how to prevent user from decrpting data file while the program is capable to read the data file freely?

Question

I want to ask for a mature model to do this. Suppose I want to deliver a program and a sensitive data file to user. The program is able to read any data stored in the data file, but user is not allowd to easly break the data file. The data file will be encrypted by standard algorithm such as AES. Now, the problem turns to how to manage the key. Putting the key in the program seems to be a bad idea, but what else I can do? Apparently I can't give the key to user directly.

Answer 1

There is no way to do this securely, ie. to really prevent the user from reading the data. As long as they have the data and they have the program that can read it a competent disassembler will be able to figure out how the program reads it and do the same thing. Or, even easier, they could let the program do it and then get the decrypted version out of its memory.

Having said that, if you just want to prevent the average user from doing it, hardcoding the key in your source code should be fine. :) Just be realistic about the level of protection this provides.

Answer 2

There is no 100% safe solution to this because at some point you have to have the key loaded into memory so that de/encryption can take place and a savvy-enough hacker will be able to capture it. The best you can do is to make it very difficult to capture and to mitigate exposure to data (by limiting access as much as possible) if the key is compromised.

As far as how to make it safer... you could have a combined key that is made up of something stored in the program and something derived from the user's passcode?

Answer 3

Is your perceived user determined? are they skilled enough to do reverse the application or the key? If the user is considered to just be a generic desktop user you probably could implement a partial key using some general encryption just to make the key non obvious, beyond that a determined individual will be able to reverse must simple means of encrypting keys and data.

Answer 4

A DVD John conundrum, eh? Why is having a key in the program bad? You could have a super-obscured function which computes it reliably once. Someone with disassembler and debugger can break your key given enough time, IMO.

Answer 5

Does it have to be pure software? If not, you could look at a solution which does decryption and storage of the key on a hardware device, e.g. a USB dongle.

You can also potentially prevent the whole problem by having your software retireve the data from a web-service instead of a data file. In this case you can control access to the data much more tightly (i.e. who gets how much of what and when). This might or might not work for your application.

Otherwise as others pointed out, there is no known good pure software solution.