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//! Implementation for encoding and decoding of ChanCells.
use super::{ChanCell, CELL_DATA_LEN};
use crate::chancell::{ChanCmd, ChanMsg, CircId};
use crate::Error;
use tor_bytes::{self, Reader, Writer};
use tor_error::internal;
use bytes::BytesMut;
/// This object can be used to encode and decode channel cells.
///
/// NOTE: only link protocol versions 3 and higher are supported.
/// VERSIONS cells are not supported via the encoder/decoder, since
/// VERSIONS cells always use a two-byte circuit-ID for backwards
/// compatibility with protocol versions < 4.
/// The implemented format is one of the following:
/// Variable-length cells (since protocol versions 2 and 3 respectively):
/// ```ignore
/// u32 circid;
/// u8 command;
/// u16 len;
/// u8 body[len];
/// ```
/// Fixed-width cells (since protocol version 1 and 4 respectively):
/// u8 body[509];
pub struct ChannelCodec {
#[allow(dead_code)] // We don't support any link versions where this matters
/// The link protocol version being used for this channel.
/// (We don't currently support any versions of the link protocol
/// where this version matters, but for protocol versions below 4, it would
/// affect the length of the circuit ID.)
link_version: u16,
}
impl ChannelCodec {
/// Create a new ChannelCodec with a given link protocol version
pub fn new(link_version: u16) -> Self {
ChannelCodec { link_version }
/// Write the given cell into the provided BytesMut object.
pub fn write_cell<M: ChanMsg>(
&mut self,
item: ChanCell<M>,
dst: &mut BytesMut,
) -> crate::Result<()> {
let ChanCell { circid, msg } = item;
let cmd = msg.cmd();
dst.write_u32(CircId::get_or_zero(circid));
dst.write_u8(cmd.into());
let pos = dst.len(); // always 5?
// now write the cell body and handle the length.
if cmd.is_var_cell() {
dst.write_u16(0);
msg.encode_onto(dst)?;
let len = dst.len() - pos - 2;
if len > u16::MAX as usize {
return Err(Error::Internal(internal!("ran out of space for varcell")));
// go back and set the length.
*(<&mut [u8; 2]>::try_from(&mut dst[pos..pos + 2])
.expect("two-byte slice was not two bytes!?")) = (len as u16).to_be_bytes();
} else {
let len = dst.len() - pos;
if len > CELL_DATA_LEN {
return Err(Error::Internal(internal!("ran out of space for cell")));
// pad to end of fixed-length cell
dst.write_zeros(CELL_DATA_LEN - len);
Ok(())
/// Try to decode a cell from the provided BytesMut object.
/// On a definite decoding error, return Err(_). On a cell that might
/// just be truncated, return Ok(None).
pub fn decode_cell<M: ChanMsg>(
src: &mut BytesMut,
) -> crate::Result<Option<ChanCell<M>>> {
/// Wrap `be` as an appropriate type.
fn wrap_err(be: tor_bytes::Error) -> crate::Error {
crate::Error::BytesErr {
err: be,
parsed: "channel cell",
if src.len() < 7 {
// Smallest possible command: varcell with len 0
return Ok(None);
let cmd: ChanCmd = src[4].into();
let varcell = cmd.is_var_cell();
let cell_len: usize = if varcell {
let msg_len = u16::from_be_bytes(
src[5..7]
.try_into()
.expect("Two-byte slice was not two bytes long!?"),
);
msg_len as usize + 7
514
};
if src.len() < cell_len {
let cell = src.split_to(cell_len).freeze();
//trace!("{:?} cell body ({}) is {:?}", cmd, cell.len(), &cell[..]);
let mut r = Reader::from_bytes(&cell);
let circid: Option<CircId> = CircId::new(r.take_u32().map_err(wrap_err)?);
r.advance(if varcell { 3 } else { 1 }).map_err(wrap_err)?;
let msg = M::decode_from_reader(cmd, &mut r).map_err(wrap_err)?;
if !cmd.accepts_circid_val(circid) {
return Err(Error::ChanProto(format!(
"Invalid circuit ID {} for cell command {}",
CircId::get_or_zero(circid),
cmd
)));
Ok(Some(ChanCell { circid, msg }))