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Peer Protocol for Contract Negotiation

Table of Contents

Contract

Definition of contract_id

Prior to a contract being accepted, a temporary_contract_id is used, which is the SHA256 hash of the offer message.

Most messages use a contract_id to identify the contract. It's derived from the funding transaction and the offer by combining the funding_txid, the funding_output_index and the temporary_contract_id, using big-endian exclusive-OR (i.e. funding_output_index alters the last 2 bytes of funding_txid XOR temporary_contract_id).

Contract Negotiation

Contract Negotiation consists of the initiator (aka offerer) sending an offer_dlc message, followed by the responding node (aka accepter) sending accept_dlc. With the contract parameters locked in, both parties are able to create the funding transaction and subsequently all contract execution transactions (CETs) and the refund transaction, as described in the transaction specification. As such, the accepter includes its signatures of the CETs and refund transaction in the accept_dlc message. The initiator is now able to generate signatures for all CETs and the refund transaction, as well as the funding transaction, and send them over using the sign_dlc message.

Once the accepter receives the sign_dlc message, it must broadcast the funding transaction to the Bitcoin network.

+-------+                    +-------+
|       |                    |       |
|       |---- (1) offer  --->|       |
|       |                    |       |
|       |<--- (2) accept ----|       |
|   A   |                    |   B   |
|       |---- (3) sign   --->|       |
|       |                    |       |
|       |                    |  (4) broadcast fund-tx
|       |                    |       |
+-------+                    +-------+

    - where node A is 'offerer' and node B is 'accepter'

If this fails at any stage, or if one node decides the contract terms offered by the other node are not suitable, the contract negotiation fails.

Note that multiple contracts can be open in parallel, as all DLC messages are identified by either a temporary_contract_id (before the funding transaction is created) or a contract_id (derived from the funding transaction).

The offer_dlc Message

This message contains information about a node and indicates its desire to enter into a new contract. This is the first step toward creating the funding transaction and CETs.

  1. type: 42778 (offer_dlc_v0)
  2. data:
    • [byte:contract_flags]
    • [chain_hash:chain_hash]
    • [contract_info:contract_info]
    • [point:funding_pubkey]
    • [spk:payout_spk]
    • [u64:payout_serial_id]
    • [u64:offer_collateral_satoshis]
    • [u16:num_funding_inputs]
    • [num_funding_inputs*funding_input:funding_inputs]
    • [spk:change_spk]
    • [u64:change_serial_id]
    • [u64:fund_output_serial_id]
    • [u64:feerate_per_vb]
    • [u32:cet_locktime]
    • [u32:refund_locktime]

No bits of contract_flags are currently defined, this field should be ignored.

The chain_hash value denotes the exact blockchain that the DLC will reside within. This is usually the genesis hash of the respective blockchain. The existence of the chain_hash allows nodes to open contracts across many distinct blockchains as well as have contracts within multiple blockchains opened to the same peer (if it supports the target chains).

contract_info specifies the contract to be constructed and the oracles to be used.

funding_pubkey is the public key in the 2-of-2 multisig script of the funding transaction output. payout_spk specifies the script pubkey that CETs and the refund transaction should use in the sender's output.

offer_collateral_satoshis is the amount the sender is putting into the contract. num_funding_inputs is the number of funding inputs contributed by the sender and funding_inputs contains outputs, outpoints, and expected weights of the sender's funding inputs. change_spk specifies the script pubkey that funding change should be sent to.

payout_serial_id is a randomly chosen number which uniquely identifies the payout output of the offering party. Outputs in the contract execution transaction will be sorted by payout_serial_id.

change_serial_id is a randomly chosen number which uniquely identifies the change output. Outputs in the funding transaction will be sorted by change_serial_id and fund_output_serial_id.

fund_output_serial_id is a randomly chosen number which uniquely identifies the funding output. Outputs in the funding transaction will be sorted by change_serial_id and fund_output_serial_id.

feerate_per_vb indicates the fee rate in satoshi per virtual byte that both sides will use to compute fees in the funding transaction, as described in the transaction specification.

cet_locktime is the nLockTime to be put on CETs. refund_locktime is the nLockTime to be put on the refund transaction.

Requirements

The sending node MUST:

  • set undefined bits in contract_flags to 0.
  • ensure the chain_hash value identifies the chain it wishes to open the contract within.
  • set payout_spk and change_spk to a standard script pubkey
  • set funding_pubkey to a valid secp256k1 pubkey in compressed format.
  • set offer_collateral_satoshis to a value greater than or equal to 1000.
  • set cet_locktime and refund_locktime to either both be UNIX timestamps, or both be block heights as distinguished here.
  • set cet_locktime to be less than refund_locktime.
  • use a unique input_serial_id for each input
  • set change_serial_id and fund_output_serial_id to different values

The sending node SHOULD:

  • set feerate_per_vb to at least the rate it estimates would cause the transaction to be immediately included in a block.
  • set cet_locktime to no later than the earliest expected oracle signature time.
  • set refund_locktime sufficiently long after the latest possible release of oracle signatures added to all other delays to closing the contract.
  • set payout_spk to a previously unused script public key.
  • set change_spk to a previously unused script public key.

The receiving node MUST:

  • ignore undefined bits in contract_flags.

The receiving node MAY reject the contract if:

  • it does not agree to the terms in contract_info.
  • the contract_info is missing relevant events.
  • it does not want to use the oracle(s) specified in contract_info.
  • offer_collateral_satoshis is too small.
  • feerate_per_vb is too small.
  • feerate_per_vb is too large.

