Hi PaulIP,
The purpose of using .snk file is to sign the assembly, so that it (strong-named assembly) can be stored in GAC. Hence .pfx file cannot instead of .snk file. About how to use .pfx file, please refer to the following links:
How to call a Web service by using a client certificate for authentication in an ASP.NET Web application
http://support.microsoft.com/kb/901183
Building Secure ASP.NET Applications: Authentication, Authorization, and Secure Communication
http://msdn2.microsoft.com/en-us/library/aa302408.aspx
Support Certificates In Your Applications With The .NET Framework 2.0
http://msdn.microsoft.com/msdnmag/issues/07/03/NETSecurity/default.aspx
Sincerely,
Benson Yu
Microsoft Online Community Support
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