[bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

[Rusty Russell]

Anthony Towns via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> writes:
> On Thu, Dec 13, 2018 at 11:07:28AM +1030, Rusty Russell via bitcoin-dev wrote:
>> And is it worthwhile doing the mask complexity, rather than just
>> removing the commitment to script with NOINPUT?  It *feels* safer to
>> restrict what scripts we can sign, but is it?
>
> If it's not safer in practice, we've spent a little extra complexity
> committing to a subset of the script in each signature to no gain. If
> it is safer in practice, we've prevented people from losing funds. I'm
> all for less complexity, but not for that tradeoff.

There are many complexities we could add, each of which would prevent
loss of funds in some theoretical case.

>From practical experience; reuse of private keys between lightning and
other things is not how people will lose funds[1].

It *is* however non-trivially more complicated for wallets; they
currently have a set of script templates which they will sign (ie. no
OP_CODESEPARATOR) and I implemented BIP 143 with only the simplest of
naive code[2].  In particular, there is no code to parse scripts.

Bitcoind developers are not in a good position to assess complexity
here.  They have to implement *everything*, so each increment seems
minor.  In addition, none of these new script versions will ever make
bitcoind simpler, since they have to support all prior ones.  Wallets,
however, do not have to.

I also think that minimal complexity for (future) wallets is an
underappreciated feature: the state of wallets in bitcoin is poor[3]
so simplicity should be a key goal.

Respectfully,
Rusty.

[1] Reusing your revocation base point across two channels will lose
    funds in a much more trivial way, as will reusing payment hashes
    across invoices.
[2] In fact, I added SIGHASH_ANYONECANPAY and SIGHASH_SINGLE recently
    for Segwit and it worked first time!  Kudos to BIP143's authors for
    such a clear guide.
[3] Bitcoind's wallet can't restore from seed; this neatly demonstrates
    how hard the wallet problem is, but there are many others.












code, as modern wallets currently don't have to parse the scripts they
sign.







I'm telling you that this is not how people are losing funds.



>
> Also, saying "I can't see how to break this, so it's probably good
> enough, even if other people have a bad feeling about it" is a crypto
> anti-pattern, isn't it?
>
> I don't see how you could feasibly commit to more information than script
> masking does for use cases where you want to be able to spend different
> scripts with the same signature [0]. If that's possible, I'd probably
> be for it.
>
> At the same time, script masking does seem feasible, both for
> lightning/eltoo, and even for possibly complex variations of scripts. So
> committing to less doesn't seem wise.
>
>> You already need both key-reuse and amount-reuse to be exploited.
>> SIGHASH_MASK only prevents you from reusing this input for a "normal"
>> output; if you used this key for multiple scripts of the same form,
>> you're vulnerable[1].
>
> For example, script masking seems general enough to prevent footguns
> even if (for some reason) key and value reuse across eltoo channels
> were a requirement, rather than prohibited: you'd make the script be
> "<eltoo-channel-id> MASK <statenum> CLTV 2DROP <a+b> CHECKSIG", and your
> signature will only apply to that channel, even if another channel has
> the same capacity and uses the same keys, a and b.
>
>> So I don't think it's worth it.  SIGHASH_NOINPUT is simply dangerous
>> with key-reuse, and Don't Do That.
>
> For my money, "NOINPUT" commits to dangerously little context, and
> doesn't really feel safe to include as a primitive -- as evidenced by
> the suggestion to add "_UNSAFE" or similar to its name. Personally, I'm
> willing to accept a bit of risk, so that feeling doesn't make me strongly
> against the idea; but it also makes it hard for me to want to support
> adding it. To me, committing to a masked script is a huge improvement.
>
> Heck, if it also makes it easier to do something safer, that's also
> probably a win...
>
> Cheers,
> aj
>
> [0] You could, perhaps, commit to knowing the private keys for all the
>     *outputs* you're spending to, as well as the inputs, which comes
>     close to saying "I know this is a scary NOINPUT transaction, but
>     we're paying to ourselves, so it will all be okay".
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