ZTM Default Mifare Keys

Default manufacturer keys for mifare are often 0000000000, ffffffffff or a1a2a3a4a5. For ZTM WKM communication cards they begin (of course) with one of the defaults and contain mostly of 0x8989 sequence (year when Solidarity's movement was born). see: http://is.gd/IU3nji

Is it really secure?