I'm searching for a database that can run on Windows and can be accessed with C#. The Database also requires that the data can be encrypted (ideally without backdoors).
Can MariaDB serve here?
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As mentioned above MS-SQL server suits you're requirement:
On how to proceed with MS SQL databases refer to below article:
For data encryption, yes its possible depending upon the edition you are going to use:
refer to this for further read on encryption via sql server : https://technet.microsoft.com/en-us/library/cc278098%28v=sql.100%29.aspx