diff --git a/src/libostree/ostree-gpg-verify-result.c b/src/libostree/ostree-gpg-verify-result.c
index 0277ce1e..059b3d56 100644
--- a/src/libostree/ostree-gpg-verify-result.c
+++ b/src/libostree/ostree-gpg-verify-result.c
@@ -237,7 +237,7 @@ ostree_gpg_verify_result_lookup (OstreeGpgVerifyResult *result,
                                  const gchar *key_id,
                                  guint *out_signature_index)
 {
-  g_autofree char *key_id_upper = NULL;
+  gpgme_key_t lookup_key = NULL;
   gpgme_signature_t signature;
   guint signature_index;
   gboolean ret = FALSE;
@@ -245,25 +245,46 @@ ostree_gpg_verify_result_lookup (OstreeGpgVerifyResult *result,
   g_return_val_if_fail (OSTREE_IS_GPG_VERIFY_RESULT (result), FALSE);
   g_return_val_if_fail (key_id != NULL, FALSE);
 
-  /* signature->fpr is always upper-case. */
-  key_id_upper = g_ascii_strup (key_id, -1);
+  /* fetch requested key_id from keyring to canonicalise ID */
+  (void) gpgme_get_key (result->context, key_id, &lookup_key, 0);
+
+  if (lookup_key == NULL)
+    {
+      g_debug ("Could not find key ID %s to lookup signature.", key_id);
+      return FALSE;
+    }
 
   for (signature = result->details->signatures, signature_index = 0;
        signature != NULL;
        signature = signature->next, signature_index++)
     {
-      if (signature->fpr == NULL)
-        continue;
+      gpgme_key_t signature_key = NULL;
 
-      if (g_str_has_suffix (signature->fpr, key_id_upper))
+      (void) gpgme_get_key (result->context, signature->fpr, &signature_key, 0);
+
+      if (signature_key == NULL)
+        {
+          g_debug ("Could not find key when looking up signature from %s.", signature->fpr);
+          continue;
+        }
+
+      /* the first subkey in the list is the primary key */
+      if (!g_strcmp0 (lookup_key->subkeys->fpr,
+                      signature_key->subkeys->fpr))
         {
           if (out_signature_index != NULL)
             *out_signature_index = signature_index;
           ret = TRUE;
-          break;
         }
+
+      gpgme_key_unref (signature_key);
+
+      if (ret)
+        break;
     }
 
+  gpgme_key_unref (lookup_key);
+
   return ret;
 }