You want to use symmetric encryption to prevent a malicious user from viewing the contents of a file.
Wrap the FileStream with a CryptoStream that encrypts data as it’s being written to disk (or decrypts it as it’s being read).
Symmetric encryption renders data indecipherable using a secret value. To decrypt the data, you need the same secret value (or key) that was used to encrypt the data. For that reason, symmetric encryption is often described as shared secret encryption.
Several classes provide symmetric encryption services in the System.Security.Cryptography namespace. These classes all derive from the base System.Security.SymmetricAlgorithm class, and they work almost identically. The difference ...