Add a ChaChaDualPolyReadAdapter that allows an optional AAD

`ChaChaPolyReadAdapter` decodes an arbitrary object and checks the
poly1305 tag. In the coming commits, we'll need a variant of this
which allows for an *optional* AAD in the poly1305 tag, accepting
either tag as valid, but indicating to the caller whether the AAD
was used.

We could use the actual AAD setup in poly1305, which puts the AAD
first in the MAC (and then pads it out to a multiple of 16 bytes),
but since we're gonna check both with and without, its nice to
instead put the AAD at the end, enabling us to only calculate most
of the hash once before cloning its state and adding the AAD block.

We do this by swapping the AAD and the data being MAC'd in the
AAD-containing MAC check (but leaving them where they belong for
the non-AAD-containing MAC check).

Author: TheBlueMatt

lightning/src/crypto/streams.rs

@@ -1,5 +1,7 @@
 use crate::crypto::chacha20::ChaCha20;
 use crate::crypto::chacha20poly1305rfc::ChaCha20Poly1305RFC;
+use crate::crypto::poly1305::Poly1305;
+use crate::crypto::fixed_time_eq;
 
 use crate::io::{self, Read, Write};
 use crate::ln::msgs::DecodeError;
@@ -49,6 +51,104 @@ impl<'a, T: Writeable> Writeable for ChaChaPolyWriteAdapter<'a, T> {
 	}
 }
 
+/// Enables the use of the serialization macros for objects that need to be simultaneously decrypted
+/// and deserialized. This allows us to avoid an intermediate Vec allocation.
+///
+/// This variant of [`ChaChaPolyReadAdapter`] calculates Poly1305 tags twice, once using the given
+/// key and once with the given 32-byte AAD appended after the encrypted stream, accepting either
+/// being correct as sufficient.
+///
+/// Note that we do *not* use the provided AAD as the standard ChaCha20Poly1305 AAD as that would
+/// require placing it first and prevent us from avoiding redundant Poly1305 rounds. Instead, the
+/// ChaCha20Poly1305 MAC check is tweaked to move the AAD to *after* the the contents being
+/// checked, effectively treating the contents as the AAD for the AAD-containing MAC but behaving
+/// like classic ChaCha20Poly1305 for the non-AAD-containing MAC.
+pub(crate) struct ChaChaDualPolyReadAdapter<R: Readable> {
+	pub readable: R,
+	pub used_aad: bool,
+}
+
+impl<T: Readable> LengthReadableArgs<([u8; 32], [u8; 32])> for ChaChaDualPolyReadAdapter<T> {
+	// Simultaneously read and decrypt an object from a LengthLimitedRead storing it in
+	// Self::readable. LengthLimitedRead must be used instead of std::io::Read because we need the
+	// total length to separate out the tag at the end.
+	fn read<R: LengthLimitedRead>(r: &mut R, params: ([u8; 32], [u8; 32])) -> Result<Self, DecodeError> {
+		if r.remaining_bytes() < 16 {
+			return Err(DecodeError::InvalidValue);
+		}
+		let (key, aad) = params;
+
+		let mut chacha = ChaCha20::new(&key[..], &[0; 12]);
+		let mut mac_key = [0u8; 64];
+		chacha.process_in_place(&mut mac_key);
+
+		let mut mac = Poly1305::new(&mac_key[..32]);
+
+		let decrypted_len = r.remaining_bytes() - 16;
+		let s = FixedLengthReader::new(r, decrypted_len);
+		let mut chacha_stream = ChaChaDualPolyReader {
+			chacha: &mut chacha,
+			poly: &mut mac,
+			read_len: 0,
+			read: s,
+		};
+
+		let readable: T = Readable::read(&mut chacha_stream)?;
+		chacha_stream.read.eat_remaining()?;
+
+		let read_len = chacha_stream.read_len;
+
+		if read_len % 16 != 0 {
+			mac.input(&[0; 16][0..16 - (read_len % 16)]);
+		}
+
+		let mut mac_aad = mac;
+
+		mac_aad.input(&aad[..]);
+		// Note that we don't need to pad the AAD since its a multiple of 16 bytes
+
+		// For the AAD-containing MAC, swap the AAD and the read data, effectively.
+		mac_aad.input(&(read_len as u64).to_le_bytes());
+		mac_aad.input(&32u64.to_le_bytes());
+
+		// For the non-AAD-containing MAC, leave the data and AAD where they belong.
+		mac.input(&0u64.to_le_bytes());
+		mac.input(&(read_len as u64).to_le_bytes());
+
+		let mut tag = [0 as u8; 16];
+		r.read_exact(&mut tag)?;
+		if fixed_time_eq(&mac.result(), &tag) {
+			Ok(Self { readable, used_aad: false })
+		} else if fixed_time_eq(&mac_aad.result(), &tag) {
+			Ok(Self { readable, used_aad: true })
+		} else {
+			return Err(DecodeError::InvalidValue);
+		}
+	}
+}
+
+struct ChaChaDualPolyReader<'a, R: Read> {
+	chacha: &'a mut ChaCha20,
+	poly: &'a mut Poly1305,
+	read_len: usize,
+	pub read: R,
+}
+
+impl<'a, R: Read> Read for ChaChaDualPolyReader<'a, R> {
+	// Decrypt bytes from Self::read into `dest`.
+	// `ChaCha20Poly1305RFC::finish_and_check_tag` must be called to check the tag after all reads
+	// complete.
+	fn read(&mut self, dest: &mut [u8]) -> Result<usize, io::Error> {
+		let res = self.read.read(dest)?;
+		if res > 0 {
+			self.poly.input(&dest[0..res]);
+			self.chacha.process_in_place(&mut dest[0..res]);
+			self.read_len += res;
+		}
+		Ok(res)
+	}
+}
+
 /// Enables the use of the serialization macros for objects that need to be simultaneously decrypted and
 /// deserialized. This allows us to avoid an intermediate Vec allocation.
 pub(crate) struct ChaChaPolyReadAdapter<R: Readable> {