Add a config option to sign all messages with Autocrypt header (#3986)

Although it does a little for security, it will help to protect from unwanted server-side
modifications and bugs. And now we have a time to test "multipart/signed" messages compatibility
with other MUAs.

src/config.rs

@@ -300,6 +300,9 @@ pub enum Config {
     /// See `crate::authres::update_authservid_candidates`.
     AuthservIdCandidates,
 
+    /// Make all outgoing messages with Autocrypt header "multipart/signed".
+    SignUnencrypted,
+
     /// Let the core save all events to the database.
     /// This value is used internally to remember the MsgId of the logging xdc
     #[strum(props(default = "0"))]

src/context.rs

@@ -763,6 +763,12 @@ impl Context {
                 .await?
                 .unwrap_or_default(),
         );
+        res.insert(
+            "sign_unencrypted",
+            self.get_config_int(Config::SignUnencrypted)
+                .await?
+                .to_string(),
+        );
 
         res.insert(
             "debug_logging",

src/e2ee.rs

@@ -124,6 +124,19 @@ impl EncryptHelper {
 
         Ok(ctext)
     }
+
+    /// Signs the passed-in `mail` using the private key from `context`.
+    /// Returns the payload and the signature.
+    pub async fn sign(
+        self,
+        context: &Context,
+        mail: lettre_email::PartBuilder,
+    ) -> Result<(lettre_email::MimeMessage, String)> {
+        let sign_key = SignedSecretKey::load_self(context).await?;
+        let mime_message = mail.build();
+        let signature = pgp::pk_calc_signature(mime_message.as_string().as_bytes(), &sign_key)?;
+        Ok((mime_message, signature))
+    }
 }
 
 /// Ensures a private key exists for the configured user.

src/mimefactory.rs

@@ -779,10 +779,36 @@ impl<'a> MimeFactory<'a> {
             };
 
             // Store protected headers in the outer message.
-            headers
+            let message = headers
                 .protected
                 .into_iter()
-                .fold(message, |message, header| message.header(header))
+                .fold(message, |message, header| message.header(header));
+
+            if self.should_skip_autocrypt()
+                || !context.get_config_bool(Config::SignUnencrypted).await?
+            {
+                message
+            } else {
+                let (payload, signature) = encrypt_helper.sign(context, message).await?;
+                PartBuilder::new()
+                    .header((
+                        "Content-Type".to_string(),
+                        "multipart/signed; protocol=\"application/pgp-signature\"".to_string(),
+                    ))
+                    .child(payload)
+                    .child(
+                        PartBuilder::new()
+                            .content_type(
+                                &"application/pgp-signature; name=\"signature.asc\""
+                                    .parse::<mime::Mime>()
+                                    .unwrap(),
+                            )
+                            .header(("Content-Description", "OpenPGP digital signature"))
+                            .header(("Content-Disposition", "attachment; filename=\"signature\";"))
+                            .body(signature)
+                            .build(),
+                    )
+            }
         };
 
