when(dc.toString()).thenReturn("SERVER123");

            NtlmChallenge nc = new NtlmChallenge(challenge, dc);
            String result = nc.toString();

            // The format is: NtlmChallenge[challenge=0x<hex>,dc=<dc_string>]
            assertTrue(result.startsWith("NtlmChallenge[challenge=0x"));
            assertTrue(result.endsWith(",dc=SERVER123]"));