A gets nothing and
> B gets it all..

You cannot force the counterparty to sign with a sighash-flag that they
don't chose themselves. We are very clear in the BIP that you should
only use sighash_noinput_unsafe in the context of protocols, that need
to be designed in such a way that these issues are excluded. In
particular, eltoo uses a public key, provided by the signing party,
which they can ensure is not reused (ensuring script
uniqueness). Finally, wallets that are not part of LN or eltoo, won't
even know how to sign with sighash-noinput (try signing anything but
sighash-all on a hardware wallet for example).

The kind of editing you describe also doesn't work, since sighash-single
is used for the late fee binding, not sighash-noinput. sighash-single
makes sure that the input is only valid if the matching output is still
intact, so redirecting funds away from the desired output doesn't work.

>   3.  by allowing certain things to change after signing. is infact
>   bringing back malleability for those that use a TXID to identify a
>   tx has been confirmed. as a TXID would change if values
>   change.. just like how malleation abused old transactions by editing
>   a tx without needing to re-sign a tx

Again, this is only to be used in the context of applications that
require it, which also means that they know how to deal with this
malleability (in fact this malleability is wanted here). If you squint
at it you can probably see that sighash-noinput is also a poor-man's
malleability fix, allowing you to take a transaction that is based on a
malleated output, and rebind it to re-establish the connection.

It seems people believe that we are advocating the use of
sighash-noinput-unsafe in general purpose wallets and in everyday
transactions, this couldn't be further from the truth: sighash-noinput
is a sharp tool, that should only be used in very specific situations,
to enable a bit more flexibility, and it can improve the safety of
off-chain protocols a lot, however general purpose wallets should not
even allow signing with it.

Cheers,
Christian