Reads and decrypts CMS enveloped-data object using the recipient's private key
public static string ReadEnvDataToString( string inputFile, string x509File, string privateKey, Cms.Options options )
Public Shared Function ReadEnvDataToString ( _ inputFile As String, _ x509File As String, _ privateKey As String, _ options As Cms.Options _ ) As String
Message text or an empty string on error