Hi

As far as I can know, there is no direct method in .net framework that can
envelope and de-envelope a message. Also you can do it by yourself, e.g.
you can import a certificate from a file and then retrieve its private key
and then negotiate with the other one about the data format.
There is some limitation in this scenario,
1.  .net's certificate class can only import certificate from file.
2. a lot of certificate can not import private key for security concern
which is essential for sign the data.

So far I think you may try to take a look at the WSE packet, which also can
be used to encrypt common data.
Web Services Enhancements (WSE) 2.0 SP1 for Microsoft .NET
http://www.microsoft.com/downloads/details.aspx?FamilyId=FC5F06C5-821F-41D3-
A4FE-6C7B56423841&displaylang=en

Best regards,

Peter Huang
Microsoft Online Partner Support