refactor(mimeparser): store only one signature fingerprint

Messages are normally not signed with more than one key
and in this case we pick an arbitrary signature later anyway.

src/mimeparser.rs

@@ -87,12 +87,12 @@ pub(crate) struct MimeMessage {
     pub chat_disposition_notification_to: Option<SingleInfo>,
     pub decrypting_failed: bool,
 
-    /// Set of valid signature fingerprints if a message is an
+    /// Valid signature fingerprint if a message is an
     /// Autocrypt encrypted and signed message.
     ///
     /// If a message is not encrypted or the signature is not valid,
-    /// this set is empty.
-    pub signatures: HashSet<Fingerprint>,
+    /// this is `None`.
+    pub signature: Option<Fingerprint>,
 
     /// The addresses for which there was a gossip header
     /// and their respective gossiped keys.
@@ -589,7 +589,7 @@ impl MimeMessage {
             decrypting_failed: mail.is_err(),
 
             // only non-empty if it was a valid autocrypt message
-            signatures,
+            signature: signatures.into_iter().last(),
             autocrypt_fingerprint,
             gossiped_keys,
             is_forwarded: false,
@@ -966,7 +966,7 @@ impl MimeMessage {
     /// This means the message was both encrypted and signed with a
     /// valid signature.
     pub fn was_encrypted(&self) -> bool {
-        !self.signatures.is_empty()
+        self.signature.is_some()
     }
 
     /// Returns whether the email contains a `chat-version` header.

src/receive_imf.rs

@@ -642,7 +642,7 @@ pub(crate) async fn receive_imf_inner(
     // For example, GitHub sends messages from `notifications@github.com`,
     // but uses display name of the user whose action generated the notification
     // as the display name.
-    let fingerprint = mime_parser.signatures.iter().next();
+    let fingerprint = mime_parser.signature.as_ref();
     let (from_id, _from_id_blocked, incoming_origin) = match from_field_to_contact_id(
         context,
         &mime_parser.from,
@@ -3662,7 +3662,10 @@ async fn has_verified_encryption(
         ));
     }
 
-    let signed_with_verified_key = mimeparser.signatures.contains(&fingerprint);
+    let signed_with_verified_key = mimeparser
+        .signature
+        .as_ref()
+        .is_some_and(|signature| *signature == fingerprint);
     if signed_with_verified_key {
         Ok(Verified)
     } else {

src/securejoin.rs

@@ -623,17 +623,19 @@ fn encrypted_and_signed(
     mimeparser: &MimeMessage,
     expected_fingerprint: &Fingerprint,
 ) -> bool {
-    if !mimeparser.was_encrypted() {
+    if let Some(signature) = mimeparser.signature.as_ref() {
+        if signature == expected_fingerprint {
+            true
+        } else {
+            warn!(
+                context,
+                "Message does not match expected fingerprint {expected_fingerprint}.",
+            );
+            false
+        }
+    } else {
         warn!(context, "Message not encrypted.",);
         false
-    } else if !mimeparser.signatures.contains(expected_fingerprint) {
-        warn!(
-            context,
-            "Message does not match expected fingerprint {}.", expected_fingerprint,
-        );
-        false
-    } else {
-        true
     }
 }