taproot-2nd-level-htlc-timeout-weights

htlc-timeout-weights.md

HTLC Timeout Transaction Weight

• htlc_timeout_script: 70 bytes

  • OP_DATA: 1 byte (local_htlcpubkey length)
  • local_htlcpubkey: 33 bytes
  • OP_CHECKSIGVERIFY: 1 byte
  • OP_DATA: 1 byte (remote_htlcpubkey length)
  • remote_htlcpubkey: 33 bytes
  • OP_CHECKSIG: 1 byte

• htlc_timeout_control_block: 65 bytes

  • c[0] & 0xfe: leaf version
  • c[1:33]: p
  • c[33:65]: hash of htlc_success

• htlc_timeout_input_witness: 269 bytes

  • number_of_witness_elements: 1 byte
  • local_schnorr_sig_length: 1 byte
  • local_schnorr_sig: 64 bytes
  • remote_schnorr_sig_length: 1 byte
  • remote_schnorr_sig: 65 bytes
  • htlc_timeout_script_length: 1 byte
  • htlc_timeout_script: 70 bytes
  • htlc_timeout_control_block_length: 1 byte
  • htlc_timeout_control_block: 65 bytes

• htlc_timeout_txin_0: 41 bytes (excl. witness)

  • previous_out_point: 36 bytes
    • hash: 32 bytes
    • index: 4 bytes
  • var_int: 1 byte (script_sig length)
  • script_sig: 0 bytes
  • witness: <---- part of the witness data
  • sequence: 4 bytes

• htlc_timeout_second_level_output: 43 bytes

  • value: 8 bytes
  • var_int: 1 byte (pk_script length)
  • pk_script (p2tr): 34 bytes

• htlc timeout transaction: 94 bytes (excl. witness)

  • version: 4 bytes
  • witness_header: <---- part of the witness data
  • count_tx_in: 1 byte
  • tx_in: 41 bytes
    • htlc_timeout_txin_0: 41 bytes
  • count_tx_out: 1 byte
  • tx_out: 43 bytes
    • htlc_timeout_second_level_output: 43 bytes
  • lock_time: 4 bytes

• Multiplying non-witness data by 4 gives a weight of:

  • htlc_timeout_transaction_weight = 94vbytes * 4 = 376WU

• Adding the witness data:

  • htlc_timeout_transaction_weight += (htlc_timeout_input_witness + witness_header)
  • htlc_timeout_transaction_weight = 647WU

• Minimum relay feerate is 250 satoshis per kWU

• 250sats/1000WU * 647WU = 161.75sats

• If 161.75sats is rounded down, then this is 161 satoshis and is below the minimum relay fee of 162 satoshis. Bitcoin Core will use a ceiling instead of the rounding down that the BOLTs currently use. See https://github.com/bitcoin/bitcoin/blob/74981aa02d2b14ad1c0b82d1eb09cf3169eaa8ae/src/policy/feerate.cpp#L29