+simulate_write_message [doesn't work, don't merge]

src/handshakestate.rs

@@ -18,6 +18,7 @@ use crate::{
 use std::{
     convert::{TryFrom, TryInto},
     fmt,
+    marker::PhantomData,
 };
 
 /// A state machine encompassing the handshake phase of a Noise session.
@@ -47,6 +48,24 @@ pub struct HandshakeState {
     pub(crate) pattern_position: usize,
 }
 
+/// Result of [`simulate_write_message`](HandshakeState::simulate_write_message) if the call was successful.
+pub struct SimulatedWriteInfo {
+    /// Would-be resulting message length of a call to [`write_message`](HandshakeState::write_message)
+    ///
+    /// This is useful to pad the message length to disguise re-handshakes.
+    pub result_length: usize,
+
+    /// Flag that indicates if the contained payload would be encrypted
+    ///
+    /// NOTE: even when the payload would be encrypted, that doesn't necessarily mean that the
+    /// encryption is as strong as the encryption in transport mode.
+    /// See: [Noise Explorer, Patterns](https://noiseexplorer.com/patterns/)
+    pub is_encrypted: bool,
+
+    /// Disallow exhausive matching
+    _non_exhausive: PhantomData<()>,
+}
+
 impl HandshakeState {
     #[allow(clippy::too_many_arguments)]
     pub(crate) fn new(
@@ -195,7 +214,7 @@ impl HandshakeState {
     }
 
     /// Construct a message from `payload` (and pending handshake tokens if in handshake state),
-    /// and writes it to the `output` buffer.
+    /// and writes it to the `message` buffer.
     ///
     /// Returns the size of the written payload.
     ///
@@ -204,11 +223,12 @@ impl HandshakeState {
     /// Will result in `Error::Input` if the size of the output exceeds the max message
     /// length in the Noise Protocol (65535 bytes).
     #[must_use]
-    pub fn write_message(&mut self, message: &[u8], payload: &mut [u8]) -> Result<usize, Error> {
+    pub fn write_message(&mut self, payload: &[u8], message: &mut [u8]) -> Result<usize, Error> {
         let checkpoint = self.symmetricstate.checkpoint();
-        match self._write_message(message, payload) {
+        match self._write_message(payload, message) {
             Ok(res) => {
                 self.pattern_position += 1;
+                self.my_turn = false;
                 Ok(res)
             },
             Err(err) => {
@@ -218,6 +238,21 @@ impl HandshakeState {
         }
     }
 
+    /// Simulates a call to [`write_message`](HandshakeState::write_message) and returns
+    /// the result and additional information.
+    pub fn simulate_write_message(&mut self, payload: &[u8]) -> Result<SimulatedWriteInfo, Error> {
+        let mut tmp = [0u8; 65535];
+        let checkpoint = self.symmetricstate.checkpoint();
+        let ret =
+            self._write_message(payload, &mut tmp[..]).map(|result_length| SimulatedWriteInfo {
+                result_length,
+                is_encrypted: self.symmetricstate.has_key(),
+                _non_exhausive: PhantomData,
+            });
+        self.symmetricstate.restore(checkpoint);
+        ret
+    }
+
     fn _write_message(&mut self, payload: &[u8], message: &mut [u8]) -> Result<usize, Error> {
         if !self.my_turn {
             bail!(StateProblem::NotTurnToWrite);
@@ -317,7 +352,6 @@ impl HandshakeState {
         if self.pattern_position == (self.message_patterns.len() - 1) {
             self.symmetricstate.split(&mut self.cipherstates.0, &mut self.cipherstates.1);
         }
-        self.my_turn = false;
         Ok(byte_index)
     }
 
@@ -338,6 +372,7 @@ impl HandshakeState {
         match self._read_message(message, payload) {
             Ok(res) => {
                 self.pattern_position += 1;
+                self.my_turn = true;
                 Ok(res)
             },
             Err(err) => {
@@ -447,7 +482,6 @@ impl HandshakeState {
         }
 
         self.symmetricstate.decrypt_and_mix_hash(ptr, payload).map_err(|_| Error::Decrypt)?;
-        self.my_turn = true;
         if last {
             self.symmetricstate.split(&mut self.cipherstates.0, &mut self.cipherstates.1);
         }
@@ -516,10 +550,7 @@ impl HandshakeState {
     pub fn dangerously_get_raw_split(&mut self) -> ([u8; CIPHERKEYLEN], [u8; CIPHERKEYLEN]) {
         let mut output = ([0u8; MAXHASHLEN], [0u8; MAXHASHLEN]);
         self.symmetricstate.split_raw(&mut output.0, &mut output.1);
-        (
-            output.0[..CIPHERKEYLEN].try_into().unwrap(),
-            output.1[..CIPHERKEYLEN].try_into().unwrap()
-        )
+        (output.0[..CIPHERKEYLEN].try_into().unwrap(), output.1[..CIPHERKEYLEN].try_into().unwrap())
     }
 
     /// Convert this `HandshakeState` into a `TransportState` with an internally stored nonce.

src/lib.rs

@@ -89,7 +89,7 @@ pub mod types;
 pub use crate::{
     builder::{Builder, Keypair},
     error::Error,
-    handshakestate::HandshakeState,
+    handshakestate::{HandshakeState, SimulatedWriteInfo},
     stateless_transportstate::StatelessTransportState,
     transportstate::TransportState,
 };