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

Chris Stewart chris at suredbits.com
Tue Feb 8 21:18:11 CET 2022

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.


On Sun, Feb 6, 2022 at 8:34 PM Thibaut Le Guilly <thibaut at cryptogarage.co.jp>

> 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
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