If I were you, I would put the data into C# structure, and serialize it to disk. 200 MB seems like an amount, which would safely fit into RAM. While serializing/deserializing, you can pass it through CryptoStream, and get encryption requirement covered that way. Whole database layer could be so simple, it would fit on one screen.
solved The Right Database (Read-Only, Tabular, Desktop, Web, C#) [closed]