Is this CRM OnPremise, or CRM Online ?
If it's OnPremise, you should be able to use the classes in the System.Security.Cryptography namespace.
If it's Online, I'm not sure if you're permitted to use the classes in the System.Security.Cryptography namespace in the plugin sandbox. If not, your best option would be to deploy the plugin in the Azure Service Bus instead
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk