How to encrypt the connection between Central（cc2540） and Peripheral(cc2540)

HI :

I use two cc2540 demoboard, one is SimpleBLECentral, and the other is SimpleBLEPeripheral,

After  connection  ，Central can read Characteristic 0xFFF1 from Peripheral. but cannot read 0xFFF5 ,

the error code is 5 : INSUFFICIENT_ENCRYPTION .

if use BTool, we can press button to Send paring request/send passkey。but how to do this in SimpleBLECentral?

Best Regards!

MarkChe