keys: warn about short key only if used by source

After restricting authentication of servers and peers to the specified
key, a short key in the key file is a security problem from the client's
point of view only if it's specified for a source.

keys.c

@@ -198,9 +198,6 @@ KEY_Reload(void)
       continue;
     }
 
-    if (key.len < MIN_SECURE_KEY_LENGTH)
-      LOG(LOGS_WARN, LOGF_Keys, "Key %"PRIu32" is too short", key_id);
-
     key.id = key_id;
     key.val = MallocArray(char, key.len);
     memcpy(key.val, keyval, key.len);
@@ -295,6 +292,21 @@ KEY_GetAuthDelay(uint32_t key_id)
 
 /* ================================================== */
 
+int
+KEY_CheckKeyLength(uint32_t key_id)
+{
+  Key *key;
+
+  key = get_key_by_id(key_id);
+
+  if (!key)
+    return 0;
+
+  return key->len >= MIN_SECURE_KEY_LENGTH;
+}
+
+/* ================================================== */
+
 int
 KEY_GenerateAuth(uint32_t key_id, const unsigned char *data, int data_len,
     unsigned char *auth, int auth_len)