rust-lightning/src/ln/chan_utils.rs

use bitcoin::blockdata::script::{Script,Builder};
use bitcoin::blockdata::opcodes;
use bitcoin::blockdata::transaction::{TxIn,TxOut,OutPoint,Transaction};
use bitcoin::util::hash::{Sha256dHash};

use bitcoin_hashes::{Hash, HashEngine};
use bitcoin_hashes::sha256::Hash as Sha256;
use bitcoin_hashes::ripemd160::Hash as Ripemd160;
use bitcoin_hashes::hash160::Hash as Hash160;

use ln::channelmanager::PaymentHash;

use secp256k1::key::{PublicKey,SecretKey};
use secp256k1::Secp256k1;
use secp256k1;

pub const HTLC_SUCCESS_TX_WEIGHT: u64 = 703;
pub const HTLC_TIMEOUT_TX_WEIGHT: u64 = 663;

// Various functions for key derivation and transaction creation for use within channels. Primarily
// used in Channel and ChannelMonitor.

pub fn build_commitment_secret(commitment_seed: [u8; 32], idx: u64) -> [u8; 32] {
	let mut res: [u8; 32] = commitment_seed;
	for i in 0..48 {
		let bitpos = 47 - i;
		if idx & (1 << bitpos) == (1 << bitpos) {
			res[bitpos / 8] ^= 1 << (bitpos & 7);
			res = Sha256::hash(&res).into_inner();
		}
	}
	res
}

pub fn derive_private_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_secret: &SecretKey) -> Result<SecretKey, secp256k1::Error> {
	let mut sha = Sha256::engine();
	sha.input(&per_commitment_point.serialize());
	sha.input(&PublicKey::from_secret_key(&secp_ctx, &base_secret).serialize());
	let res = Sha256::from_engine(sha).into_inner();

	let mut key = base_secret.clone();
	key.add_assign(&res)?;
	Ok(key)
}

pub fn derive_public_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_point: &PublicKey) -> Result<PublicKey, secp256k1::Error> {
	let mut sha = Sha256::engine();
	sha.input(&per_commitment_point.serialize());
	sha.input(&base_point.serialize());
	let res = Sha256::from_engine(sha).into_inner();

	let hashkey = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&res)?);
	base_point.combine(&hashkey)
}

/// Derives a revocation key from its constituent parts
pub fn derive_private_revocation_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_secret: &SecretKey, revocation_base_secret: &SecretKey) -> Result<SecretKey, secp256k1::Error> {
	let revocation_base_point = PublicKey::from_secret_key(&secp_ctx, &revocation_base_secret);
	let per_commitment_point = PublicKey::from_secret_key(&secp_ctx, &per_commitment_secret);

	let rev_append_commit_hash_key = {
		let mut sha = Sha256::engine();
		sha.input(&revocation_base_point.serialize());
		sha.input(&per_commitment_point.serialize());

		Sha256::from_engine(sha).into_inner()
	};
	let commit_append_rev_hash_key = {
		let mut sha = Sha256::engine();
		sha.input(&per_commitment_point.serialize());
		sha.input(&revocation_base_point.serialize());

		Sha256::from_engine(sha).into_inner()
	};

	let mut part_a = revocation_base_secret.clone();
	part_a.mul_assign(&rev_append_commit_hash_key)?;
	let mut part_b = per_commitment_secret.clone();
	part_b.mul_assign(&commit_append_rev_hash_key)?;
	part_a.add_assign(&part_b[..])?;
	Ok(part_a)
}

pub fn derive_public_revocation_key<T: secp256k1::Verification>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, revocation_base_point: &PublicKey) -> Result<PublicKey, secp256k1::Error> {
	let rev_append_commit_hash_key = {
		let mut sha = Sha256::engine();
		sha.input(&revocation_base_point.serialize());
		sha.input(&per_commitment_point.serialize());

		Sha256::from_engine(sha).into_inner()
	};
	let commit_append_rev_hash_key = {
		let mut sha = Sha256::engine();
		sha.input(&per_commitment_point.serialize());
		sha.input(&revocation_base_point.serialize());

