openpgp: Serialize TSKs without secrets with appropriate label.

  - Previously, we emitted the PRIVATE KEY label when armoring TSKs
    that don't in fact contain secrets, confusing users.

  - See https://gitlab.com/sequoia-pgp/sequoia-sq/-/issues/14

  - Fixes #1075.

2 files changed, 44 insertions, 3 deletions

diff --git a/openpgp/src/serialize/cert.rs b/openpgp/src/serialize/cert.rs
index 2702fda6..764a16ac 100644
--- a/openpgp/src/serialize/cert.rs
+++ b/openpgp/src/serialize/cert.rs
@@ -280,6 +280,17 @@ impl<'a> TSK<'a> {
         }
     }
 
+    /// Returns whether we're emitting secret (sub)key packets when
+    /// serializing.
+    ///
+    /// Computes whether we have secrets, taking the filter into
+    /// account, and whether we are emitting stubs.  This can be used
+    /// to determine the correct armor label to use.
+    pub(crate) fn emits_secret_key_packets(&self) -> bool {
+        self.emit_stubs
+            || self.cert.keys().secret().any(|skb| (self.filter)(skb.key()))
+    }
+
     /// Filters which secret keys to export using the given predicate.
     ///
     /// Note that the given filter replaces any existing filter.
@@ -985,4 +996,27 @@ mod test {
 
         Ok(())
     }
+
+    #[test]
+    fn issue_1075() -> Result<()> {
+        let cert = Cert::from_bytes(crate::tests::key("testy.pgp"))?;
+        let tsk = Cert::from_bytes(crate::tests::key("testy-private.pgp"))?;
+
+        // Filters out secrets.
+        let no_secrets = |_| false;
+
+        assert!(! cert.as_tsk().emits_secret_key_packets());
+        assert!(cert.as_tsk().emit_secret_key_stubs(true)
+                .emits_secret_key_packets());
+        assert!(tsk.as_tsk().emits_secret_key_packets());
+        assert!(! tsk.as_tsk().set_filter(no_secrets)
+                .emits_secret_key_packets());
+
+        assert!(cert.armored().to_vec()? != tsk.as_tsk().armored().to_vec()?);
+        assert_eq!(cert.armored().to_vec()?,
+                   cert.as_tsk().armored().to_vec()?);
+        assert_eq!(cert.armored().to_vec()?,
+                   tsk.as_tsk().set_filter(no_secrets).armored().to_vec()?);
+        Ok(())
+    }
 }
diff --git a/openpgp/src/serialize/cert_armored.rs b/openpgp/src/serialize/cert_armored.rs
index a3060e8d..63642954 100644
--- a/openpgp/src/serialize/cert_armored.rs
+++ b/openpgp/src/serialize/cert_armored.rs
@@ -143,8 +143,11 @@ impl<'a> Encoder<'a> {
         let (prelude, headers) = match self {
             Encoder::Cert(cert) =>
                 (armor::Kind::PublicKey, cert.armor_headers()),
-            Encoder::TSK(ref tsk) =>
-                (armor::Kind::SecretKey, tsk.cert.armor_headers()),
+            Encoder::TSK(tsk) => if tsk.emits_secret_key_packets() {
+                (armor::Kind::SecretKey, tsk.cert.armor_headers())
+            } else {
+                (armor::Kind::PublicKey, tsk.cert.armor_headers())
+            },
         };
 
         // Convert the Vec<String> into Vec<(&str, &str)>
@@ -202,7 +205,11 @@ impl<'a> MarshalInto for Encoder<'a> {
 
         let word = match self {
             Self::Cert(_) => "PUBLIC",
-            Self::TSK(_) => "PRIVATE",
+            Self::TSK(tsk) => if tsk.emits_secret_key_packets() {
+                "PRIVATE"
+            } else {
+                "PUBLIC"
+            },
         }.len();
 
         "-----BEGIN PGP ".len() + word + " KEY BLOCK-----\n\n".len()