From 7fcc612e1efa7f9afc80b5957cfce835377e2af0 Mon Sep 17 00:00:00 2001
From: link2xt <link2xt@testrun.org>
Date: Mon, 8 Jun 2026 00:23:04 +0200
Subject: [PATCH 1/3] refactor: remove Context argument from
 secret_key_to_public_key()

---
 src/key.rs                           | 5 +----
 src/mimefactory/mimefactory_tests.rs | 1 -
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/src/key.rs b/src/key.rs
index e7d69eb5fe..ae6f9994ac 100644
--- a/src/key.rs
+++ b/src/key.rs
@@ -124,14 +124,11 @@ pub trait DcKey: Serialize + Deserializable + Clone {
 
 /// Converts secret key to public key.
 pub(crate) fn secret_key_to_public_key(
-    context: &Context,
     mut signed_secret_key: SignedSecretKey,
     timestamp: u32,
     addr: &str,
     relay_addrs: &str,
 ) -> Result<SignedPublicKey> {
-    info!(context, "Converting secret key to public key.");
-
     // Make sure timestamp of created signatures
     // is not in the past compared to the primary key timestamp.
     let timestamp = std::cmp::max(
@@ -304,7 +301,7 @@ pub(crate) async fn load_self_public_key_opt(context: &Context) -> Result<Option
     let addr = context.get_primary_self_addr().await?;
     let all_addrs = context.get_self_addrs().await?.join(",");
     let signed_public_key =
-        secret_key_to_public_key(context, signed_secret_key, timestamp, &addr, &all_addrs)?;
+        secret_key_to_public_key(signed_secret_key, timestamp, &addr, &all_addrs)?;
     *lock = Some(signed_public_key.clone());
 
     Ok(Some(signed_public_key))
diff --git a/src/mimefactory/mimefactory_tests.rs b/src/mimefactory/mimefactory_tests.rs
index 9420f2c385..5989aa36df 100644
--- a/src/mimefactory/mimefactory_tests.rs
+++ b/src/mimefactory/mimefactory_tests.rs
@@ -341,7 +341,6 @@ async fn test_mdn_sent_to_all_relays() -> Result<()> {
     // Bob's key gets a second relay address and Alice merges the newer key.
     let bob_secret_key = load_self_secret_key(bob).await?;
     let bob_public_key = secret_key_to_public_key(
-        bob,
         bob_secret_key,
         u32::try_from(time())? + 100,
         "bob@example.net",

From 36384f977f0ba0f63883b675a60c2d9a9d12e1e6 Mon Sep 17 00:00:00 2001
From: link2xt <link2xt@testrun.org>
Date: Mon, 8 Jun 2026 00:23:04 +0200
Subject: [PATCH 2/3] feat: take key flags and expiration into account when
 selecting the key

---
 src/pgp.rs | 64 +++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 58 insertions(+), 6 deletions(-)

diff --git a/src/pgp.rs b/src/pgp.rs
index 31b7c27215..62a5416fdd 100644
--- a/src/pgp.rs
+++ b/src/pgp.rs
@@ -1,5 +1,6 @@
 //! OpenPGP helper module using [rPGP facilities](https://github.com/rpgp/rpgp).
 
+use std::cmp::Ordering;
 use std::collections::{HashMap, HashSet};
 use std::io::Cursor;
 
@@ -83,13 +84,62 @@ pub(crate) fn create_keypair(addr: EmailAddress) -> Result<SignedSecretKey> {
 
