Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

[Johnson Lau <jl2012@xbt.hk>]

I don’t think this has been mentioned: without signing the script or masked script, OP_CODESEPARATOR becomes unusable or insecure with NOINPUT.

In the new sighash proposal, we will sign the hash of the full script (or masked script), without any truncation. To make OP_CODESEPARATOR works like before, we will commit to the position of the last executed OP_CODESEPARATOR. If NOINPUT doesn’t commit to the masked script, it will just blindly committing to a random OP_CODESEPARATOR position, which a wallet couldn’t know what codes are actually being executed.

> On 14 Dec 2018, at 5:30 PM, Anthony Towns via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
> 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.
> 
> 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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