Message Signature Validation

[Pivotal Issue 78632442][1]

[1]: https://www.pivotaltracker.com/n/projects/1163646/stories/78632442
Personally for me this ought to be something that is done up front, when creating the configuration object rather than after the fact when the configured values are required. That would however be quite a large change from the current flow in SSP... Been pondering about it but for now held off of it to not change too much from the original implementation.