Mifare and iCLASS smart cards require cryptographic keys to encrypt data stream communication between cards and card readers. You can set the site key on BioStar Client. Only when the key stored in the reader matches the key stored in the card, the encrypted data is properly decrypted.
Go to Option > Mifare Sitekey or iCLASS Sitekey. This brings up the site key setting dialog as shown in the figure below:
Current Primary Key: Enter the current primary site key. If you haven't set it, just leave it blank.
New Primary Key: Enter a key you want to use as a primary cryptographic key.
Retype Primary Key: Enter the new primary key again.
Secondary Key: Enter a key you want to use as a secondary cryptographic key. If you set the secondary key, the reader will also try the secondary key when the primary key doesn't match a key in a card. The secondary key can be set as a site key of old cards to allow users with old cards to gain access.