[bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

Russell O'Connor roconnor at blockstream.io
Thu Nov 22 22:10:11 UTC 2018

On Thu, Nov 22, 2018 at 3:53 PM Johnson Lau <jl2012 at xbt.hk> wrote:

> Assuming a script size of 128 bytes (including SHA256 padding), 2^20
> scripts is 134MB. Double it to 268MB for the merkle branch hashes. With
> roughly 100MB/s, this should take 2.5s (or 42min for 30 levels). However,
> memory use is not considered.
> >each call to this operation effectively takes O(script-size) time
> I’m not sure if this is correct. Actually,
> CTransactionSignatureSerializer() scans every script for OP_CODESEPARATOR.
> Scripts with and without OP_CODESEPARATOR should take exactly the same
> O(script-size) time (see https://github.com/bitcoin/bitcoin/pull/14786)
> Also, this is no longer a concern under segwit (BIP143), which
> CTransactionSignatureSerializer() is not used. Actually, OP_CODESEPARATOR
> under segwit is way simpler than the proposed OP_MASK. If one finds OP_MASK
> acceptable, there should be no reason to reject OP_CODESEPARATOR.

Even still, each call to OP_CODESEPARATOR / OP_CHECKSIG pair requires
recomputing a new #5. scriptCode from BIP 143, and hence computes a new
transaction digest.  I understood that this issue was the main motivation
for wanting to deprecate OP_CODESEPARATOR and remove it from later versions
of script.

However, given that we are looking at a combinatorial explosion in SIGHASH
flag combinations already, coupled with existing SigOp limitations, maybe
the cost of recomputing scriptCode with OP_CODESEPARATOR isn't such a big

And even if we choose remove the behavior of OP_CODESEPARATOR in new
versions of Script, it seems more than 30 layers of sequential OP_IFs can
be MASTified, so there is no need to use OP_CODESEPARATOR within that limit.

>One suggestion I heard (I think I heard it from Pieter) to achieve the
above is to add an internal counter that increments on every control flow

> If I have to choose among OP_CODESEPARATOR and “flow operator counting”,
> I’d rather choose OP_CODESEPARATOR. At least we don’t need to add more
> lines to the consensus code, just for something that is mostly archivable
> with MAST.
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