feat: Add Config::StdHeaderProtectionComposing (enables composing as defined in RFC 9788) (#7130)

And enable it by default as the standard Header Protection is backward-compatible.

Also this tests extra IMF header removal when a message has standard Header Protection since now we
can send such messages.

src/config.rs

@@ -445,6 +445,12 @@ pub enum Config {
 
     /// Return an error from `receive_imf_inner()` for a fully downloaded message. For tests.
     FailOnReceivingFullMsg,
+
+    /// Enable composing emails with Header Protection as defined in
+    /// <https://www.rfc-editor.org/rfc/rfc9788.html> "Header Protection for Cryptographically
+    /// Protected Email".
+    #[strum(props(default = "1"))]
+    StdHeaderProtectionComposing,
 }
 
 impl Config {

src/context.rs

@@ -1077,6 +1077,13 @@ impl Context {
                 .await?
                 .unwrap_or_default(),
         );
+        res.insert(
+            "std_header_protection_composing",
+            self.sql
+                .get_raw_config("std_header_protection_composing")
+                .await?
+                .unwrap_or_default(),
+        );
 
         let elapsed = time_elapsed(&self.creation_time);
         res.insert("uptime", duration_to_str(elapsed));

src/mimefactory.rs

@@ -1083,6 +1083,9 @@ impl MimeFactory {
             }
         }
 
+        let use_std_header_protection = context
+            .get_config_bool(Config::StdHeaderProtectionComposing)
+            .await?;
         let outer_message = if let Some(encryption_pubkeys) = self.encryption_pubkeys {
             // Store protected headers in the inner message.
             let message = protected_headers
@@ -1098,6 +1101,22 @@ impl MimeFactory {
                     message.header(header, value)
                 });
 
+            if use_std_header_protection {
+                message = unprotected_headers
+                    .iter()
+                    // Structural headers shouldn't be added as "HP-Outer". They are defined in
+                    // <https://www.rfc-editor.org/rfc/rfc9787.html#structural-header-fields>.
+                    .filter(|(name, _)| {
+                        !(name.eq_ignore_ascii_case("mime-version")
+                            || name.eq_ignore_ascii_case("content-type")
+                            || name.eq_ignore_ascii_case("content-transfer-encoding")
+                            || name.eq_ignore_ascii_case("content-disposition"))
+                    })
+                    .fold(message, |message, (name, value)| {
+                        message.header(format!("HP-Outer: {name}"), value.clone())
+                    });
+            }
+
             // Add gossip headers in chats with multiple recipients
             let multiple_recipients =
                 encryption_pubkeys.len() > 1 || context.get_config_bool(Config::BccSelf).await?;
@@ -1187,7 +1206,13 @@ impl MimeFactory {
             for (h, v) in &mut message.headers {
                 if h == "Content-Type" {
                     if let mail_builder::headers::HeaderType::ContentType(ct) = v {
-                        *ct = ct.clone().attribute("protected-headers", "v1");
+                        let mut ct_new = ct.clone();
+                        ct_new = ct_new.attribute("protected-headers", "v1");
+                        if use_std_header_protection {
+                            ct_new = ct_new.attribute("hp", "cipher");
+                        }
+                        *ct = ct_new;
+                        break;
                     }
                 }
             }
@@ -1325,7 +1350,13 @@ impl MimeFactory {
                 for (h, v) in &mut message.headers {
                     if h == "Content-Type" {
                         if let mail_builder::headers::HeaderType::ContentType(ct) = v {
-                            *ct = ct.clone().attribute("protected-headers", "v1");
+                            let mut ct_new = ct.clone();
+                            ct_new = ct_new.attribute("protected-headers", "v1");
+                            if use_std_header_protection {
+                                ct_new = ct_new.attribute("hp", "clear");
+                            }
+                            *ct = ct_new;
+                            break;
                         }
                     }
                 }

src/mimefactory/mimefactory_tests.rs

@@ -832,8 +832,8 @@ async fn test_protected_headers_directive() -> Result<()> {
             .count(),
         1
     );
-    assert_eq!(part.match_indices("Subject:").count(), 1);
-
+    assert_eq!(part.match_indices("Subject:").count(), 2);
+    assert_eq!(part.match_indices("HP-Outer: Subject:").count(), 1);
     Ok(())
 }
 

src/mimeparser/mimeparser_tests.rs

@@ -1401,22 +1401,28 @@ async fn test_x_microsoft_original_message_id_precedence() -> Result<()> {
 }
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn test_extra_imf_chat_header() -> Result<()> {
+async fn test_extra_imf_headers() -> Result<()> {
     let mut tcm = TestContextManager::new();
     let t = &tcm.alice().await;
     let chat_id = t.get_self_chat().await.id;
 
-    chat::send_text_msg(t, chat_id, "hi!".to_string()).await?;
-    let sent_msg = t.pop_sent_msg().await;
-    // Check removal of some nonexistent "Chat-*" header to protect the code from future breakages.
-    let payload = sent_msg
-        .payload
-        .replace("Message-ID:", "Chat-Forty-Two: 42\r\nMessage-ID:");
-    let msg = MimeMessage::from_bytes(t, payload.as_bytes(), None)
-        .await
-        .unwrap();
-    assert!(msg.headers.contains_key("chat-version"));
-    assert!(!msg.headers.contains_key("chat-forty-two"));
+    for std_hp_composing in [false, true] {
+        t.set_config_bool(Config::StdHeaderProtectionComposing, std_hp_composing)
+            .await?;
+        chat::send_text_msg(t, chat_id, "hi!".to_string()).await?;
+        let sent_msg = t.pop_sent_msg().await;
+        // Check removal of some nonexistent "Chat-*" header to protect the code from future
+        // breakages. But headers not prefixed with "Chat-" remain unless a message has standard
+        // Header Protection.
+        let payload = sent_msg.payload.replace(
+            "Message-ID:",
+            "Chat-Forty-Two: 42\r\nForty-Two: 42\r\nMessage-ID:",
+        );
+        let msg = MimeMessage::from_bytes(t, payload.as_bytes(), None).await?;
+        assert!(msg.headers.contains_key("chat-version"));
+        assert!(!msg.headers.contains_key("chat-forty-two"));
+        assert_ne!(msg.headers.contains_key("forty-two"), std_hp_composing);
+    }
     Ok(())
 }