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

But disable it by default for now. We will enable it when most users update to versions capable to
receive protected headers.

Author: iequidoo

src/config.rs

@@ -440,6 +440,11 @@ 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".
+    StdHeaderProtectionComposing,
 }
 
 impl Config {

src/context.rs

@@ -1067,6 +1067,13 @@ 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

@@ -1054,6 +1054,9 @@ impl MimeFactory {
             }
         }
 
+        let use_std_header_protection = context
+            .get_config_bool(Config::StdHeaderProtectionComposing)
+            .await?;
         let outer_message = if let Some(encryption_keys) = self.encryption_keys {
             // Store protected headers in the inner message.
             let message = protected_headers
@@ -1069,6 +1072,22 @@ 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_keys.len() > 1 || context.get_config_bool(Config::BccSelf).await?;
@@ -1158,7 +1177,13 @@ 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;
                     }
                 }
             }
@@ -1244,7 +1269,13 @@ 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;
                         }
                     }
                 }