fix #41

Adds a check for empty messages as well as unit test verifying that
empty messages are handled as desired.

flake.nix

@@ -62,6 +62,7 @@
               gawk
               wireguard-tools
             ];
+
             # a function to generate a nix derivation for rosenpass against any
             # given set of nixpkgs
             rpDerivation = p:
@@ -89,6 +90,11 @@
                 # otherwise pkg-config tries to link non-existent dynamic libs
                 PKG_CONFIG_ALL_STATIC = true;
 
+                # liboqs requires quite a lot of stack memory, thus we adjust
+                # the default stack size picked for new threads (which is used
+                # by `cargo test`) to be _big enough_
+                RUST_MIN_STACK = 8 * 1024 * 1024; # 8 MiB
+
                 # nix defaults to building for aarch64 _without_ the armv8-a
                 # crypto extensions, but liboqs depens on these
                 preBuild =
@@ -241,6 +247,7 @@
           #
           devShells.default = pkgs.mkShell {
             inherit (packages.proof-proverif) CRYPTOVERIF_LIB;
+            inherit (packages.rosenpass) RUST_MIN_STACK;
             inputsFrom = [ packages.default ];
             nativeBuildInputs = with pkgs; [
               cargo-release
@@ -252,6 +259,7 @@
           };
           devShells.coverage = pkgs.mkShell {
             inputsFrom = [ packages.default ];
+            inherit (packages.rosenpass) RUST_MIN_STACK;
             nativeBuildInputs = with pkgs; [ inputs.fenix.packages.${system}.complete.toolchain cargo-llvm-cov ];
           };
 

src/protocol.rs

@@ -775,6 +775,8 @@ impl CryptoServer {
         let mut len = 0;
         let mut exchanged = false;
 
+        ensure!(!rx_buf.is_empty(), "received empty message, ignoring it");
+
         let peer = match rx_buf[0].try_into() {
             Ok(MsgType::InitHello) => {
                 let msg_in = rx_buf.envelope::<InitHello<&[u8]>>()?;
@@ -1693,3 +1695,32 @@ impl CryptoServer {
         Ok(hs.peer())
     }
 }
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn init_crypto_server() -> CryptoServer {
+        // always init libsodium before anything
+        crate::sodium::sodium_init().unwrap();
+
+        // initialize public and private key for the crypto server
+        let (mut sk, mut pk) = (SSk::zero(), SPk::zero());
+        StaticKEM::keygen(sk.secret_mut(), pk.secret_mut()).expect("unable to generate keys");
+
+        CryptoServer::new(sk, pk)
+    }
+
+    /// The determination of the message type relies on reading the first byte of the message. Only
+    /// after that the length of the message is checked against the specified message type. This
+    /// test ensures that nothing breaks in the case of an empty message.
+    #[test]
+    #[should_panic = "called `Result::unwrap()` on an `Err` value: received empty message, ignoring it"]
+    fn handle_empty_message() {
+        let mut crypt = init_crypto_server();
+        let empty_rx_buf = [0u8; 0];
+        let mut tx_buf = [0u8; 0];
+
+        crypt.handle_msg(&empty_rx_buf, &mut tx_buf).unwrap();
+    }
+}