 /// Selects a subkey of the public key to use for encryption.
 ///
-/// Returns `None` if the public key cannot be used for encryption.
+/// The key is selected according to
+/// <https://www.ietf.org/archive/id/draft-autocrypt-openpgp-v2-cert-03.html#section-4.3-4>.
+/// If multiple keys are available, the one that will expire sooner is selected.
 ///
-/// TODO: take key flags and expiration dates into account
-fn select_pk_for_encryption(key: &SignedPublicKey) -> Option<&SignedPublicSubKey> {
+/// Returns `None` if the public key cannot be used for encryption.
+fn select_pk_for_encryption(now: u32, key: &SignedPublicKey) -> Option<&SignedPublicSubKey> {
     key.public_subkeys
         .iter()
-        .find(|subkey| subkey.algorithm().can_encrypt())
+        .filter(|subkey| subkey.algorithm().can_encrypt())
+        .filter_map(|subkey| {
+            let signature = subkey.signatures.first()?;
+
+            let key_flags = signature.key_flags();
+            if !key_flags.encrypt_comms() {
+                return None;
+            }
+
+            if let Some(expiration_duration) = signature
+                .key_expiration_time()
+                .filter(|duration| duration.as_secs() != 0)
+                && now
+                    > subkey
+                        .created_at()
+                        .as_secs()
+                        .saturating_add(expiration_duration.as_secs())
+            {
+                // Key is expired.
+                return None;
+            }
+            Some((subkey, signature))
+        })
+        .min_by(|(subkey1, signature1), (subkey2, signature2)| {
+            match (
+                signature1
+                    .key_expiration_time()
+                    .filter(|duration| duration.as_secs() != 0),
+                signature2
+                    .key_expiration_time()
+                    .filter(|duration| duration.as_secs() != 0),
+            ) {
+                (None, None) => Ordering::Equal,
+                (None, Some(_)) => Ordering::Greater,
+                (Some(_), None) => Ordering::Less,
+                (Some(expiration1), Some(expiration2)) => (subkey1
+                    .created_at()
+                    .as_secs()
+                    .saturating_add(expiration1.as_secs()))
+                .cmp(
+                    &(subkey2
+                        .created_at()
+                        .as_secs()
+                        .saturating_add(expiration2.as_secs())),
+                ),
+            }
+        })
+        .map(|(subkey, _signature)| subkey)
 }
 
 /// Version of SEIPD packet to use.
@@ -151,10 +201,11 @@ pub fn pk_encrypt(
 ) -> Result<String> {
     tokio::task::block_in_place(|| {
         let mut rng = thread_rng();
+        let now = pgp::types::Timestamp::now();
 
         let pkeys = public_keys_for_encryption
             .iter()
-            .filter_map(select_pk_for_encryption);
+            .filter_map(|key| select_pk_for_encryption(now.as_secs(), key));
 
         let msg = MessageBuilder::from_bytes("", plain);
         let encoded_msg = match seipd_version {
@@ -485,7 +536,8 @@ pub(crate) fn relay_addrs(public_key: &SignedPublicKey, addr: &str) -> Vec<Strin
 
 /// Returns true if the key can be encrypted to, i.e. has an encryption subkey.
 pub(crate) fn pubkey_can_encrypt(public_key: &SignedPublicKey) -> bool {
-    select_pk_for_encryption(public_key).is_some()
+    let now = pgp::types::Timestamp::now();
+    select_pk_for_encryption(now.as_secs(), public_key).is_some()
 }
 
 /// Returns true if public key advertises SEIPDv2 feature.

From a1675f46bc95be14901819c5d76e6108486ff8fa Mon Sep 17 00:00:00 2001
From: link2xt <link2xt@testrun.org>
Date: Mon, 8 Jun 2026 15:05:38 +0200
Subject: [PATCH 3/3] feat: merging and minimization of subkeys

---
 src/mimefactory.rs |   7 +-
 src/pgp.rs         | 212 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 213 insertions(+), 6 deletions(-)

diff --git a/src/mimefactory.rs b/src/mimefactory.rs
index 82f9823faf..bd52b8a1e1 100644
--- a/src/mimefactory.rs
+++ b/src/mimefactory.rs
@@ -331,9 +331,10 @@ pub(crate) fn render_queued_mail(
     outer_headers.extend(b"MIME-Version: 1.0\r\n");
 
     if should_attach_pubkey {
+        let public_key = crate::pgp::minimize_autocrypt_certificate(public_key);
         let aheader = Aheader {
             addr: from_addr,
-            public_key: public_key.clone(),
+            public_key,
             prefer_encrypt: EncryptPreference::Mutual,
         };
         let autocrypt_header = mail_builder::headers::raw::Raw::new(aheader.to_string());
@@ -1472,9 +1473,11 @@ impl MimeFactory {
                                 continue;
                             }
 
+                            let public_key = crate::pgp::minimize_autocrypt_certificate(key);
+
                             let header = Aheader {
                                 addr: addr.clone(),
-                                public_key: key.clone(),
+                                public_key,
                                 // Autocrypt 1.1.0 specification says that
                                 // `prefer-encrypt` attribute SHOULD NOT be included.
                                 prefer_encrypt: EncryptPreference::NoPreference,
diff --git a/src/pgp.rs b/src/pgp.rs
index 62a5416fdd..70137bcca0 100644
--- a/src/pgp.rs
+++ b/src/pgp.rs
@@ -1,7 +1,8 @@
 //! OpenPGP helper module using [rPGP facilities](https://github.com/rpgp/rpgp).
 
