Creating a cryptographic connection strings file

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C#3.5

encryption

connection

hi

how to Creating a cryptographic connection strings  file and using the file at the beginning of the application.
connection string .

The connection string is not inside the program and is separated by a file
and in login form by combobox select for ever

What I have tried:

The connection string is not inside the program and is separated by a file
and in login form by combobox select for ever