[dlc-dev] Contract id breaks on any changes to offer serialization

Chris Stewart chris at suredbits.com
Wed Feb 9 00:41:53 CET 2022


Yes but was is the protocol version when we merge #163? Apologies if I am
missing it. I definitely think it should _not_ be zero as that is what the
contract flags are serialized as currently I believe.

On Tue, Feb 8, 2022 at 5:28 PM Thibaut Le Guilly <thibaut at cryptogarage.co.jp>
wrote:

> > From a quick glance through #163, I don't see a protocol version defined.
> It's actually one of the main things I wanted to get as part of the change
> :)! It's here:
> https://github.com/discreetlogcontracts/dlcspecs/pull/163/files#diff-e0f5b925f91a1c09c6daf26f9a7d28816cb9ff9f08863faca719b7ee0a1cc065R69
>
> On Wed, Feb 9, 2022 at 5:18 AM Chris Stewart <chris at suredbits.com> wrote:
>
>> If we add the `tempContractId` to the offer message, this will break
>> existing offer messages. Maybe this isn't such a problem as the protocol
>> version can be read off the front of the offer message on #163. If the
>> protocol version is `0`, hash the offer message. If the protocol version is
>> >= 1, use the tempContractId embedded in the offer message. From a quick
>> glance through #163, I don't see a protocol version defined.
>>
>> -Chris
>>
>>
>> On Sun, Feb 6, 2022 at 8:34 PM Thibaut Le Guilly <
>> thibaut at cryptogarage.co.jp> wrote:
>>
>>> Hi Chris,
>>>
>>> I've tried to consider other approaches than the random temporary
>>> contract id, but it seems to me that it is the best approach. One thing
>>> that I found appealing was simply having a counter between the parties
>>> which they would increment for each contract, but this leads to quite some
>>> issues if one party gets out of sync or misbehaves so I think the random id
>>> is just simpler.
>>>
>>> One thing to note is that this change will also require a change in the
>>> offer message since we will have to include the temporary contract id
>>> inside it (at the moment we don't since it is computed as a hash of the
>>> offer message). If we agree on this I think it would be good to get the
>>> change to the offer message together with the serialization update in #163.
>>>
>>> Cheers,
>>>
>>> Thibaut
>>>
>>> On Wed, Jan 12, 2022 at 11:53 PM Chris Stewart <chris at suredbits.com>
>>> wrote:
>>>
>>>> For reference of our implementation specific issue, please see
>>>> https://github.com/bitcoin-s/bitcoin-s/issues/3969
>>>>
>>>> On Wed, Jan 12, 2022 at 8:51 AM Chris Stewart <chris at suredbits.com>
>>>> wrote:
>>>>
>>>>> When implementing 163
>>>>> <https://github.com/discreetlogcontracts/dlcspecs/pull/163> inside of
>>>>> bitcoin-s, i discovered that any existing wallets deployed with previous
>>>>> serialization versions will have their contract ids
>>>>> <https://github.com/discreetlogcontracts/dlcspecs/blob/master/Protocol.md#definition-of-contract_id>
>>>>> broken and require database level migrations if you are persisting contract
>>>>> ids.
>>>>>
>>>>> The offer message
>>>>> <https://github.com/discreetlogcontracts/dlcspecs/blob/master/Protocol.md#the-offer_dlc-message>
>>>>> is what contract ids are derived off of. If any of the nested data
>>>>> structures have serialization changes inside the offer, this will break
>>>>> contract ids. Things included in the offer are contract descriptors,
>>>>> announcements etc. Thus if we make any changes to the binary level
>>>>> serialization of the DLC spec, contract ids will break most likely.
>>>>>
>>>>> It was talked about on our open source developer call last night what
>>>>> we can do to prevent this from happening. When we first wrote the DLC spec
>>>>> we tried to copy as much stuff as we could from Lightning with two hopes
>>>>>
>>>>> 1. Learning from their deployment of Lightning
>>>>> 2. Reducing the amount of bikeshedding
>>>>>
>>>>> The equivalent to `temp_contract_id` in Lightning is the
>>>>> temporary_channel_id
>>>>> <https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#definition-of-channel_id>.
>>>>> It appears that they just pick a random nonce now. Here is the relevant
>>>>> section from BOLT2
>>>>>
>>>>> >Prior to channel establishment, a temporary_channel_id is used,
>>>>> which is a random nonce.
>>>>>
>>>>> >Note that as duplicate temporary_channel_ids may exist from
>>>>> different peers, APIs which reference channels by their channel id before
>>>>> the funding transaction is created are inherently unsafe. The only
>>>>> protocol-provided identifier for a channel before funding_created has been
>>>>> exchanged is the (source_node_id, destination_node_id,
>>>>> temporary_channel_id) tuple. Note that any such APIs which reference
>>>>> channels by their channel id before the funding transaction is confirmed
>>>>> are also not persistent - until you know the script pubkey corresponding to
>>>>> the funding output nothing prevents duplicative channel ids.
>>>>>
>>>>> So it appears that they are just randomly generating a 32 byte nonce.
>>>>> It seems we should just do the same. This means that whenever we introduce
>>>>> a binary level serialization change, we don't have to run database
>>>>> migrations to recalculate all contract ids.
>>>>>
>>>>> It does appear that our `contract_id` calculation will work as that is
>>>>> calculated with
>>>>>
>>>>> funding_txid XOR temp_contract_id
>>>>>
>>>>> which seems to adhere to the wisdom on the Lightning RFC. Once we know
>>>>> the funding_txid we have a way to reasonably guarantee unique entropy that
>>>>> leads to a stable contract id identifier, just as lightning can calculate a
>>>>> stable channel_id when they have the funding tx.
>>>>>
>>>>> This seems like an easy backward compatible deployment as well, as we
>>>>> can just assume all existing `temp_contract_id` were randomly generated
>>>>> (even though in reality they are hash(offer_bytes)).
>>>>>
>>>>> -Chris
>>>>>
>>>>>
>>>>>
>>>> dlc-dev mailing list
>>>> dlc-dev at mailmanlists.org
>>>> https://mailmanlists.org/mailman/listinfo/dlc-dev
>>>>
>>>
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