 use std::cmp::Ordering;
-use std::collections::{HashMap, HashSet};
+use std::collections::btree_map::Entry as BTreeMapEntry;
+use std::collections::{BTreeMap, HashMap, HashSet};
 use std::io::Cursor;
 
 use anyhow::{Context as _, Result, ensure};
@@ -15,7 +16,7 @@ use pgp::crypto::aead::{AeadAlgorithm, ChunkSize};
 use pgp::crypto::ecc_curve::ECCCurve;
 use pgp::crypto::hash::HashAlgorithm;
 use pgp::crypto::sym::SymmetricKeyAlgorithm;
-use pgp::packet::{Signature, Subpacket, SubpacketData};
+use pgp::packet::{Signature, SignatureType, Subpacket, SubpacketData};
 use pgp::types::{
     CompressionAlgorithm, Imprint, KeyDetails, KeyVersion, Password, SignedUser, SigningKey as _,
     StringToKey,
@@ -361,6 +362,161 @@ pub fn symm_encrypt_message(
     })
 }
 
+/// Minimizes the signatures of a subkey.
+///
+/// Keeps at most one subkey binding signature
+/// and at most one revocation signature,
+/// preferring the newest signatures.
+///
+/// Subkey binding signature is kept
+/// even if the revocation signature exists
+/// because according to
+/// <https://www.rfc-editor.org/rfc/rfc9580.html#name-openpgp-version-6-certifica>
+/// "Every subkey MUST have at least one Subkey Binding signature."
+/// Distributing subkey with only a revocation signature
+/// is not allowed according to the standard,
+/// so we keep a subkey binding signature next to it
+/// for interoperability.
+///
+/// This function does not check if the signatures are valid.
+/// Such properties should be validated when importing OpenPGP certificates.
+fn minimize_subpacket_signatures(signatures: Vec<Signature>) -> Vec<Signature> {
+    let mut newest_revocation_signature: Option<Signature> = None;
+    let mut newest_binding_signature: Option<Signature> = None;
+    for signature in signatures {
+        let Some(config) = signature.config() else {
+            // Skip unknown signatures.
+            continue;
+        };
+        match config.typ {
+            SignatureType::SubkeyBinding => {
+                if newest_binding_signature
+                    .as_ref()
+                    .is_none_or(|s| s.created() < signature.created())
+                {
+                    newest_binding_signature = Some(signature)
+                }
+            }
+            SignatureType::SubkeyRevocation => {
+                if newest_revocation_signature
+                    .as_ref()
+                    .is_none_or(|s| s.created() < signature.created())
+                {
+                    newest_revocation_signature = Some(signature)
+                }
+            }
+            _ => continue,
+        }
+    }
+    newest_revocation_signature
+        .into_iter()
+        .chain(newest_binding_signature)
+        .collect()
+}
+
+/// Minimizes OpenPGP certificate for Autocrypt and Autocrypt-Gossip headers.
+pub fn minimize_autocrypt_certificate(certificate: &SignedPublicKey) -> SignedPublicKey {
+    let primary_key = certificate.primary_key.clone();
+    let details = certificate.details.clone();
+
+    // Select the newest non-expiring subkey and the newest expiring subkey.
+    let fallback_subkey = certificate
+        .public_subkeys
+        .iter()
+        .filter(|subkey| {
+            subkey
+                .signatures
+                .iter()
+                .find(|signature| {
+                    signature
+                        .config()
+                        .is_some_and(|config| config.typ == SignatureType::SubkeyBinding)
+                })
+                .is_some_and(|signature| signature.key_expiration_time().is_none())
+        })
+        .max_by_key(|subkey| {
+            subkey
+                .signatures
+                .iter()
+                .find(|signature| {
+                    signature
+                        .config()
+                        .is_some_and(|config| config.typ == SignatureType::SubkeyBinding)
+                })
+                .map(|signature| signature.created().unwrap_or(subkey.created_at()))
+        });
+    let rotating_subkey = certificate
+        .public_subkeys
+        .iter()
+        .filter(|subkey| {
+            subkey
+                .signatures
+                .iter()
+                .find(|signature| {
+                    signature
+                        .config()
+                        .is_some_and(|config| config.typ == SignatureType::SubkeyBinding)
+                })
+                .is_some_and(|signature| signature.key_expiration_time().is_some())
+        })
+        .max_by_key(|subkey| {
+            subkey
+                .signatures
+                .iter()
+                .find(|signature| {
+                    signature
+                        .config()
+                        .is_some_and(|config| config.typ == SignatureType::SubkeyBinding)
+                })
+                .map(|signature| signature.created().unwrap_or(subkey.created_at()))
+        });
+    let public_subkeys: Vec<_> = fallback_subkey
+        .into_iter()
+        .chain(rotating_subkey)
+        .cloned()
+        .collect();
+
+    // We do not want to ever gossip more than two subkeys
+    // to save the traffic.
+    debug_assert!(public_subkeys.len() <= 2);
+
+    SignedPublicKey {
+        primary_key,
+        details,
+        public_subkeys,
+    }
+}
+
+/// Merges two OpenPGP subkeys.
+fn merge_openpgp_subkey(old_subkey: &mut SignedPublicSubKey, new_subkey: SignedPublicSubKey) {
+    debug_assert_eq!(old_subkey.fingerprint(), new_subkey.fingerprint());
+    old_subkey.signatures = minimize_subpacket_signatures(
+        std::mem::take(&mut old_subkey.signatures)
+            .into_iter()
+            .chain(new_subkey.signatures)
+            .collect(),
+    );
+}
+
+/// Merges OpenPGP subkey vectors.
+pub fn merge_openpgp_subkeys(
+    subkeys: impl IntoIterator<Item = SignedPublicSubKey>,
+) -> Result<Vec<SignedPublicSubKey>> {
+    let mut merged_subkeys: BTreeMap<_, SignedPublicSubKey> = BTreeMap::new();
+    for subkey in subkeys {
+        let imprint = subkey.imprint::<Sha256>()?;
+        match merged_subkeys.entry(imprint) {
+            BTreeMapEntry::Vacant(entry) => {
+                entry.insert(subkey);
+            }
+            BTreeMapEntry::Occupied(entry) => {
+                merge_openpgp_subkey(entry.into_mut(), subkey);
+            }
+        }
+    }
+    Ok(merged_subkeys.into_values().collect())
+}
+
 /// Merges and minimizes OpenPGP certificates.
 ///
 /// Keeps at most one direct key signature and
@@ -397,7 +553,7 @@ pub fn merge_openpgp_certificates(
     let SignedPublicKey {
         primary_key: new_primary_key,
         details: new_details,
-        public_subkeys: _new_public_subkeys,
+        public_subkeys: new_public_subkeys,
     } = new_certificate;
 