		Sha256::from_engine(sha).into_inner()
	};

	let mut part_a = revocation_base_point.clone();
	part_a.mul_assign(&secp_ctx, &rev_append_commit_hash_key)?;
	let mut part_b = per_commitment_point.clone();
	part_b.mul_assign(&secp_ctx, &commit_append_rev_hash_key)?;
	part_a.combine(&part_b)
}

pub struct TxCreationKeys {
	pub per_commitment_point: PublicKey,
	pub revocation_key: PublicKey,
	pub a_htlc_key: PublicKey,
	pub b_htlc_key: PublicKey,
	pub a_delayed_payment_key: PublicKey,
	pub b_payment_key: PublicKey,
}

impl TxCreationKeys {
	pub fn new<T: secp256k1::Signing + secp256k1::Verification>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, a_delayed_payment_base: &PublicKey, a_htlc_base: &PublicKey, b_revocation_base: &PublicKey, b_payment_base: &PublicKey, b_htlc_base: &PublicKey) -> Result<TxCreationKeys, secp256k1::Error> {
		Ok(TxCreationKeys {
			per_commitment_point: per_commitment_point.clone(),
			revocation_key: derive_public_revocation_key(&secp_ctx, &per_commitment_point, &b_revocation_base)?,
			a_htlc_key: derive_public_key(&secp_ctx, &per_commitment_point, &a_htlc_base)?,
			b_htlc_key: derive_public_key(&secp_ctx, &per_commitment_point, &b_htlc_base)?,
			a_delayed_payment_key: derive_public_key(&secp_ctx, &per_commitment_point, &a_delayed_payment_base)?,
			b_payment_key: derive_public_key(&secp_ctx, &per_commitment_point, &b_payment_base)?,
		})
	}
}

/// Gets the "to_local" output redeemscript, ie the script which is time-locked or spendable by
/// the revocation key
pub fn get_revokeable_redeemscript(revocation_key: &PublicKey, to_self_delay: u16, delayed_payment_key: &PublicKey) -> Script {
	Builder::new().push_opcode(opcodes::all::OP_IF)
	              .push_slice(&revocation_key.serialize())
	              .push_opcode(opcodes::all::OP_ELSE)
	              .push_int(to_self_delay as i64)
	              .push_opcode(opcodes::OP_CSV)
	              .push_opcode(opcodes::all::OP_DROP)
	              .push_slice(&delayed_payment_key.serialize())
	              .push_opcode(opcodes::all::OP_ENDIF)
	              .push_opcode(opcodes::all::OP_CHECKSIG)
	              .into_script()
}

#[derive(Clone, PartialEq)]
pub struct HTLCOutputInCommitment {
	pub offered: bool,
	pub amount_msat: u64,
	pub cltv_expiry: u32,
	pub payment_hash: PaymentHash,
	pub transaction_output_index: Option<u32>,
}

