combine_dtls_fragments helper

Author: algesten

assets/dtls_cert_frag01.bin

@@ -95,6 +95,12 @@ pub struct DTLSMessageHandshake<'a> {
     pub body: DTLSMessageHandshakeBody<'a>,
 }
 
+impl<'a> DTLSMessageHandshake<'a> {
+    pub fn is_fragment(&self) -> bool {
+        matches!(self.body, DTLSMessageHandshakeBody::Fragment(_))
+    }
+}
+
 /// DTLS Generic handshake message
 #[derive(Debug, PartialEq)]
 pub enum DTLSMessageHandshakeBody<'a> {
@@ -133,7 +139,7 @@ impl<'a> DTLSMessage<'a> {
     /// fragments to be a complete message.
     pub fn is_fragment(&self) -> bool {
         match self {
-            DTLSMessage::Handshake(h) => matches!(h.body, DTLSMessageHandshakeBody::Fragment(_)),
+            DTLSMessage::Handshake(h) => h.is_fragment(),
             _ => false,
         }
     }

assets/dtls_cert_frag02.bin

@@ -0,0 +1,218 @@
+use core::array;
+
+use nom::error::{make_error, ErrorKind};
+use nom::{Err, IResult};
+
+use crate::{
+    parse_dtls_message_handshake, DTLSMessage, DTLSMessageHandshake, DTLSMessageHandshakeBody,
+};
+
+const MAX_FRAGMENTS: usize = 50;
+
+/// Combine the given fragments into one. Returns true if the fragments made a complete output.
+/// The fragments are combined in such a way that the output constitutes a complete DTLSMessage.
+///
+/// Returns `None` if the fragments are not complete.
+///
+/// Errors if:
+///
+/// 1. The output is not big enough to hold the reconstituted messages
+/// 2. Fragments are not of the same type (for example ClientHello mixed with Certificate)
+/// 3. (Total) length field differs between the fragments
+/// 4. Fragment offset/length are not consistent with total length
+/// 5. The DTLSMessageHandshakeBody in the message is not a Fragment
+/// 6. The message_seq differs between the fragments.
+///
+/// Panics if there are more than 50 fragments.
+pub fn combine_dtls_fragments<'a>(
+    fragments: &[DTLSMessageHandshake],
+    out: &'a mut [u8],
+) -> IResult<&'a [u8], Option<DTLSMessage<'a>>> {
+    if fragments.is_empty() {
+        return Ok((&[], None));
+    }
+
+    if fragments.len() > MAX_FRAGMENTS {
+        panic!("More than max fragments");
+    }
+
+    const MESSAGE_HEADER_OFFSET: usize = 12;
+
+    // The header all of the fragments share the same DTLSMessage start, apart from the
+    // fragment information. This goes into the first 12 bytes.
+    if out.len() < MESSAGE_HEADER_OFFSET {
+        // Error case 1
+        return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+    }
+
+    // Helper to iterate the fragments in order.
+    let ordered = Ordered::new(fragments);
+
+    // Investigate each fragment_offset + fragment_length to figure out
+    // the max contiguous range over the fragments.
+    let max = ordered.max_contiguous();
+
+    // Unwrap is OK, because we have at least one item (checked above).
+    let first_handshake = ordered.iter().next().unwrap();
+
+    if first_handshake.fragment_offset != 0 {
+        // The first fragment must start at 0, or we might have
+        // missing packets or arriving out of order.
+        return Ok((&[], None));
+    }
+
+    let msg_type = first_handshake.msg_type;
+    let message_seq = first_handshake.message_seq;
+    let length = first_handshake.length;
+
+    if max > length {
+        // Error case 4
+        return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+    } else if max < length {
+        // We do not have all fragments yet
+        return Ok((&[], None));
+    }
+
+    // Write the header into output.
+    {
+        out[0] = msg_type.into(); // The type.
+        (&mut out[1..4]).copy_from_slice(&length.to_be_bytes()[1..]); // 24 bit length
+        (&mut out[4..6]).copy_from_slice(&message_seq.to_be_bytes()); // 16 bit message sequence
+        (&mut out[6..9]).copy_from_slice(&[0, 0, 0]); // 24 bit fragment_offset, which is 0 for the entire message.
+        (&mut out[9..12]).copy_from_slice(&length.to_be_bytes()[1..]); // 24 bit fragment_length, which is entire length.
+    }
+
+    let data = &mut out[MESSAGE_HEADER_OFFSET..];
+
+    if data.len() < length as usize {
+        // Error case 1
+        return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+    }
+
+    // Loop the fragments, in order and output the data.
+    for handshake in ordered.iter() {
+        if msg_type != handshake.msg_type {
+            // Error case 2
+            return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+        }
