We are laying  foundation for an  internet hub based on XML. Basically, this will be an information exchange kind of a site that will rely on XML technology to provide the data transport mechanism.  We are concerned about the security issues surrounding this whole approach. There should be a way to encrypt and decrypt  XML documents. Is there any white paper or book which talks about the security issues involved in transporting XML documents?  The fundamental characteristic of an XML document, which  is well formed and readable document looks like a double edged sword to us. I am looking for good solutions to overcome the security issues associated with transporting XML documents.

I would appreciate any ideas or anything that will put us in the right direction.