#[inline]
pub fn get_htlc_redeemscript_with_explicit_keys(htlc: &HTLCOutputInCommitment, a_htlc_key: &PublicKey, b_htlc_key: &PublicKey, revocation_key: &PublicKey) -> Script {
	let payment_hash160 = Ripemd160::hash(&htlc.payment_hash.0[..]).into_inner();
	if htlc.offered {
		Builder::new().push_opcode(opcodes::all::OP_DUP)
		              .push_opcode(opcodes::all::OP_HASH160)
		              .push_slice(&Hash160::hash(&revocation_key.serialize())[..])
		              .push_opcode(opcodes::all::OP_EQUAL)
		              .push_opcode(opcodes::all::OP_IF)
		              .push_opcode(opcodes::all::OP_CHECKSIG)
		              .push_opcode(opcodes::all::OP_ELSE)
		              .push_slice(&b_htlc_key.serialize()[..])
		              .push_opcode(opcodes::all::OP_SWAP)
		              .push_opcode(opcodes::all::OP_SIZE)
		              .push_int(32)
		              .push_opcode(opcodes::all::OP_EQUAL)
		              .push_opcode(opcodes::all::OP_NOTIF)
		              .push_opcode(opcodes::all::OP_DROP)
		              .push_int(2)
		              .push_opcode(opcodes::all::OP_SWAP)
		              .push_slice(&a_htlc_key.serialize()[..])
		              .push_int(2)
		              .push_opcode(opcodes::all::OP_CHECKMULTISIG)
		              .push_opcode(opcodes::all::OP_ELSE)
		              .push_opcode(opcodes::all::OP_HASH160)
		              .push_slice(&payment_hash160)
		              .push_opcode(opcodes::all::OP_EQUALVERIFY)
		              .push_opcode(opcodes::all::OP_CHECKSIG)
		              .push_opcode(opcodes::all::OP_ENDIF)
		              .push_opcode(opcodes::all::OP_ENDIF)
		              .into_script()
	} else {
		Builder::new().push_opcode(opcodes::all::OP_DUP)
		              .push_opcode(opcodes::all::OP_HASH160)
		              .push_slice(&Hash160::hash(&revocation_key.serialize())[..])
		              .push_opcode(opcodes::all::OP_EQUAL)
		              .push_opcode(opcodes::all::OP_IF)
		              .push_opcode(opcodes::all::OP_CHECKSIG)
		              .push_opcode(opcodes::all::OP_ELSE)
		              .push_slice(&b_htlc_key.serialize()[..])
		              .push_opcode(opcodes::all::OP_SWAP)
		              .push_opcode(opcodes::all::OP_SIZE)
		              .push_int(32)
		              .push_opcode(opcodes::all::OP_EQUAL)
		              .push_opcode(opcodes::all::OP_IF)
		              .push_opcode(opcodes::all::OP_HASH160)
		              .push_slice(&payment_hash160)
		              .push_opcode(opcodes::all::OP_EQUALVERIFY)
		              .push_int(2)
		              .push_opcode(opcodes::all::OP_SWAP)
		              .push_slice(&a_htlc_key.serialize()[..])
		              .push_int(2)
		              .push_opcode(opcodes::all::OP_CHECKMULTISIG)
		              .push_opcode(opcodes::all::OP_ELSE)
		              .push_opcode(opcodes::all::OP_DROP)
		              .push_int(htlc.cltv_expiry as i64)
		              .push_opcode(opcodes::OP_CLTV)
		              .push_opcode(opcodes::all::OP_DROP)
		              .push_opcode(opcodes::all::OP_CHECKSIG)
		              .push_opcode(opcodes::all::OP_ENDIF)
		              .push_opcode(opcodes::all::OP_ENDIF)
		              .into_script()
	}
}

/// note here that 'a_revocation_key' is generated using b_revocation_basepoint and a's
/// commitment secret. 'htlc' does *not* need to have its previous_output_index filled.
#[inline]
pub fn get_htlc_redeemscript(htlc: &HTLCOutputInCommitment, keys: &TxCreationKeys) -> Script {
	get_htlc_redeemscript_with_explicit_keys(htlc, &keys.a_htlc_key, &keys.b_htlc_key, &keys.revocation_key)
}

/// panics if htlc.transaction_output_index.is_none()!
pub fn build_htlc_transaction(prev_hash: &Sha256dHash, feerate_per_kw: u64, to_self_delay: u16, htlc: &HTLCOutputInCommitment, a_delayed_payment_key: &PublicKey, revocation_key: &PublicKey) -> Transaction {
	let mut txins: Vec<TxIn> = Vec::new();
	txins.push(TxIn {
		previous_output: OutPoint {
			txid: prev_hash.clone(),
			vout: htlc.transaction_output_index.expect("Can't build an HTLC transaction for a dust output"),
		},
		script_sig: Script::new(),
		sequence: 0,
		witness: Vec::new(),
	});

	let total_fee = if htlc.offered {
			feerate_per_kw * HTLC_TIMEOUT_TX_WEIGHT / 1000
		} else {
			feerate_per_kw * HTLC_SUCCESS_TX_WEIGHT / 1000
		};

	let mut txouts: Vec<TxOut> = Vec::new();
	txouts.push(TxOut {
		script_pubkey: get_revokeable_redeemscript(revocation_key, to_self_delay, a_delayed_payment_key).to_v0_p2wsh(),
		value: htlc.amount_msat / 1000 - total_fee //TODO: BOLT 3 does not specify if we should add amount_msat before dividing or if we should divide by 1000 before subtracting (as we do here)
	});

	Transaction {
		version: 2,
		lock_time: if htlc.offered { htlc.cltv_expiry } else { 0 },
		input: txins,
		output: txouts,
	}
}