Storing static string secure

You never told the level of security you are looking for.

Is it to protect a bank account access ?

If so, probably need some strong crypto, but also export authorizations

Or just protect some private information so that they are not visible to anyone ?

If so, you could design your own encryption.

I suggest to make it specific to each device

For instance (to be adapted if String contains non ASCII), to encode a String s:

- select a codeS string which is device specific (e.g, MAC address)

- append codeS to s

- select a number N between 3 and 6 (may be also device dependant)

- pad the resulting s to have a length multiple of N

- convert each char of String to another char with a f func (you could use a parameter to make it depend on device) - give a name that does not show it is part of encryption ; build g, the reverse func

- mangle the resulting String with a reversible algo (for instance by moving packets of N char)

Decoding is just the reverse order.

Level of security for storing api-secret key: Free from exposing or making the api-secret key non human-readable .

So, there is no risk for users sensitive data ?

In this case, probably don't need strong encryption. Could look to what I proposed.