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

From: Johnson Lau <jl2012@xbt•hk>
To: Russell O'Connor <roconnor@blockstream•io>
Cc: bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer sighashes and more granular SIGHASH_NOINPUT
Date: Fri, 23 Nov 2018 04:52:54 +0800	[thread overview]
Message-ID: <8CD6C248-9ADF-4324-B4E3-DE41A1ED49A9@xbt.hk> (raw)
In-Reply-To: <CAMZUoKkhrLKOaquP1_M9GwuKT1u7d+GoyW6tcK-t2uv+5VRfyA@mail.gmail.com>

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.

>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 operator,……...
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.

> On 23 Nov 2018, at 12:23 AM, Russell O'Connor <roconnor@blockstream•io> wrote:
> 
> I see, so your suggestion is that a sequence of OP_IF ... OP_ENDIF can be replaced by a Merklized Script tree of that depth in practice.
> 
> I'm concerned that at script creation time it takes exponential time to complete a Merkle root of depth 'n'.  Can anyone provide benchmarks or estimates of how long it takes to compute a Merkle root of a full tree of various depths on typical consumer hardware?  I would guess things stop becoming practical at a depth of 20-30.
> 
> On Thu, Nov 22, 2018 at 9:28 AM Johnson Lau <jl2012@xbt•hk> wrote:
> With MAST in taproot, OP_IF etc become mostly redundant, with worse privacy. To maximise fungibility, we should encourage people to use MAST, instead of improve the functionality of OP_IF and further complicate the protocol.
> 
> 
>> On 22 Nov 2018, at 1:07 AM, Russell O'Connor via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
>> 
>> On Mon, Nov 19, 2018 at 10:22 PM Pieter Wuille via bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org> wrote:
>> So my question is whether anyone can see ways in which this introduces
>> redundant flexibility, or misses obvious use cases?
>> 
>> Hopefully my comment is on-topic for this thread:
>> 
>> Given that we want to move away from OP_CODESEPARATOR, because each call to this operation effectively takes O(script-size) time, we need a replacement for the functionality it currently provides.  While perhaps the original motivation for OP_CODESEPARTOR is surrounded in mystery, it currently can be used (or perhaps abused) for the task of creating signature that covers, not only which input is being signed, but which specific branch within that input Script code is being signed for.
>> 
>> For example, one can place an OP_CODESEPARATOR within each branch of an IF block, or by placing an OP_CODESEPARATOR before each OP_CHECKSIG operation.  By doing so, signatures created for one clause cannot be used as signatures for another clause.  Since different clauses in Bitcoin Script may be enforcing different conditions (such as different time-locks, hash-locks, etc), it is useful to be able to sign in such a way that your signature is only valid when the conditions for a particular branch are satisfied.  In complex Scripts, it may not be practical or possible to use different public keys for every different clause. (In practice, you will be able to get away with fewer OP_CODESEPARATORS than one in every IF block).
>> 
>> 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 operator, OP_IF, OP_NOTIF, OP_ELSE, OP_ENDIF, and have the signature cover the value of this counter.  Equivalently we divide every Bitcoin Script program into blocks deliminated by these control flow operator and have the signature cover the index of the block that the OP_CHECKSIG occurs within.  More specifically, we will want a SigHash flag to enables/disable the signature covering this counter.
>> 
>> There are many different ways one might go about replacing the remaining useful behaviour of OP_CODESEPARATOR than the one I gave above. I would be happy with any solution.
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