I need two methods one to encrypt and one to decrypt an xml file with a key= \"hello world\",the key hello world should be used to encrypt and decrypt the xml file.These met
First of all, if you want to use the same key for encrypting and decrypting, you should look at symmetric cryptography. Asymmetric cryptography is when the keys for encrypting and decrypting are different. Just so that you know - RSA is asymmetric, TripleDES and Rijndael are symmetric. There are others too, but .NET does not have default implementations for them.
I'd advise studying the System.Security.Cryptography namespace. And learning a bit about all that stuff. It has all you need to encrypt and decrypt files, as well as generate a password. In particular, you might be interested in these classes:
CryptoStreamPasswordDeriveBytesRijndaelManagedThere are also examples for usage in MSDN for each of them. You can use these classes to encrypt any file, not just XML. If however you want to encrypt just a select few elements, you can take a look at System.Security.Cryptography.Xml namespace. I see you've already found one article about it. Keep following the links on that page and you will learn more about those classes.