Add MessagePaddingPrototype feature and UserConfig option

We add prototypical support for the `option_message_padding` feature
while the BOLTs PR is still underway.

Author: tnull

lightning-types/src/features.rs

@@ -83,6 +83,8 @@
 //!   (see [BOLT PR #1160](https://github.com/lightning/bolts/pull/1160) for more information).
 //! - `HtlcHold` - requires/supports holding HTLCs and forwarding on receipt of an onion message
 //!   (see [BOLT-2](https://github.com/lightning/bolts/pull/989/files) for more information).
+//! - `MessagePaddingPrototype` - requires/supports padding of network messages for improved privacy
+//!   (see [BOLT-1](TODO) for more information).
 //!
 //! LDK knows about the following features, but does not support them:
 //! - `AnchorsNonzeroFeeHtlcTx` - the initial version of anchor outputs, which was later found to be
@@ -174,7 +176,7 @@ mod sealed {
 			// Byte 18
 			,
 			// Byte 19
-			HtlcHold | SplicePrototype,
+			HtlcHold | SplicePrototype | MessagePaddingPrototype,
 		]
 	);
 	define_context!(
@@ -732,6 +734,17 @@ mod sealed {
 		supports_splicing,
 		requires_splicing
 	);
+	define_feature!(
+		165, // BOLTs PR uses 64/65, so add +100
+		MessagePaddingPrototype,
+		[InitContext],
+		"Feature flags for network message padding.",
+		set_message_padding_optional,
+		set_message_padding_required,
+		clear_message_padding,
+		supports_message_padding,
+		requires_message_padding
+	);
 	define_feature!(
 		259,
 		DnsResolver,

lightning/src/ln/channelmanager.rs

@@ -15876,6 +15876,14 @@ pub fn provided_init_features(config: &UserConfig) -> InitFeatures {
 		features.set_htlc_hold_optional();
 	}
 
+	if config.enable_htlc_hold {
+		features.set_htlc_hold_optional();
+	}
+
+	if config.enable_message_padding {
+		features.set_message_padding_optional();
+	}
+
 	features
 }
 

lightning/src/util/config.rs

@@ -973,6 +973,14 @@ pub struct UserConfig {
 	///
 	/// [`ChannelManager::splice_channel`]: crate::ln::channelmanager::ChannelManager::splice_channel
 	pub reject_inbound_splices: bool,
+	/// If this set to `true`, then we will negotiate support for padding network messages with our
+	/// counterparty. If both parties agree, network messages will be padded to a fixed length to
+	/// improve privacy in the face of an adversary monitoring network traffic.
+	///
+	/// Nodes which are heavily bandwidth-restricted might want to set this to `false`.
+	///
+	/// Default value: `true`
+	pub enable_message_padding: bool,
 }
 
 impl Default for UserConfig {
@@ -990,6 +998,7 @@ impl Default for UserConfig {
 			enable_htlc_hold: false,
 			hold_outbound_htlcs_at_next_hop: false,
 			reject_inbound_splices: true,
+			enable_message_padding: true,
 		}
 	}
 }