+
+        if handshake.length != length {
+            // Error case 3
+            return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+        }
+
+        if handshake.message_seq != message_seq {
+            // Error case 6
+            return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+        }
+
+        let from = handshake.fragment_offset as usize;
+        let to = from + handshake.fragment_length as usize;
+
+        let body = match &handshake.body {
+            DTLSMessageHandshakeBody::Fragment(v) => v,
+            _ => {
+                // Error case 5
+                return Err(Err::Error(make_error(&*out, ErrorKind::Fail)));
+            }
+        };
+
+        // Copy into output.
+        (&mut data[from..to]).copy_from_slice(&body[..]);
+    }
+
+    // This parse should succeed now and produce a complete message.
+    let (rest, message) = parse_dtls_message_handshake(out)?;
+
+    Ok((rest, Some(message)))
+}
+
+struct Ordered<'a, 'b>([usize; MAX_FRAGMENTS], &'a [DTLSMessageHandshake<'b>]);
+
+impl<'a, 'b> Ordered<'a, 'b> {
+    fn new(fragments: &'a [DTLSMessageHandshake<'b>]) -> Self {
+        // Indexes that will point into handshakes
+        let mut order: [usize; MAX_FRAGMENTS] = array::from_fn(|i| i);
+
+        // Sort the index for fragment_offset starts.
+        order.sort_by_key(|idx| {
+            fragments
+                .get(*idx)
+                .map(|h| h.fragment_offset)
+                // Somewhere outside the fragments length.
+                .unwrap_or(*idx as u32 + 1000)
+        });
+
+        Self(order, fragments)
+    }
+
+    fn iter(&self) -> impl Iterator<Item = &'a DTLSMessageHandshake<'b>> + '_ {
+        let len = self.1.len();
+        self.0
+            .iter()
+            .take_while(move |idx| **idx < len)
+            .map(move |idx| &self.1[*idx])
+    }
+
+    // Find the max contiguous fragment_offset/fragment_length.
+    fn max_contiguous(&self) -> u32 {
+        let mut max = 0;
+
+        for h in self.iter() {
+            // DTLS fragments can overlap, which means the offset might not start at the previous end.
+            if h.fragment_offset <= max {
+                let start = h.fragment_offset;
+                max = start + h.fragment_length;
+            } else {
+                // Not contiguous.
+                return 0;
+            }
+        }
+
+        max
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::parse_dtls_plaintext_record;
+
+    use super::*;
+
+    #[test]
+    fn read_dtls_certifiate_fragments() {
+        // These are complete packets dumped with wireshark.
+        const DTLS_CERT01: &[u8] = include_bytes!("../assets/dtls_cert_frag01.bin");
+        const DTLS_CERT02: &[u8] = include_bytes!("../assets/dtls_cert_frag02.bin");
+        const DTLS_CERT03: &[u8] = include_bytes!("../assets/dtls_cert_frag03.bin");
+        const DTLS_CERT04: &[u8] = include_bytes!("../assets/dtls_cert_frag04.bin");
+
+        let mut fragments = vec![];
+
+        for c in &[DTLS_CERT01, DTLS_CERT02, DTLS_CERT03, DTLS_CERT04] {
+            let (_, record) = parse_dtls_plaintext_record(c).expect("parsing failed");
+
+            for message in record.messages {
+                // All of these should be fragments.
+                assert!(message.is_fragment());
+
+                let handshake = match message {
+                    DTLSMessage::Handshake(v) => v,
+                    _ => panic!("Expected Handshake"),
+                };
+
+                assert!(handshake.is_fragment());
+                fragments.push(handshake);
+            }
+        }
+
+        // Temporary output to combine the fragments into.
+        let mut out = vec![0_u8; 4192];
+        let (_, message) = combine_dtls_fragments(&fragments, &mut out).expect("combine fragments");
+
+        // This optional should hold Some(DTLSMessage) indicating a complete parse.
+        let message = message.expect("Combined fragments");
+
+        println!("{:02x?}", message);
+    }
+}

assets/dtls_cert_frag03.bin

@@ -142,6 +142,7 @@ extern crate alloc;
 
 mod certificate_transparency;
 mod dtls;
+mod dtls_combine;
 mod tls;
 mod tls_alert;
 mod tls_ciphers;
@@ -154,6 +155,7 @@ mod tls_states;
 
 pub use certificate_transparency::*;
 pub use dtls::*;
+pub use dtls_combine::*;
 pub use tls::*;
 pub use tls_alert::*;
 pub use tls_ciphers::*;