         // Store the unprotected headers on the outer message.
@@ -2140,6 +2166,96 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_selfavatar_unencrypted_signed() {
+        // create chat with bob, set selfavatar
+        let t = TestContext::new_alice().await;
+        t.set_config(Config::SignUnencrypted, Some("1"))
+            .await
+            .unwrap();
+        let chat = t.create_chat_with_contact("bob", "bob@example.org").await;
+
+        let file = t.dir.path().join("avatar.png");
+        let bytes = include_bytes!("../test-data/image/avatar64x64.png");
+        tokio::fs::write(&file, bytes).await.unwrap();
+        t.set_config(Config::Selfavatar, Some(file.to_str().unwrap()))
+            .await
+            .unwrap();
+
+        // send message to bob: that should get multipart/mixed because of the avatar moved to inner header;
+        // make sure, `Subject:` stays in the outer header (imf header)
+        let mut msg = Message::new(Viewtype::Text);
+        msg.set_text(Some("this is the text!".to_string()));
+
+        let sent_msg = t.send_msg(chat.id, &mut msg).await;
+        let mut payload = sent_msg.payload().splitn(4, "\r\n\r\n");
+
+        let part = payload.next().unwrap();
+        assert_eq!(part.match_indices("multipart/signed").count(), 1);
+        assert_eq!(part.match_indices("Subject:").count(), 0);
+        assert_eq!(part.match_indices("Autocrypt:").count(), 1);
+        assert_eq!(part.match_indices("Chat-User-Avatar:").count(), 0);
+
+        let part = payload.next().unwrap();
+        assert_eq!(part.match_indices("multipart/mixed").count(), 1);
+        assert_eq!(part.match_indices("Subject:").count(), 1);
+        assert_eq!(part.match_indices("Autocrypt:").count(), 0);
+        assert_eq!(part.match_indices("Chat-User-Avatar:").count(), 0);
+
+        let part = payload.next().unwrap();
+        assert_eq!(part.match_indices("text/plain").count(), 1);
+        assert_eq!(part.match_indices("Chat-User-Avatar:").count(), 1);
+        assert_eq!(part.match_indices("Subject:").count(), 0);
+
+        let body = payload.next().unwrap();
+        assert_eq!(body.match_indices("this is the text!").count(), 1);
+
+        let bob = TestContext::new_bob().await;
+        bob.recv_msg(&sent_msg).await;
+        let alice_id = Contact::lookup_id_by_addr(&bob.ctx, "alice@example.org", Origin::Unknown)
+            .await
+            .unwrap()
+            .unwrap();
+        let alice_contact = Contact::load_from_db(&bob.ctx, alice_id).await.unwrap();
+        assert!(alice_contact
+            .get_profile_image(&bob.ctx)
+            .await
+            .unwrap()
+            .is_some());
+
+        // if another message is sent, that one must not contain the avatar
+        // and no artificial multipart/mixed nesting
+        let sent_msg = t.send_msg(chat.id, &mut msg).await;
+        let mut payload = sent_msg.payload().splitn(3, "\r\n\r\n");
+
+        let part = payload.next().unwrap();
+        assert_eq!(part.match_indices("multipart/signed").count(), 1);
+        assert_eq!(part.match_indices("Subject:").count(), 0);
+        assert_eq!(part.match_indices("Autocrypt:").count(), 1);
+        assert_eq!(part.match_indices("Chat-User-Avatar:").count(), 0);
+
+        let part = payload.next().unwrap();
+        assert_eq!(part.match_indices("text/plain").count(), 1);
+        assert_eq!(part.match_indices("Subject:").count(), 1);
+        assert_eq!(part.match_indices("Autocrypt:").count(), 0);
+        assert_eq!(part.match_indices("multipart/mixed").count(), 0);
+        assert_eq!(part.match_indices("Chat-User-Avatar:").count(), 0);
+
+        let body = payload.next().unwrap();
+        assert_eq!(body.match_indices("this is the text!").count(), 1);
+        assert_eq!(body.match_indices("text/plain").count(), 0);
+        assert_eq!(body.match_indices("Chat-User-Avatar:").count(), 0);
+        assert_eq!(body.match_indices("Subject:").count(), 0);
+
+        bob.recv_msg(&sent_msg).await;
+        let alice_contact = Contact::load_from_db(&bob.ctx, alice_id).await.unwrap();
+        assert!(alice_contact
+            .get_profile_image(&bob.ctx)
+            .await
+            .unwrap()
+            .is_some());
+    }
+
     /// Test that removed member address does not go into the `To:` field.
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_remove_member_bcc() -> Result<()> {

src/pgp.rs

@@ -262,6 +262,20 @@ pub async fn pk_encrypt(
         .await?
 }
 
+/// Signs `plain` text using `private_key_for_signing`.
+pub fn pk_calc_signature(
+    plain: &[u8],
+    private_key_for_signing: &SignedSecretKey,
+) -> Result<String> {
+    let msg = Message::new_literal_bytes("", plain).sign(
+        private_key_for_signing,
+        || "".into(),
+        Default::default(),
+    )?;
+    let signature = msg.into_signature().to_armored_string(None)?;
+    Ok(signature)
+}
+
 /// Decrypts the message with keys from the private key keyring.
 ///
 /// Receiver private keys are provided in