diff --git a/pySim/sysmocom_sja2.py b/pySim/sysmocom_sja2.py
index 8b1fc04e..1bd4a8a8 100644
--- a/pySim/sysmocom_sja2.py
+++ b/pySim/sysmocom_sja2.py
@@ -207,7 +207,7 @@ class EF_USIM_AUTH_KEY(TransparentEF):
                                    'tuak_cfg'/TuakCfgByte,
                                    'num_of_keccak_iterations'/Int8ub,
                                    'op_opc'/HexAdapter(Bytes(32)),
-                                   'k'/HexAdapter(Bytes(32)))
+                                   'k'/HexAdapter(Bytes(this.cfg.key_length//8)))
 
     def _decode_bin(self, raw_bin_data: bytearray) -> dict:
         if raw_bin_data[0] & 0x0F == 0x06: