Repair encrypted mails turned into attachments

Google Workspace has an option "Append footer" which appends standard
footer defined by administrator to all outgoing messages. However,
there is no plain text part in encrypted messages sent by Delta Chat,
so Google Workspace turn the message into multipart/mixed MIME, where
the first part is an empty plaintext part with a footer and the second
part is the original encrypted message.

This commit makes Delta Chat attempt to repair such messages,
similarly to how it already repairs "Mixed Up" MIME structure in
`get_mixed_up_mime`.

src/e2ee.rs

@@ -266,13 +266,40 @@ fn get_mixed_up_mime<'a, 'b>(mail: &'a ParsedMail<'b>) -> Option<&'a ParsedMail<
     }
 }
 
+/// Returns a reference to the encrypted payload of a message turned into attachment.
+///
+/// Google Workspace has an option "Append footer" which appends standard footer defined
+/// by administrator to all outgoing messages. However, there is no plain text part in
+/// encrypted messages sent by Delta Chat, so Google Workspace turns the message into
+/// multipart/mixed MIME, where the first part is an empty plaintext part with a footer
+/// and the second part is the original encrypted message.
+fn get_attachment_mime<'a, 'b>(mail: &'a ParsedMail<'b>) -> Option<&'a ParsedMail<'b>> {
+    if mail.ctype.mimetype != "multipart/mixed" {
+        return None;
+    }
+    if let [first_part, second_part] = &mail.subparts[..] {
+        if first_part.ctype.mimetype == "text/plain"
+            && second_part.ctype.mimetype == "multipart/encrypted"
+        {
+            get_autocrypt_mime(second_part)
+        } else {
+            None
+        }
+    } else {
+        None
+    }
+}
+
 async fn decrypt_if_autocrypt_message(
     context: &Context,
     mail: &ParsedMail<'_>,
     private_keyring: Keyring<SignedSecretKey>,
     public_keyring_for_validate: Keyring<SignedPublicKey>,
 ) -> Result<Option<(Vec<u8>, HashSet<Fingerprint>)>> {
-    let encrypted_data_part = match get_autocrypt_mime(mail).or_else(|| get_mixed_up_mime(mail)) {
+    let encrypted_data_part = match get_autocrypt_mime(mail)
+        .or_else(|| get_mixed_up_mime(mail))
+        .or_else(|| get_attachment_mime(mail))
+    {
         None => {
             // not an autocrypt mime message, abort and ignore
             return Ok(None);
@@ -389,6 +416,7 @@ pub async fn ensure_secret_key_exists(context: &Context) -> Result<String> {
 #[cfg(test)]
 mod tests {
     use crate::chat;
+    use crate::dc_receive_imf::dc_receive_imf;
     use crate::message::{Message, Viewtype};
     use crate::param::Param;
     use crate::peerstate::ToSave;
@@ -592,8 +620,8 @@ Sent with my Delta Chat Messenger: https://delta.chat";
         assert!(!encrypt_helper.should_encrypt(&t, false, &ps).unwrap());
     }
 
-    #[test]
-    fn test_mixed_up_mime() -> Result<()> {
+    #[async_std::test]
+    async fn test_mixed_up_mime() -> Result<()> {
         // "Mixed Up" mail as received when sending an encrypted
         // message using Delta Chat Desktop via ProtonMail IMAP/SMTP
         // Bridge.
@@ -601,6 +629,7 @@ Sent with my Delta Chat Messenger: https://delta.chat";
         let mail = mailparse::parse_mail(mixed_up_mime)?;
         assert!(get_autocrypt_mime(&mail).is_none());
         assert!(get_mixed_up_mime(&mail).is_some());
+        assert!(get_attachment_mime(&mail).is_none());
 
         // Same "Mixed Up" mail repaired by Thunderbird 78.9.0.
         //
@@ -611,6 +640,20 @@ Sent with my Delta Chat Messenger: https://delta.chat";
         let mail = mailparse::parse_mail(repaired_mime)?;
         assert!(get_autocrypt_mime(&mail).is_some());
         assert!(get_mixed_up_mime(&mail).is_none());
+        assert!(get_attachment_mime(&mail).is_none());
+
+        // Another form of "Mixed Up" mail created by Google Workspace,
+        // where original message is turned into attachment to empty plaintext message.
+        let attachment_mime = include_bytes!("../test-data/message/google-workspace-mixed-up.eml");
+        let mail = mailparse::parse_mail(attachment_mime)?;
+        assert!(get_autocrypt_mime(&mail).is_none());
+        assert!(get_mixed_up_mime(&mail).is_none());
+        assert!(get_attachment_mime(&mail).is_some());
+
+        let bob = TestContext::new_bob().await;
+        dc_receive_imf(&bob, attachment_mime, false).await?;
+        let msg = bob.get_last_msg().await;
+        assert_eq!(msg.text.as_deref(), Some("Hello from Thunderbird!"));
 
         Ok(())
     }