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Implement Script for Witness and Add Tweak in PSBT.
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Adding Witness Script and key tweaks makes
a Partially Signed Bitcoin Transaction the single data
source needed for a Signer to produce valid signatures.
A Signer is not required to be able to generate L2 keys,
e.g delayed payment basepoint.
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@yellowred
yellowred committed Apr 18, 2024
1 parent 6264a44 commit e08f7de
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Showing 3 changed files with 188 additions and 48 deletions.
1 change: 1 addition & 0 deletions lightning/src/chain/channelmonitor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4330,6 +4330,7 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
revocation_pubkey: broadcasted_holder_revokable_script.2,
channel_keys_id: self.channel_keys_id,
channel_value_satoshis: self.channel_value_satoshis,
channel_transaction_parameters: Some(self.onchain_tx_handler.channel_transaction_parameters.clone()),
}));
}
}
Expand Down
110 changes: 79 additions & 31 deletions lightning/src/ln/channel_keys.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,19 +10,19 @@
//! Keys used to generate commitment transactions.
//! See: <https://github.com/lightning/bolts/blob/master/03-transactions.md#keys>
use bitcoin::hashes::Hash;
use bitcoin::hashes::HashEngine;
use bitcoin::secp256k1::Scalar;
use bitcoin::secp256k1::SecretKey;
use bitcoin::secp256k1::Secp256k1;
use bitcoin::secp256k1;
use crate::io;
use crate::ln::msgs::DecodeError;
use crate::util::ser::Readable;
use crate::io;
use crate::util::ser::Writer;
use crate::util::ser::Writeable;
use bitcoin::secp256k1::PublicKey;
use crate::util::ser::Writer;
use bitcoin::hashes::sha256::Hash as Sha256;
use bitcoin::hashes::Hash;
use bitcoin::hashes::HashEngine;
use bitcoin::secp256k1;
use bitcoin::secp256k1::PublicKey;
use bitcoin::secp256k1::Scalar;
use bitcoin::secp256k1::Secp256k1;
use bitcoin::secp256k1::SecretKey;

macro_rules! doc_comment {
($x:expr, $($tt:tt)*) => {
Expand All @@ -37,15 +37,28 @@ macro_rules! basepoint_impl {
pub fn to_public_key(&self) -> PublicKey {
self.0
}

/// Derives a per-commitment-transaction (eg an htlc key or delayed_payment key) private key addition tweak
/// from a basepoint and a per_commitment_point:
/// `privkey = basepoint_secret + SHA256(per_commitment_point || basepoint)`
/// This calculates the hash part in the tweak derivation process, which is used to ensure
/// that each key is unique and cannot be guessed by an external party. It is equivalent
/// to the `from_basepoint` method, but without the addition operation, providing just the
/// tweak from the hash of the per_commitment_point and the basepoint.
pub fn derive_add_tweak(&self, per_commitment_point: &PublicKey) -> [u8; 32] {
let mut sha = Sha256::engine();
sha.input(&per_commitment_point.serialize());
sha.input(&self.to_public_key().serialize());
Sha256::from_engine(sha).to_byte_array()
}
}

impl From<PublicKey> for $BasepointT {
fn from(value: PublicKey) -> Self {
Self(value)
}
}

}
};
}
macro_rules! key_impl {
($BasepointT:ty, $KeyName:expr) => {
Expand Down Expand Up @@ -87,11 +100,9 @@ macro_rules! key_read_write {
Ok(Self(key))
}
}
}
};
}



/// Base key used in conjunction with a `per_commitment_point` to generate a [`DelayedPaymentKey`].
///
/// The delayed payment key is used to pay the commitment state broadcaster their
Expand All @@ -102,7 +113,6 @@ pub struct DelayedPaymentBasepoint(pub PublicKey);
basepoint_impl!(DelayedPaymentBasepoint);
key_read_write!(DelayedPaymentBasepoint);


/// A derived key built from a [`DelayedPaymentBasepoint`] and `per_commitment_point`.
///
/// The delayed payment key is used to pay the commitment state broadcaster their
Expand Down Expand Up @@ -150,14 +160,26 @@ key_read_write!(HtlcKey);
/// Derives a per-commitment-transaction public key (eg an htlc key or a delayed_payment key)
/// from the base point and the per_commitment_key. This is the public equivalent of
/// derive_private_key - using only public keys to derive a public key instead of private keys.
fn derive_public_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_point: &PublicKey) -> PublicKey {
fn derive_public_key<T: secp256k1::Signing>(
secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_point: &PublicKey,
) -> PublicKey {
let mut sha = Sha256::engine();
sha.input(&per_commitment_point.serialize());
sha.input(&base_point.serialize());
let res = Sha256::from_engine(sha).to_byte_array();

let hashkey = PublicKey::from_secret_key(&secp_ctx,
&SecretKey::from_slice(&res).expect("Hashes should always be valid keys unless SHA-256 is broken"));
add_public_key_tweak(secp_ctx, base_point, &res)
}

