Connection between two hidden NRF51822 modules

[vldzero]

Hello

First of all, I would like to apologize if a similar question was already asked but all this BLE matter is so broad that it defeats me. I am planning to put two NRF51822 devices into my project. I want one of the devices to pull one of the GPIO pins down as soon as the other one is in the range of 10-30m. Also, I want it to be secure, I don't want anyone else to scan my devices, copy them and control the pin with another one. I want slave device to consume as low energy as possible.

My idea is:

1. make both devices undiscoverable so it will be difficult to scan for them.
2. The master device (the one that controls the GPIO pin) will advertise only to the slave one and it will be visible only for the slave one.
3. The slave device will check if it is in range of 10-30-m and if so it will send back a message/password or whatever to master. If it is not in range it won't send any message.
4. If master receives the message it pulls the pin down.
5. Repeat points 2-4 each 1s.

Is it possible to implement such routine? Or are there any better ideas? I am a total newbie in BLE. I managed to run simple advertising example but nothing more. I would appreciate a guidance, not a readymade solution.

vldzero

[vldzero]

Thanks Petter The device will be a simple wirelessly controlled relay. While the slave is in range of central one the relay controlled by the master will energize. It is all part of a bigger system but BLE will play an essential role. With your input and my tiny knowledge about BLE, I simplified the list.

1. The slave will advertise with a whitelist. I won't use random resolvable address for now since it would take me too much time. When the device is functional I will have time to improve the safety.
2. Slave will run service where an ID will be stored. If the ID is accepted by the master it will energize the relay.
3. If the device is out of range master will de-energize the relay.