Add WalletInterface trait to handle keys management, first part: when ChannelMonitor

detects a spendable outputs onchain it will send back an Event to user
wallet with needed data to be at disposal

Add CustomOutputScriptDescriptor to ease user wallet spending of onchain
lightning outputs

Extend KeyStorage with delayed_payment_base_key and per_commitment_point
to derive local_delayed private key

Author: Antoine Riard

src/chain/keysinterface.rs

@@ -0,0 +1,68 @@
+//! WalletInterface is *not* a wallet, only an interface to bridge between
+//! user wallet and ChannelMonitor. If this last one discover on-chain outputs they will
+//! be send with associate data as events::Event::SpendableOutputs to be at the disposal of wallet.
+//!
+//! KeysInterface is no more a wallet, just an entity to get secret from user wallet and derive
+//! appropriate keyring materials to others lightning components, as such node_id, destination_script.
+//!
+
+use bitcoin::blockdata::transaction::OutPoint;
+use bitcoin::blockdata::script::Script;
+
+use secp256k1::key::SecretKey;
+
+use util::events;
+
+/// A trait to describe a wallet which sould receive data to be able to spend onchain outputs
+/// fron a lightning channel
+pub trait WalletInterface: Send + Sync {
+	/// Handle an incoming SpendableOutputs event from SimpleManyChannelMonitor containing a 
+	/// CustomOutputScriptDesctitpor. Follow doc of the latter to know how to spend the output.
+	fn handle_spendable_output(&self, event: events::Event);
+}
+
+/// Hacky custom output script descriptors to ease spending of onchain outputs by user wallet
+/// Maybe should be changed by real ones when merged into rust-bitcoin.
+/// StaticOutputs commit to a static pubkey, i.e one derived once for node operation lifetime.
+/// DynamicOutputs commit to a dynamic local_delayedpubkey, i.e one which change for each per_commitment_point
+pub enum CustomOutputScriptDescriptor {
+	/// Outpoint commits to a P2PWKH, should be spend by the following witness :
+	/// <signature> <pubkey>
+	/// With pubkey being bip32 /1' from HMAC-Sha512 of user-provided seed as master private key
+	StaticOutput {
+		/// Outpoint spendable by user wallet
+		outpoint: OutPoint,
+	},
+	/// Outpoint commits to a P2WSH, should be spend by the following witness :
+	/// <local_delayedsig> 0 <witnessScript>
+	/// With input nSequence set to_self_delay.
+	DynamicOutput {
+		/// Outpoint spendable by user wallet
+		outpoint: OutPoint,
+		/// local_delayedkey = delayed_payment_basepoint_secret + SHA256(per_commitment_point || delayed_payment_basepoint
+		local_delayedkey: SecretKey,
+		/// witness redeemScript encumbering output
+		witness_script: Script,
+		/// nSequence input must commit to self_delay to satisfy script's OP_CSV
+		to_self_delay: u16,
+	}
+}
+
+impl CustomOutputScriptDescriptor {
+	/// Build a StaticOuput descriptor
+	pub fn static_key(outpoint: OutPoint) -> Self {
+		CustomOutputScriptDescriptor::StaticOutput {
+			outpoint,
+		}
+	}
+
+	/// Build a DynamicOuput descriptor
+	pub fn dynamic_key(outpoint: OutPoint, local_delayedkey: SecretKey, witness_script: Script, to_self_delay: u16) -> Self {
+		CustomOutputScriptDescriptor::DynamicOutput {
+			outpoint,
+			local_delayedkey,
+			witness_script,
+			to_self_delay,
+		}
+	}
+}

src/chain/mod.rs

@@ -2,3 +2,4 @@
 
 pub mod chaininterface;
 pub mod transaction;
+pub mod keysinterface;

src/ln/channel.rs

@@ -491,7 +491,7 @@ impl Channel {
 		let our_channel_monitor_claim_script = Builder::new().push_opcode(opcodes::All::OP_PUSHBYTES_0).push_slice(&our_channel_monitor_claim_key_hash[..]).into_script();
 		let channel_monitor = ChannelMonitor::new(&chan_keys.revocation_base_key,
 		                                          &PublicKey::from_secret_key(&secp_ctx, &chan_keys.delayed_payment_base_key),
-		                                          &chan_keys.htlc_base_key,
+		                                          &chan_keys.htlc_base_key, &chan_keys.delayed_payment_base_key,
 		                                          BREAKDOWN_TIMEOUT, our_channel_monitor_claim_script);
 
 		Ok(Channel {
@@ -653,7 +653,7 @@ impl Channel {
 		let our_channel_monitor_claim_script = Builder::new().push_opcode(opcodes::All::OP_PUSHBYTES_0).push_slice(&our_channel_monitor_claim_key_hash[..]).into_script();
 		let mut channel_monitor = ChannelMonitor::new(&chan_keys.revocation_base_key,
 		                                              &PublicKey::from_secret_key(&secp_ctx, &chan_keys.delayed_payment_base_key),
-		                                              &chan_keys.htlc_base_key,
+		                                              &chan_keys.htlc_base_key, &chan_keys.delayed_payment_base_key,
 		                                              BREAKDOWN_TIMEOUT, our_channel_monitor_claim_script);
 		channel_monitor.set_their_base_keys(&msg.htlc_basepoint, &msg.delayed_payment_basepoint);
 		channel_monitor.set_their_to_self_delay(msg.to_self_delay);