I am a bit confused about securing the secret encyption key. Here is the scenari

Question

I am a bit confused about securing the secret encyption key. Here is the scenario:

1. I need to encrypt some data (file)

2. This data will be read by a program (say written in C/C++).

3. Both the program binary and encrypted data will be accesible to some arbitrary user.

However, in order to read the data the program needs to have the secret key that was used to encrypt the data. One possibility would be to hard code the secret key into the source code of the program itself. However, the key can be found out even from the compiled binary! I also do not have any possibility of taking input from the user for password/secret key etc. I also don't have any network access to request a server to send the key. Please, assume the OS to be linux/linux like.

So, the question is, how can I secure this key?

Explanation / Answer

First, do not bake the key into the binary. As Xander noted, a HSM is designed for this purpose such that a key is only accessible to the application and is not on the system. You may be able to setup a bastion host you locked down on a separate management network to serve this purpose instead, which is a little less secure. The next level would be to at least keep the key in a secure area on the local file system. You then use access control and privileged identity management and monitoring to limit the possibility of the root user getting the key and not being detected.

If the data is never used by computers and only be explicitly identified users/subjects, you could try something like Mylar from MIT.

You need to find the solution that will maximize your return on security investment relative to the risk of the encrypted data being exposed. There will always be threats, but the likelihood and impact will need to be considered before selecting any countermeasures.

It's also not clear if this is a client-server application, a web application, or a desktop program which uses a local data file. These specifics would further impact your strategy.

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