[bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT

Rusty Russell rusty at rustcorp.com.au
Wed Dec 12 23:49:02 UTC 2018

Johnson Lau <jl2012 at xbt.hk> writes:
>> On 12 Dec 2018, at 5:42 PM, Rusty Russell via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> wrote:
>> Pieter Wuille via bitcoin-dev <bitcoin-dev at lists.linuxfoundation.org> writes:
>>> Here is a combined proposal:
>>> * Three new sighash flags are added: SIGHASH_NOINPUT, SIGHASH_NOFEE,
>>> * A new opcode OP_MASK is added, which acts as a NOP during execution.
>>> * The sighash is computed like in BIP143, but:
>>>  * If SIGHASH_SCRIPTMASK is present, for every OP_MASK in scriptCode
>>> the subsequent opcode/push is removed.
>> Having the SIGHASH_SCRIPTMASK flag is redundant AFAICT: why not always
>> perform mask-removal for signing?
> Because a hardware wallet may want to know what exact script it is signing?

OK, removing OP_MASKs unconditionally would introduce a hole without
some explicit flag to say they've been removed (the "real script" could
be something different with OP_MASKs).  We could have the signature
commit to the outputscript, but that's a bit meh.

> Masked script has reduced security, but this is a tradeoff with
> functionality (e.g. eltoo can’t work without masking part of the
> script). So when you don’t need that extra functionality, you go back
> to better security
> However, I’m not sure if there is any useful NOINPUT case with unmasked script.

This is *not* true of Eltoo; the script itself need not change for the
rebinding (Christian, did something change?).

So, can we find an example where OP_MASK is useful?

>> If you're signing arbitrary scripts, you're surely in trouble already?
>> And I am struggling to understand the role of scriptmask in a taproot
>> world, where the alternate script is both hidden and general?
> It makes sure that your signature is applicable to a specific script branch, not others (assuming you use the same pubkey in many branches, which is avoidable)

If I'm using SIGHASH_NOINPUT, I'm already required to take care with key

Without a concrete taproot proposal it's hard to make assertions, but
if the signature flags that it's using the taproot script, it's
no less safe, and more general AFAICT.


More information about the bitcoin-dev mailing list