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

@@ -20,7 +20,7 @@ use crate::ln::msgs::DecodeError;
 use crate::ln::onion_utils;
 use crate::onion_message::messenger::Destination;
 use crate::crypto::streams::ChaChaPolyWriteAdapter;
-use crate::util::ser::{Readable, Writeable};
+use crate::util::ser::{Readable, Writeable, Writer};
 
 use crate::io;
 
@@ -135,17 +135,44 @@ fn encrypt_payload<P: Writeable>(payload: P, encrypted_tlvs_rho: [u8; 32]) -> Ve
 	write_adapter.encode()
 }
 
-/// Blinded path encrypted payloads may be padded to ensure they are equal length.
-///
-/// Reads padding to the end, ignoring what's read.
-pub(crate) struct Padding {}
+/// Represents optional padding for encrypted payloads.
+/// Padding is used to ensure payloads have a consistent length.
+pub(crate) struct Padding {
+	length: usize,
+}
+
+impl Padding {
+	/// Creates a new [`Padding`] 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 Readable for Padding {
 	#[inline]
 	fn read<R: io::Read>(reader: &mut R) -> Result<Self, DecodeError> {
 		loop {
 			let mut buf = [0; 8192];
 			if reader.read(&mut buf[..])? == 0 { break; }
 		}
-		Ok(Self {})
+		Ok(Self::new(0))
+	}
+}
+
+impl Writeable for Padding {
+	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(())
 	}
 }