feat: use SEIPDv2 if all recipients support it

src/pgp.rs

@@ -495,6 +495,35 @@ pub(crate) fn addresses_from_public_key(public_key: &SignedPublicKey) -> Option<
     None
 }
 
+/// Returns true if public key advertises SEIPDv2 feature.
+pub(crate) fn pubkey_supports_seipdv2(public_key: &SignedPublicKey) -> bool {
+    // If any Direct Key Signature or any User ID signature has SEIPDv2 feature,
+    // assume that recipient can handle SEIPDv2.
+    //
+    // Third-party User ID signatures are dropped during certificate merging.
+    // We don't check if the User ID is primary User ID.
+    // Primary User ID is preferred during merging
+    // and if some key has only non-primary User ID
+    // it is acceptable. It is anyway unlikely that SEIPDv2
+    // is advertised in a key without DKS or primary User ID.
+    public_key
+        .details
+        .direct_signatures
+        .iter()
+        .chain(
+            public_key
+                .details
+                .users
+                .iter()
+                .flat_map(|user| user.signatures.iter()),
+        )
+        .any(|signature| {
+            signature
+                .features()
+                .is_some_and(|features| features.seipd_v2())
+        })
+}
+
 #[cfg(test)]
 mod tests {
     use std::sync::LazyLock;