ExFacto

A pure Elixir library for Schnorr & Taproot DLCs on Bitcoin.

This is nowhere near ready to be used, much less secure enough for real money. Use at your own risk. All contributions welcome.

Standards Compliance

For several reasons, we chose not to fully implement the DLC spec being worked on here.

• They use ECDSA for the parties and Schnorr only for the Oracle.
• Schnorr and ECDSA have different data requirements for Adaptor Signatures (Schnorr's are much smaller)
• The DLC spec is unnecessarily complex and verbose. We chose to drop the extra TLVs and Negotiation Fields from Offers and Accepts.
• We opted to sort inputs lexicographically by outpoint (txid:vout) and outputs lexicographically by scriptpubkey instead of using serial_ids to determine input and output ordering. See: BIP-69
• We will not support P2SH-wrapped SegWit inputs to funding transactions.
• We replaced temp_contract_id with offer_id. The offer_id is calcualted by taking the serialized Offer (with the offer_id field empty) as the preimage to a BIP340 tagged hash using DLC/contractor/offer/v0 as the tag.
• We also changed how the contract_id is calculated. The DLCSpec says that the vout should only affect the last 2 bytes of the id, but vout is a 4-byte field. Instead of only taking the last 2 bytes of the vout, we allow it to affect all 4 bytes. This might be a mistake in the DLCSpec. The contract_id is thus calculated as

XOR( XOR(funding_txid, offer_id), funding_vout )

With this said, we closely followed this spec as a guideline and significant thanks are owed to the creators of the spec for showing us how to build a DLC platform.

Installation

If available in Hex, the package can be installed by adding exfacto to your list of dependencies in mix.exs:

def deps do
  [
    {:exfacto, "~> 0.1.0"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/exfacto.