feat: MimeMessage: Put intended recipient fingerprints into signature

src/mimeparser.rs

@@ -89,11 +89,12 @@ pub(crate) struct MimeMessage {
     pub decrypting_failed: bool,
 
     /// Valid signature fingerprint if a message is an
-    /// Autocrypt encrypted and signed message.
+    /// Autocrypt encrypted and signed message and corresponding intended recipient fingerprints
+    /// (<https://www.rfc-editor.org/rfc/rfc9580.html#name-intended-recipient-fingerpr>) if any.
     ///
     /// If a message is not encrypted or the signature is not valid,
     /// this is `None`.
-    pub signature: Option<Fingerprint>,
+    pub signature: Option<(Fingerprint, HashSet<Fingerprint>)>,
 
     /// The addresses for which there was a gossip header
     /// and their respective gossiped keys.
@@ -529,12 +530,16 @@ impl MimeMessage {
         let mut signatures = if let Some(ref decrypted_msg) = decrypted_msg {
             crate::pgp::valid_signature_fingerprints(decrypted_msg, &public_keyring)
         } else {
-            HashSet::new()
+            HashMap::new()
         };
 
         let mail = mail.as_ref().map(|mail| {
             let (content, signatures_detached) = validate_detached_signature(mail, &public_keyring)
                 .unwrap_or((mail, Default::default()));
+            let signatures_detached = signatures_detached
+                .into_iter()
+                .map(|fp| (fp, Vec::new()))
+                .collect::<HashMap<_, _>>();
             signatures.extend(signatures_detached);
             content
         });
@@ -640,6 +645,10 @@ impl MimeMessage {
             };
         }
 
+        let signature = signatures
+            .into_iter()
+            .last()
+            .map(|(fp, recipient_fps)| (fp, recipient_fps.into_iter().collect::<HashSet<_>>()));
         let mut parser = MimeMessage {
             parts: Vec::new(),
             headers,
@@ -655,7 +664,7 @@ impl MimeMessage {
             decrypting_failed: mail.is_err(),
 
             // only non-empty if it was a valid autocrypt message
-            signature: signatures.into_iter().last(),
+            signature,
             autocrypt_fingerprint,
             gossiped_keys,
             is_forwarded: false,

src/pgp.rs

@@ -1,6 +1,6 @@
 //! OpenPGP helper module using [rPGP facilities](https://github.com/rpgp/rpgp).
 
-use std::collections::{BTreeMap, HashSet};
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::io::{BufRead, Cursor};
 
 use anyhow::{Context as _, Result, bail};
@@ -370,19 +370,28 @@ fn check_symmetric_encryption(msg: &Message<'_>) -> std::result::Result<(), &'st
 
 /// Returns fingerprints
 /// of all keys from the `public_keys_for_validation` keyring that
-/// have valid signatures there.
+/// have valid signatures in `msg` and corresponding intended recipient fingerprints
+/// (<https://www.rfc-editor.org/rfc/rfc9580.html#name-intended-recipient-fingerpr>) if any.
 ///
-/// If the message is wrongly signed, HashSet will be empty.
+/// If the message is wrongly signed, returns an empty map.
 pub fn valid_signature_fingerprints(
     msg: &pgp::composed::Message,
     public_keys_for_validation: &[SignedPublicKey],
-) -> HashSet<Fingerprint> {
-    let mut ret_signature_fingerprints: HashSet<Fingerprint> = Default::default();
+) -> HashMap<Fingerprint, Vec<Fingerprint>> {
+    let mut ret_signature_fingerprints = HashMap::new();
     if msg.is_signed() {
         for pkey in public_keys_for_validation {
-            if msg.verify(&pkey.primary_key).is_ok() {
+            if let Ok(signature) = msg.verify(&pkey.primary_key) {
                 let fp = pkey.dc_fingerprint();
-                ret_signature_fingerprints.insert(fp);
+                let mut recipient_fps = Vec::new();
+                if let Some(cfg) = signature.config() {
+                    for subpkt in &cfg.hashed_subpackets {
+                        if let SubpacketData::IntendedRecipientFingerprint(fp) = &subpkt.data {
+                            recipient_fps.push(fp.clone().into());
+                        }
+                    }
+                }
+                ret_signature_fingerprints.insert(fp, recipient_fps);
             }
         }
     }
@@ -497,13 +506,14 @@ mod tests {
     use pgp::composed::Esk;
     use pgp::packet::PublicKeyEncryptedSessionKey;
 
+    #[expect(clippy::type_complexity)]
     fn pk_decrypt_and_validate<'a>(
         ctext: &'a [u8],
         private_keys_for_decryption: &'a [SignedSecretKey],
         public_keys_for_validation: &[SignedPublicKey],
     ) -> Result<(
         pgp::composed::Message<'static>,
-        HashSet<Fingerprint>,
+        HashMap<Fingerprint, Vec<Fingerprint>>,
         Vec<u8>,
     )> {
         let mut msg = decrypt(ctext.to_vec(), private_keys_for_decryption, &[])?;
@@ -611,7 +621,7 @@ mod tests {
     }
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-    async fn test_decrypt_singed() {
+    async fn test_decrypt_signed() {
         // Check decrypting as Alice
         let decrypt_keyring = vec![KEYS.alice_secret.clone()];
         let sig_check_keyring = vec![KEYS.alice_public.clone()];
@@ -623,6 +633,10 @@ mod tests {
         .unwrap();
         assert_eq!(content, CLEARTEXT);
         assert_eq!(valid_signatures.len(), 1);
+        for recipient_fps in valid_signatures.values() {
+            // Intended Recipient Fingerprint subpackets aren't added currently.
+            assert_eq!(recipient_fps.len(), 0);
+        }
 
         // Check decrypting as Bob
         let decrypt_keyring = vec![KEYS.bob_secret.clone()];
@@ -635,6 +649,9 @@ mod tests {
         .unwrap();
         assert_eq!(content, CLEARTEXT);
         assert_eq!(valid_signatures.len(), 1);
+        for recipient_fps in valid_signatures.values() {
+            assert_eq!(recipient_fps.len(), 0);
+        }
     }
 
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]

src/receive_imf.rs

@@ -577,7 +577,7 @@ 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.signature.as_ref();
+    let fingerprint = mime_parser.signature.as_ref().map(|(fp, _)| fp);
     let (from_id, _from_id_blocked, incoming_origin) = match from_field_to_contact_id(
         context,
         &mime_parser.from,
@@ -3901,7 +3901,7 @@ async fn has_verified_encryption(
     let signed_with_verified_key = mimeparser
         .signature
         .as_ref()
-        .is_some_and(|signature| *signature == fingerprint);
+        .is_some_and(|(signature, _)| *signature == fingerprint);
     if signed_with_verified_key {
         Ok(Verified)
     } else {

src/securejoin.rs

@@ -784,7 +784,7 @@ fn encrypted_and_signed(
     mimeparser: &MimeMessage,
     expected_fingerprint: &Fingerprint,
 ) -> bool {
-    if let Some(signature) = mimeparser.signature.as_ref() {
+    if let Some((signature, _)) = mimeparser.signature.as_ref() {
         if signature == expected_fingerprint {
             true
         } else {