[bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

Russell O'Connor roconnor at blockstream.io
Thu Dec 13 16:50:10 UTC 2018

On Wed, Dec 12, 2018 at 2:53 PM Johnson Lau <jl2012 at xbt.hk> wrote:

> I think the root cause of witness weight malleability is some opcodes
> accept variable size input (without affecting the output), and that input
> is provided by the puzzle solver. Going through the opcode list, I think
> such opcodes include IF, NOTIF, VERIFY, DROP, 2DROP, NIP, DEPTH, and all
> arithmetic opcode that accepts CScriptNum (including CHECKMULTISIG)
> VERIFY, DROP, 2DROP, NIP are not real problem, since they should not be
> the first opcode to interact with data directly provided by the puzzle
> solver.
> CHECKMULTISIG is fixed by BIP147. For the key number and sig number, they
> should be part of the script, so not malleable.
> DEPTH is a problem only if its inputs are not later examined by other
> opcodes. Again, this is pointless.
> The liberally example should be protected by the MINIMAL_IF policy, which
> requires the input of OP_IF be minimal. As you note, OP_IF could be
> replaced by taproot in many cases
> Non-minimal CScriptNum is also banned as BIP62 policy.
> For the purpose of preventing malicious third party witness bloating, all
> we need is the miners to enforce the policy. There is no reason for miners
> to accept size malleated txs, as that will reduce the usable block space.
> If they hate a tx, they would simply drop it, instead of wasting the block
> space.

I don't know if it such a clear cut case for miner's policy.  A miner is
passed a malleated tx.  They know that there is probably a non-malleated
variant floating around out there somewhere, and they would rather have
it.  But right now they don't, and they probably not going to try to
unmalleate it themselves.  So, why not stick it into their mempool?  If it
eventually makes it into one of their blocks, then it will because it has
the best fee rate available, and to reject it outright is harmful to their
bottom line.  If they find the non-malleated variant later, great, they can
replace it and gain a higher-fee rate tx.  Of course, such a policy opens
them up to a Denial of Service attack.

So what do they do?  Do they accept malleated tx's and implement an RBF
policy that requires sufficient fee rate increases?  Do they reject
malleated txs outright to avoid falling in this trap in the first place as
you suggest?  I don't know, but I don't think things are as clear cut as
you present.

That aside, your list of weight malleable opcodes is shorter than I
imagined and I'm grateful you've compiled it.  Perhaps the best solution is
to make MINIMAL_IF and minimal CScriptNum consensus enforced in the next
version of Script and all but eliminate weight malleability in practice?
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