feat: mimeparser: Omit Legacy Display Elements from text/plain (#7130)

Implement 4.5.3.2 of https://www.rfc-editor.org/rfc/rfc9788 "Header Protection for Cryptographically
Protected Email".

Author: iequidoo

src/mimeparser.rs

@@ -1344,16 +1344,30 @@ impl MimeMessage {
 
                         let (simplified_txt, simplified_quote) = if mime_type.type_() == mime::TEXT
                             && mime_type.subtype() == mime::PLAIN
-                            && is_format_flowed
                         {
-                            let delsp = if let Some(delsp) = mail.ctype.params.get("delsp") {
-                                delsp.as_str().eq_ignore_ascii_case("yes")
-                            } else {
-                                false
+                            // Don't check that we're inside an encrypted or signed part for
+                            // simplicity and ease of testing.
+                            let simplified_txt = match mail
+                                .ctype
+                                .params
+                                .get("hp-legacy-display")
+                                .is_some_and(|v| v == "1")
+                            {
+                                false => simplified_txt,
+                                true => rm_legacy_display_elements(&simplified_txt),
                             };
-                            let unflowed_text = unformat_flowed(&simplified_txt, delsp);
-                            let unflowed_quote = top_quote.map(|q| unformat_flowed(&q, delsp));
-                            (unflowed_text, unflowed_quote)
+                            if is_format_flowed {
+                                let delsp = if let Some(delsp) = mail.ctype.params.get("delsp") {
+                                    delsp.as_str().eq_ignore_ascii_case("yes")
+                                } else {
+                                    false
+                                };
+                                let unflowed_text = unformat_flowed(&simplified_txt, delsp);
+                                let unflowed_quote = top_quote.map(|q| unformat_flowed(&q, delsp));
+                                (unflowed_text, unflowed_quote)
+                            } else {
+                                (simplified_txt, top_quote)
+                            }
                         } else {
                             (simplified_txt, top_quote)
                         };
@@ -1970,6 +1984,20 @@ impl MimeMessage {
     }
 }
 
+fn rm_legacy_display_elements(text: &str) -> String {
+    let mut res = None;
+    for l in text.lines() {
+        res = res.map(|r: String| match r.is_empty() {
+            true => l.to_string(),
+            false => r + "\r\n" + l,
+        });
+        if l.is_empty() {
+            res = Some(String::new());
+        }
+    }
+    res.unwrap_or_default()
+}
+
 fn remove_header(
     headers: &mut HashMap<String, String>,
     key: &str,

src/mimeparser/mimeparser_tests.rs

@@ -1751,6 +1751,43 @@ async fn test_time_in_future() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_hp_legacy_display() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let bob = &tcm.bob().await;
+
+    let msg = MimeMessage::from_bytes(
+        bob,
+        br#"Date: Fri, 21 Jan 2022 20:40:48 -0500
+From: Alice <alice@example.net>
+To: Bob <bob@example.net>
+Subject: Dinner plans
+Message-ID: <text-plain-legacy-display@lhp.example>
+MIME-Version: 1.0
+Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
+ hp="cipher"
+HP-Outer: Date: Fri, 21 Jan 2022 20:40:48 -0500
+HP-Outer: From: Alice <alice@example.net>
+HP-Outer: To: Bob <bob@example.net>
+HP-Outer: Subject: [...]
+HP-Outer: Message-ID: <text-plain-legacy-display@lhp.example>
+
+Subject: Dinner plans
+
+Let's meet at Rama's Roti Shop at 8pm and go to the park
+from there.
+"#,
+        None,
+    )
+    .await?;
+    assert_eq!(
+        msg.parts[0].msg,
+        "Dinner plans — Let's meet at Rama's Roti Shop at 8pm and go to the park\r\n\
+        from there."
+    );
+    Ok(())
+}
+
 /// Tests that subject is not prepended to the message
 /// when bot receives it.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]