[Lightning-dev] Lightning, the death of BIP62, and Segregated Witness

Tadge Dryja tadge at lightning.network
Thu Nov 19 19:12:24 UTC 2015


I've joked that BIP62 is the "whack-a-mole" BIP in that it addresses many
vectors for txid malleability, but maybe there are more.  And more
importantly, it addresses 3rd party malleability.  It's not helpful in the
context of lightning channel creation because ECDSA sigs are inherently
malleable.  You can always re-sign the same message with a different
k-value and get a different signature.

The functionality that's needed is to be able to reliably spend from
unconfirmed transactions.  Segregated witness can accomplish that, but it
quite a large hard-fork change.  sighash_noinput can also accomplish that:
as input txids are not signed, if they change, the spending transaction can
be modified while leaving counterparty signatures intact.

I'm hoping to start a new "testnet-L" similar to testnet3, with this
sighash type so that we can test malleability mitigation out.

(Oh also, hi mailing list, sorry I have not posted till now!  But I will
start posting!)

-Tadge

On Thu, Nov 19, 2015 at 9:56 AM, Mark Friedenbach <mark at friedenbach.org>
wrote:

> The basic idea of the soft-fork plan is very simple --- have the
> scriptPubKey be just the 20-byte hash of the redeem script. The scriptSig
> of the spending input is empty. The actual scriptSig, with the redeem
> script and signatures, is contained in a separate Merkle tree committed to
> elsewhere in the block (e.g. in the last output of the coinbase, or the
> last output of the last transaction).
>
> On Thu, Nov 19, 2015 at 7:31 AM, Greg Sanders <gsanders87 at gmail.com>
> wrote:
>
>> The hardfork variant is quite simple, if I understood it correctly. You
>> just stick the signatures in another parallel merkle tree. So if you don't
>> want to validate signatures, just don't download them, and validate
>> everything else. TXIDs don't use the signature at all. Nothing to malleate,
>> AFAIK. Not sure what the softfork plan is, but it will be a talk at Scaling
>> Bitcoin HK.
>>
>> On Thu, Nov 19, 2015 at 10:28 AM, Glenn Tarbox, PhD <glenn at tarbox.org>
>> wrote:
>>
>>>
>>> On Thu, Nov 19, 2015 at 4:33 AM, sickpig at gmail.com <sickpig at gmail.com>
>>> wrote:
>>>
>>>> Hi Pierre
>>>>
>>>> you could start here
>>>>
>>>>
>>>> https://github.com/ElementsProject/elementsproject.github.io#segregated-witness
>>>>
>>>> https://people.xiph.org/~greg/blockstream.gmaxwell.elements.talk.060815.pdf
>>>> https://github.com/ElementsProject/elements
>>>
>>>
>>> There was a brief blip on Reddit:
>>>
>>>
>>> https://www.reddit.com/r/Bitcoin/comments/3ngtx5/could_the_segregated_witness_part_of_the/cwnthlh
>>>
>>> Its weird how little information there is on Segregated Witness.  I'm
>>> guessing its a simple concept and those working on it (sipa / gmaxwell)
>>> haven't felt the need to write it up.
>>>
>>> That it "apparently" can be done with a soft fork similar to P2SH is
>>> good news... I guess...
>>>
>>>
>>> --
>>> Glenn H. Tarbox, PhD
>>>  =]|[=
>>>
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>>> Lightning-dev at lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
>>>
>>>
>>
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>
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