Posting message to Salesforce Chatter via Javascript SDK

I am using this application as a basis for what i am looking to do. https://github.com/sgshawnblanchard/CanvasNodeJS

The issue I am having is that I don't think I quite understand what needs to happen in order to be able to post to the Chatter feed via the Canvas application. I have everything working, however on this page: https://github.com/sgshawnblanchard/CanvasNodeJS/blob/master/public/index.html

you will see that he is posting a "signed request", here is where I am getting confused. I understand that I need to post, and that value will let salesforce know that I have permission to post to the feed, however how do I get that value? In the example you see.

<form action="canvas" method="post">
            <input id="signed_request" name="signed_request" type="text" value="Pxy%2FPFsGb%2FfH1ORo%2B0vqdJjCi97Bypek0hzST8UIGzY%3D.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%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%3D%3D" />
            <input type="submit" value="submit"/>
        </form>

I understand that the this value is comprised on 2 things, found here http://www.salesforce.com/us/developer/docs/platform_connect/Content/canvas_app_signed_req_authentication.htm

The signed request is a string of the following elements concatenated:

The canvas app consumer secret encrypted with HMAC SHA–256 algorithm A period (“.”)

The context and authorization token JSON encoded in Base64

The signed request looks similar to this, although it will be much longer: 9Rpl6rE7R2bSNjoSfYdERk8nffmgtKQNhr5U/5eSJPI=.eyJjb250ZXh0Ijp7InVzZXIiOnsibGFuZ3V....

I know how to get my secret from inside of SalesForce setting, however I have no idea what "The context and authorization token JSON encoded in Base64" means

Am I correct in thinking that I need to get these static values and create a long string much like what the developer did in the example and post it, along with whatever message I want to send to Chatter? Or am I missing something?

Here is another video that I watched, however the guy ran out of time and didn't show how it actually worked https://www.youtube.com/watch?v=kcNBwOqj2qA