I wrote a little password checker for kobo readers. The main idea is to intercept the start up script at a certain point and ask for the password. If the correct password is entered the start up script continues, if a wrong password is entered (you get three chances), the device is powered off.
The security level is rather low.
- I don't encrypt the password. If anybody has access to your running reader, he can find your password.
- Anybody can factory reset your reader. (S)he will be able to use your reader as (s)he likes, any personal information however will be deleted.
- If the device has a removable sd card, anybody with the necessary knowledge can open the device and grap any information from the sd.
As you see the protection level is not very high, but might suffice for some situations. As for the first security loophole, you might consider activating "Automatically power off after" in the Sleep & power settings.The second security loophole (factory reset) might actually be welcomed in case you forget the password.