@node-dlc/core/lib/lightning/CommitmentNumber.ts

DLC Core

import { LockTime, Sequence } from '@node-dlc/bitcoin';
import { bigFromBufBE } from '@node-dlc/bufio';
import { sha256 } from '@node-dlc/crypto';

export class CommitmentNumber {
  /**
   * Using the obscured commitment number, creates a Sequence with
   * the upper byte set to 0x80 and the lower three bytes set to the
   * upper 3 bytes of the 48-bit commitment number.
   *
   * The upper most byte is set to 0x80 so that the relative lock time
   * is disabled.
   *
   * @param commitment commitment number to obscure
   */
  public static getSequence(obscurred: bigint): Sequence {
    return new Sequence(
      Number((BigInt(0x80) << BigInt(24)) | (obscurred >> BigInt(24))),
    );
  }

  /**
   * Using the obscured commitment number, creates a LockTime with
   * the upper byte set to 0x20 and the lower three bytes set to the
   * lower 3 bytes of the 48-bit commitment number.
   *
   * The uppermost byte is set to 0x20 so the LockTime uses a time
   * based absolute locktime that has already expired.
   *
   * @param obscurred obscurred commitment number
   */
  public static getLockTime(obscurred: bigint): LockTime {
    return new LockTime(
      Number((BigInt(0x20) << BigInt(24)) | (obscurred & BigInt(0xffffff))),
    );
  }

  /**
   * Reveals the commitment number provided by the nSequence and
   * nLockTime values. This function reverses the obscurring using the
   * known payment base points.
   *
   * @param locktime
   * @param sequence
   */
  public static reveal(
    locktime: LockTime,
    sequence: Sequence,
    openBasePoint: Buffer,
    acceptBasePoint: Buffer,
  ): number {
    const hash = CommitmentNumber.getHash(openBasePoint, acceptBasePoint);
    const upper = BigInt(sequence.value & 0xffffff) << BigInt(24);
    const lower = BigInt(locktime.value & 0xffffff);
    return Number(hash ^ (upper | lower));
  }

  /**
   * Defined in BOLT3, the obscurred commitment number is attached to the
   * commitment transaction. The 48-bit commitment number is broken into
   * two pieces, the lower 3-bytes are obscured in nLocketime and the
   * upper 3-bytes are obscured in the funding input's nSequence.
   *
   * The obscurred commitment number is calculated as the:
   * sha256(open_channel payment_basepoint || accept_channel payment_basepoint) ^ commitment_number
   * @param commitment
   * @param openBasePoint
   * @param acceptBasePoint
   */
  public static obscure(
    commitment: number,
    openBasePoint: Buffer,
    acceptBasePoint: Buffer,
  ): bigint {
    return (
      CommitmentNumber.getHash(openBasePoint, acceptBasePoint) ^
      BigInt(commitment)
    );
  }

  /**
   * Obtains the lower 6-bytes from the sha256 of the open_channel
   * basepoint and the accept_channel basepoint.
   * @param openBasePoint
   * @param acceptBasePoint
   */
  private static getHash(
    openBasePoint: Buffer,
    acceptBasePoint: Buffer,
  ): bigint {
    const hash = sha256(Buffer.concat([openBasePoint, acceptBasePoint]));
    return bigFromBufBE(hash.slice(hash.length - 6));
  }
}