openpgp: Reject short key IDs.

  - When parsing a key ID string, reject short key IDs.

  - Note: we can't reject short key IDs in `KeyID::from_bytes`,
    because that function in infallible.  But, that function does return
    `KeyID::Invalid` when presented with a short key ID.

  - Fixes #388.

3 files changed, 20 insertions, 2 deletions

diff --git a/openpgp/NEWS b/openpgp/NEWS
index d5b7debf..c68fd0d8 100644
--- a/openpgp/NEWS
+++ b/openpgp/NEWS
@@ -17,6 +17,7 @@
    - Cert::exportable
    - CertBuilder::set_exportable
    - UserID::from_static_bytes
+   - Error::ShortKeyID
 * Changes in 1.17.0
 ** Notable fixes
    - Sequoia now ignores some formatting errors when reading secret
diff --git a/openpgp/src/keyid.rs b/openpgp/src/keyid.rs
index 2d880706..1dc08fe8 100644
--- a/openpgp/src/keyid.rs
+++ b/openpgp/src/keyid.rs
@@ -107,6 +107,8 @@ impl std::str::FromStr for KeyID {
         // A KeyID is exactly 8 bytes long.
         if bytes.len() == 8 {
             Ok(KeyID::from_bytes(&bytes[..]))
+        } else if bytes.len() == 4 {
+            Err(Error::ShortKeyID(s.to_string()).into())
         } else {
             // Maybe a fingerprint was given.  Try to parse it and
             // convert it to a KeyID.
@@ -456,8 +458,19 @@ mod test {
         "GB3751F1587DAEF1".parse::<KeyID>().unwrap_err();
         "EFB3751F1587DAEF1".parse::<KeyID>().unwrap_err();
         "%FB3751F1587DAEF1".parse::<KeyID>().unwrap_err();
-        assert_match!(KeyID::Invalid(_) = "587DAEF1".parse().unwrap());
-        assert_match!(KeyID::Invalid(_) = "0x587DAEF1".parse().unwrap());
+    }
+
+    #[test]
+    fn from_hex_short_keyid() {
+        for s in &[ "FB3751F1", "0xFB3751F1", "fb3751f1",  "0xfb3751f1" ] {
+            match s.parse::<KeyID>() {
+                Ok(_) => panic!("Failed to reject short Key ID."),
+                Err(err) => {
+                    let err = err.downcast_ref::<Error>().unwrap();
+                    assert!(matches!(err, Error::ShortKeyID(_)));
+                }
+            }
+        }
     }
 
     #[test]
diff --git a/openpgp/src/lib.rs b/openpgp/src/lib.rs
index 6f792c88..09e1bfca 100644
--- a/openpgp/src/lib.rs
+++ b/openpgp/src/lib.rs
@@ -345,6 +345,10 @@ pub enum Error {
             })
             .unwrap_or_else(|| "".into()))]
     PolicyViolation(String, Option<std::time::SystemTime>),
+
+    /// Short key IDs are insecure, and not supported.
+    #[error("Short key IDs are insecure, and not supported: {0}")]
+    ShortKeyID(String),
 }
 
 assert_send_and_sync!(Error);