The receiving node MUST reject the contract if:

  • the chain_hash value is set to a hash of a chain that is unknown to the receiver.
  • the contract_info refers to events unknown to the receiver.
  • the contract_info refers to an oracle unknown or inaccessible to the receiver.
  • payout_spk or change_spk are not a standard script pubkey.
  • it considers feerate_per_vb too small for timely processing or unreasonably large.
  • funding_pubkey is not a valid secp256k1 pubkey in compressed format.
  • funding_inputs do not contribute at least total_collateral_satoshis plus full fee payment.
  • Any input_serial_id is duplicated
  • The fund_output_serial_id and change_serial_id are not set to different value
  • Any input in funding_inputs is not a BIP141 (Segregated Witness) input.

The accept_dlc Message

This message contains information about a node and indicates its acceptance of the new DLC, as well as its CET and refund transaction signatures. This is the second step toward creating the funding transaction and closing transactions.

  1. type: 42780 (accept_dlc_v0)
  2. data:
    • [32*byte:temporary_contract_id]
    • [u64:accept_collateral_satoshis]
    • [point:funding_pubkey]
    • [spk:payout_spk]
    • [u64:payout_serial_id]
    • [u16:num_funding_inputs]
    • [num_funding_inputs*funding_input:funding_inputs]
    • [spk:change_spk]
    • [u64:change_serial_id]
    • [cet_adaptor_signatures:cet_adaptor_signatures]
    • [signature:refund_signature]
    • [negotiation_fields:negotiation_fields]

Requirements

The temporary_contract_id MUST be the SHA256 hash of the offer_dlc message.

The sender MUST:

  • set accept_collateral_satoshis to equal the offer_dlc's contract_info total_collateral minus the offer_collateral_satoshis.
  • set payout_spk and change_spk to a standard script pubkey
  • set cet_adaptor_signatures to valid adaptor signatures, using its funding_pubkey for each CET, as defined in the transaction specification and using signature public keys computed using the offer_dlc's contract_info and oracle_info as adaptor points.
  • include an adaptor signature in cet_adaptor_signatures for every event specified in the offer_dlc's contract_info.
  • set refund_signature to the valid signature, using its funding_pubkey for the refund transaction, as defined in the transaction specification.
  • use a unique input_serial_id for each input
  • use a unique change_serial_id
  • set change_serial_id so that it is not equal to the fund_output_serial_id
  • use a unique payout_serial_id

The sender SHOULD:

  • set payout_spk to a previously unused script public key.
  • set change_spk to a previously unused script public key.

The receiver:

  • if accept_collateral_satoshis is not total_collateral - offer_collateral_satoshis:
    • MAY reject the contract.
  • if payout_spk or change_spk are not a standard script pubkey
    • MUST reject the contract.
  • if any input in funding_inputs is not a BIP141 (Segregated Witness) input.
    • MUST reject the contract.
  • if cet_adaptor_signatures or refund_signature fail validation:
    • MUST reject the contract.
  • if funding_inputs do not contribute at least accept_collateral_satoshis plus fee payment
    • MUST reject the contract.
  • if any input_serial_id is duplicated
    • MUST reject the contract.
  • if any change_serial_id is duplicated
    • MUST reject the contract.
  • if any change_serial_id is equal to the fund_output_serial_id
    • MUST reject the contract.
  • if any payout_serial_id is duplicated
    • MUST reject the contract.

Other fields have the same requirements as their counterparts in offer_dlc.

The sign_dlc Message

This message gives all of the initiator's signatures, which allows the receiver to broadcast the funding transaction with both parties being fully committed to all closing transactions.

This message introduces the contract_id to identify the contract.

  1. type: 42782 (sign_dlc_v0)
  2. data:
    • [contract_id:contract_id]
    • [cet_adaptor_signatures:cet_adaptor_signatures]
    • [signature:refund_signature]
    • [funding_signatures:funding_signatures]

Requirements

The sender MUST:

  • set contract_id by exclusive-OR of the funding_txid, the funding_output_index and the temporary_contract_id from the offer_dlc and accept_dlc messages.
  • set cet_adaptor_signatures to valid adaptor signatures, using its funding_pubkey for each CET, as defined in the transaction specification and using signature public keys computed using the offer_dlc's contract_info and oracle_info as adaptor points.
  • include an adaptor signature in cet_adaptor_signatures for every event specified in the offer_dlc's contract_info.
  • set refund_signature to the valid signature, using its funding_pubkey for the refund transaction, as defined in the transaction specification.
  • set funding_signatures to contain valid witnesses for every funding input specified in the offer_dlc message and in the same order.

The recipient:

  • if any signature or witness is incorrect:
    • MUST reject the contract.
  • if any witness exceeds its corresponding max_witness_len from the offer_dlc message:
    • MAY reject the contract.
    • MUST NOT broadcast the funding transaction before receipt of a valid sign_dlc.
    • on receipt of a valid sign_dlc:
      • SHOULD broadcast the funding transaction.

Script Pubkey Standardness Definition

For a script pub key to be valid it must be in one of the following forms:

1. `OP_DUP` `OP_HASH160` `20` 20-bytes `OP_EQUALVERIFY` `OP_CHECKSIG` (pay to pubkey hash), OR
2. `OP_HASH160` `20` 20-bytes `OP_EQUAL` (pay to script hash), OR
3. `OP_0` `20` 20-bytes (version 0 pay to witness pubkey hash), OR
4. `OP_0` `32` 32-bytes (version 0 pay to witness script hash), OR
5. `OP_1` through `OP_16` inclusive, followed by a single push of 2 to 40 bytes
   (witness program versions 1 through 16)

These script pub key forms include only standard forms accepted by the wider set of deployed Bitcoin clients in the network, which increase the chances of successful propagation to miners.

Authors

Nadav Kohen [email protected]

Ben Carman [email protected]

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.