/// Adds a tweak to a public key to derive a new public key.
pub fn add_public_key_tweak<T: secp256k1::Signing>(
secp_ctx: &Secp256k1<T>, base_point: &PublicKey, tweak: &[u8; 32],
) -> PublicKey {
let hashkey = PublicKey::from_secret_key(
&secp_ctx,
&SecretKey::from_slice(tweak)
.expect("Hashes should always be valid keys unless SHA-256 is broken"),
);
base_point.combine(&hashkey)
.expect("Addition only fails if the tweak is the inverse of the key. This is not possible when the tweak contains the hash of the key.")
}
Expand All @@ -169,7 +191,6 @@ pub struct RevocationBasepoint(pub PublicKey);
basepoint_impl!(RevocationBasepoint);
key_read_write!(RevocationBasepoint);


/// The revocation key is used to allow a channel party to revoke their state - giving their
/// counterparty the required material to claim all of their funds if they broadcast that state.
///
Expand All @@ -192,8 +213,7 @@ impl RevocationKey {
///
/// [`chan_utils::derive_private_revocation_key`]: crate::ln::chan_utils::derive_private_revocation_key
pub fn from_basepoint<T: secp256k1::Verification>(
secp_ctx: &Secp256k1<T>,
countersignatory_basepoint: &RevocationBasepoint,
secp_ctx: &Secp256k1<T>, countersignatory_basepoint: &RevocationBasepoint,
per_commitment_point: &PublicKey,
) -> Self {
let rev_append_commit_hash_key = {
Expand Down Expand Up @@ -227,28 +247,56 @@ impl RevocationKey {
}
key_read_write!(RevocationKey);


#[cfg(test)]
mod test {
use bitcoin::secp256k1::{Secp256k1, SecretKey, PublicKey};
use bitcoin::hashes::hex::FromHex;
use super::derive_public_key;
use bitcoin::hashes::hex::FromHex;
use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};

#[test]
fn test_key_derivation() {
// Test vectors from BOLT 3 Appendix E:
let secp_ctx = Secp256k1::new();

let base_secret = SecretKey::from_slice(&<Vec<u8>>::from_hex("000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f").unwrap()[..]).unwrap();
let per_commitment_secret = SecretKey::from_slice(&<Vec<u8>>::from_hex("1f1e1d1c1b1a191817161514131211100f0e0d0c0b0a09080706050403020100").unwrap()[..]).unwrap();
let base_secret = SecretKey::from_slice(
&<Vec<u8>>::from_hex(
"000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f",
)
.unwrap()[..],
)
.unwrap();
let per_commitment_secret = SecretKey::from_slice(
&<Vec<u8>>::from_hex(
"1f1e1d1c1b1a191817161514131211100f0e0d0c0b0a09080706050403020100",
)
.unwrap()[..],
)
.unwrap();

let base_point = PublicKey::from_secret_key(&secp_ctx, &base_secret);
assert_eq!(base_point.serialize()[..], <Vec<u8>>::from_hex("036d6caac248af96f6afa7f904f550253a0f3ef3f5aa2fe6838a95b216691468e2").unwrap()[..]);
assert_eq!(
base_point.serialize()[..],
<Vec<u8>>::from_hex(
"036d6caac248af96f6afa7f904f550253a0f3ef3f5aa2fe6838a95b216691468e2"
)
.unwrap()[..]
);

let per_commitment_point = PublicKey::from_secret_key(&secp_ctx, &per_commitment_secret);
assert_eq!(per_commitment_point.serialize()[..], <Vec<u8>>::from_hex("025f7117a78150fe2ef97db7cfc83bd57b2e2c0d0dd25eaf467a4a1c2a45ce1486").unwrap()[..]);

assert_eq!(derive_public_key(&secp_ctx, &per_commitment_point, &base_point).serialize()[..],
<Vec<u8>>::from_hex("0235f2dbfaa89b57ec7b055afe29849ef7ddfeb1cefdb9ebdc43f5494984db29e5").unwrap()[..]);
assert_eq!(
per_commitment_point.serialize()[..],
<Vec<u8>>::from_hex(
"025f7117a78150fe2ef97db7cfc83bd57b2e2c0d0dd25eaf467a4a1c2a45ce1486"
)
.unwrap()[..]
);

assert_eq!(
derive_public_key(&secp_ctx, &per_commitment_point, &base_point).serialize()[..],
<Vec<u8>>::from_hex(
"0235f2dbfaa89b57ec7b055afe29849ef7ddfeb1cefdb9ebdc43f5494984db29e5"
)
.unwrap()[..]
);
}
}
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