CPLDs such as MAX II have a function called a security bit to prevent leakage of write data.
(The security bit can be set on the Quartus II programmer.)

After checking and programming the security bit, if you Examine the device to which you wrote, you will be able to extract some data, but it will be EXamined as different data than the previously written data. increase.
Therefore, it becomes impossible to know what data was written to the device.
　
It is possible to prevent data leakage by the above method.
 
 
When the security bit is checked and programming and verification are executed at the same time, a verify check is possible, but if only the verify check is performed, a verify error will occur.
 
If you want to update new data, you can update it by writing a new pof file.