     // Public keys may be serialized differently, e.g. using old and new packet type,
@@ -473,7 +629,55 @@ pub fn merge_openpgp_certificates(
         });
     let users: Vec<SignedUser> = best_user.into_iter().collect();
 
-    let public_subkeys = old_public_subkeys;
+    let (fallback_subkeys, mut rotating_subkeys): (Vec<_>, Vec<_>) =
+        merge_openpgp_subkeys(old_public_subkeys.into_iter().chain(new_public_subkeys))?
+            .into_iter()
+            .filter_map(|subkey| {
+                // Select the newest subkey binding signature.
+                //
+                // There is at most one subkey binding signature at this point
+                // because older subkey binding signatures are removed during merging.
+                let signature = subkey.signatures.iter().find(|signature| {
+                    signature
+                        .config()
+                        .is_some_and(|config| config.typ == SignatureType::SubkeyBinding)
+                })?;
+
+                let created_at_secs = signature.created().unwrap_or(subkey.created_at()).as_secs();
+                let expires_at_secs: Option<u32> = signature
+                    .key_expiration_time()
+                    .map(|duration| duration.as_secs())
+                    .filter(|duration_secs| *duration_secs != 0)
+                    .map(|duration_secs| {
+                        subkey.created_at().as_secs().saturating_add(duration_secs)
+                    });
+
+                Some((subkey, created_at_secs, expires_at_secs))
+            })
+            .partition(|(_subkey, _created_at_secs, expires_at_secs)| expires_at_secs.is_none());
+    let fallback_subkey: Option<SignedPublicSubKey> = fallback_subkeys
+        .into_iter()
+        .max_by_key(|(_subkey, created_at_secs, _)| *created_at_secs)
+        .map(|(subkey, _, _)| subkey);
+
+    rotating_subkeys
+        .sort_by_key(|(_subkey, created_at_secs, _)| std::cmp::Reverse(*created_at_secs));
+
+    // Put the fallback subkey first so it is gossiped first.
+    //
+    // We want to always gossip non-expiring key first
+    // for older versions that always encrypted to the first subkey.
+    //
+    // Keep 10 newest rotating subkeys to avoid storing indefinitely growing number of subkeys locally.
+    let public_subkeys = fallback_subkey
+        .into_iter()
+        .chain(
+            rotating_subkeys
+                .into_iter()
+                .take(10)
+                .map(|(subkey, _, _)| subkey),
+        )
+        .collect();
 
     Ok(SignedPublicKey {
         primary_key: old_primary_key,