rustls/rustls/src/msgs/fragmenter.rs

use crate::enums::ContentType;
use crate::enums::ProtocolVersion;
use crate::msgs::message::{OutboundChunks, OutboundMessage, PlainMessage};
use crate::Error;
pub(crate) const MAX_FRAGMENT_LEN: usize = 16384;
pub(crate) const PACKET_OVERHEAD: usize = 1 + 2 + 2;
pub(crate) const MAX_FRAGMENT_SIZE: usize = MAX_FRAGMENT_LEN + PACKET_OVERHEAD;

pub struct MessageFragmenter {
    max_frag: usize,
}

impl Default for MessageFragmenter {
    fn default() -> Self {
        Self {
            max_frag: MAX_FRAGMENT_LEN,
        }
    }
}

impl MessageFragmenter {
    /// Take the Message `msg` and re-fragment it into new
    /// messages whose fragment is no more than max_frag.
    /// Return an iterator across those messages.
    /// Payloads are borrowed.
    pub fn fragment_message<'a>(
        &self,
        msg: &'a PlainMessage,
    ) -> impl Iterator<Item = OutboundMessage<'a>> + 'a {
        self.fragment_payload(msg.typ, msg.version, msg.payload.bytes().into())
    }

    /// Enqueue borrowed fragments of (version, typ, payload) which
    /// are no longer than max_frag onto the `out` deque.
    pub(crate) fn fragment_payload<'a>(
        &self,
        typ: ContentType,
        version: ProtocolVersion,
        payload: OutboundChunks<'a>,
    ) -> impl ExactSizeIterator<Item = OutboundMessage<'a>> {
        Chunker::new(payload, self.max_frag).map(move |payload| OutboundMessage {
            typ,
            version,
            payload,
        })
    }

    /// Set the maximum fragment size that will be produced.
    ///
    /// This includes overhead. A `max_fragment_size` of 10 will produce TLS fragments
    /// up to 10 bytes long.
    ///
    /// A `max_fragment_size` of `None` sets the highest allowable fragment size.
    ///
    /// Returns BadMaxFragmentSize if the size is smaller than 32 or larger than 16389.
    pub fn set_max_fragment_size(&mut self, max_fragment_size: Option<usize>) -> Result<(), Error> {
        self.max_frag = match max_fragment_size {
            Some(sz @ 32..=MAX_FRAGMENT_SIZE) => sz - PACKET_OVERHEAD,
            None => MAX_FRAGMENT_LEN,
            _ => return Err(Error::BadMaxFragmentSize),
        };
        Ok(())
    }
}

/// An iterator over borrowed fragments of a payload
struct Chunker<'a> {
    payload: OutboundChunks<'a>,
    limit: usize,
}

impl<'a> Chunker<'a> {
    fn new(payload: OutboundChunks<'a>, limit: usize) -> Self {
        Self { payload, limit }
    }
}

impl<'a> Iterator for Chunker<'a> {
    type Item = OutboundChunks<'a>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.payload.is_empty() {
            return None;
        }

        let (before, after) = self.payload.split_at(self.limit);
        self.payload = after;
        Some(before)
    }
}

impl<'a> ExactSizeIterator for Chunker<'a> {
    fn len(&self) -> usize {
        (self.payload.len() + self.limit - 1) / self.limit
    }
}

#[cfg(test)]
mod tests {
    use super::{MessageFragmenter, PACKET_OVERHEAD};
    use crate::enums::ContentType;
    use crate::enums::ProtocolVersion;
    use crate::msgs::base::Payload;
    use crate::msgs::message::{
        BorrowedPlainMessage, OutboundChunks, OutboundMessage, PlainMessage,
    };

    fn msg_eq(
        m: &OutboundMessage,
        total_len: usize,
        typ: &ContentType,
        version: &ProtocolVersion,
        bytes: &[u8],
    ) {
        assert_eq!(&m.typ, typ);
        assert_eq!(&m.version, version);
        assert_eq!(m.payload.to_vec(), bytes);

        let buf = m.to_unencrypted_opaque().encode();

        assert_eq!(total_len, buf.len());
    }

    #[test]
    fn smoke() {
        let typ = ContentType::Handshake;
        let version = ProtocolVersion::TLSv1_2;
        let data: Vec<u8> = (1..70u8).collect();
        let m = PlainMessage {
            typ,
            version,
            payload: Payload::new(data),
        };

        let mut frag = MessageFragmenter::default();
        frag.set_max_fragment_size(Some(32))
            .unwrap();
        let q = frag
            .fragment_message(&m)
            .collect::<Vec<_>>();
        assert_eq!(q.len(), 3);
        msg_eq(
            &q[0],
            32,
            &typ,
            &version,
            &[
                1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
                24, 25, 26, 27,
            ],
        );
        msg_eq(
            &q[1],
            32,
            &typ,
            &version,
            &[
                28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
                49, 50, 51, 52, 53, 54,
            ],
        );
        msg_eq(
            &q[2],
            20,
            &typ,
            &version,
            &[55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69],
        );
    }

    #[test]
    fn non_fragment() {
        let m = PlainMessage {
            typ: ContentType::Handshake,
            version: ProtocolVersion::TLSv1_2,
            payload: Payload::new(b"\x01\x02\x03\x04\x05\x06\x07\x08".to_vec()),
        };

        let mut frag = MessageFragmenter::default();
        frag.set_max_fragment_size(Some(32))
            .unwrap();
        let q = frag
            .fragment_message(&m)
            .collect::<Vec<_>>();
        assert_eq!(q.len(), 1);
        msg_eq(
            &q[0],
            PACKET_OVERHEAD + 8,
            &ContentType::Handshake,
            &ProtocolVersion::TLSv1_2,
            b"\x01\x02\x03\x04\x05\x06\x07\x08",
        );
    }

    #[test]
    fn fragment_multiple_slices() {
        let typ = ContentType::Handshake;
        let version = ProtocolVersion::TLSv1_2;
        let payload_owner: Vec<&[u8]> = vec![&[b'a'; 8], &[b'b'; 12], &[b'c'; 32], &[b'd'; 20]];
        let borrowed_payload = OutboundChunks::new(&payload_owner);
        let mut frag = MessageFragmenter::default();
        frag.set_max_fragment_size(Some(37)) // 32 + packet overhead
            .unwrap();

        let fragments = frag
            .fragment_payload(typ, version, borrowed_payload)
            .collect::<Vec<_>>();
        assert_eq!(fragments.len(), 3);
        msg_eq(
            &fragments[0],
            37,
            &typ,
            &version,
            b"aaaaaaaabbbbbbbbbbbbcccccccccccc",
        );
        msg_eq(
            &fragments[1],
            37,
            &typ,
            &version,
            b"ccccccccccccccccccccdddddddddddd",
        );
        msg_eq(&fragments[2], 13, &typ, &version, b"dddddddd");
    }
}