Why do you want to encrypt / decrypt your connection string?
Is the web.config public?
I am using .NET with a Sql-Server as backend in windows authentication mode, so no password is required.
If you really need this encryption: Decrypt the connection string in the Global.asax and store it in the application object:
Application("connectionstring-decrypted") = yourDecryptFunction(Input)
and use it:
Dim conn As New Connection(CType(Application("connectionstring-decrypted"), String))