Update Padding struct to contain padding length.

1. This allows setting the length of padding at the time of writing.
2. This will be used in the following commit to allow setting the
   padding for blinded message paths, and blinded payment paths.

Author: shaavan

lightning/src/blinded_path/utils.rs

@@ -18,9 +18,10 @@ use bitcoin::secp256k1::{self, PublicKey, Scalar, Secp256k1, SecretKey};
 use super::message::BlindedMessagePath;
 use super::{BlindedHop, BlindedPath};
 use crate::crypto::streams::ChaChaPolyWriteAdapter;
+use crate::io;
 use crate::ln::onion_utils;
 use crate::onion_message::messenger::Destination;
-use crate::util::ser::Writeable;
+use crate::util::ser::{Writeable, Writer};
 
 use core::borrow::Borrow;
 
@@ -197,4 +198,33 @@ fn encrypt_payload<P: Writeable>(payload: P, encrypted_tlvs_rho: [u8; 32]) -> Ve
 /// equal length. Padding is written at Type 1 for compatibility with the lightning specification.
 ///
 /// For more details, see the [BOLTs Specification - Encrypted Recipient Data](https://github.com/lightning/bolts/blob/8707471dbc23245fb4d84c5f5babac1197f1583e/04-onion-routing.md#inside-encrypted_recipient_data-encrypted_data_tlv).
-pub(crate) struct BlindedPathPadding {}
+pub(crate) struct BlindedPathPadding {
+	length: usize,
+}
+
+impl BlindedPathPadding {
+	/// Creates a new [`BlindedPathPadding`] instance with a specified size.
+	/// Use this method when defining the padding size before writing
+	/// an encrypted payload.
+	pub fn new(length: usize) -> Self {
+		Self { length }
+	}
+}
+
+impl Writeable for BlindedPathPadding {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		const BUFFER_SIZE: usize = 1024;
+		let buffer = [0u8; BUFFER_SIZE];
+
+		let mut remaining = self.length;
+		loop {
+			let to_write = core::cmp::min(remaining, BUFFER_SIZE);
+			writer.write_all(&buffer[..to_write])?;
+			remaining -= to_write;
+			if remaining == 0 {
+				break;
+			}
+		}
+		Ok(())
+	}
+}