Passing Custom Attributes in Canvas App Signed Request

Question

I have set up a Canvas App that is using the Signed Request Access Method. I have defined a Custom Attribute for the app to pull from a Custom User Field. But when I look at the Signed Request from my target application, I don't see the custom attribute/value anywhere. How do I access that custom attribute from the Signed Request?

The workaround I have for now is to call back using the canvas API to get the User Object and read the value I need, but didn't think I should need to do that with the Custom Attribute.

Am I misunderstanding what the Custom Attributes are? Am I missing something in my setup or just looking in the wrong place for those attributes?

Edit:

Another option may be to pass parameters in the VS Page opening the canvas app. But I haven't found how to pull from an existing SF Object field. Again, in this case it's a Custom Field on the User object Something like:

<apex:page >
  <apex:canvasApp applicationName="AppName" parameters="{parmName: User.customField}"/>
</apex:page>

Edit 2 Code/Setup:

VS Page:

<apex:page >
  <apex:canvasApp applicationName="AppName"/>
</apex:page>

Canvas App receiving Signed Request:

    public void authenticate(HttpServletRequest request, HttpServletResponse response) throws GatewayException
    {
        logger.debug("Authenticating Salesforce login attempt.");

        String signedRequest = request.getParameter("signed_request");

        if(logger.isDebugEnabled())
        {
            logger.debug("Authenticating signed request: " + signedRequest);
        }

        if (signedRequest == null)
        {
            throw new GatewayException(SalesforceConstants.UNSIGNED_REQUEST, "The request must be signed.");
        }

        try
        {
            //Custom attribute not in the JSON string
            String signedRequestJSON = SignedRequest.verifyAndDecodeAsJson(signedRequest, consumerSecret);
            CanvasRequest canvasRequest = SignedRequest.verifyAndDecode(signedRequest, consumerSecret);
            //canvasParms is empty
            Map<String, Object> canvasParms = canvasRequest.getContext().getEnvironmentContext().getParameters();

            if(logger.isDebugEnabled())
            {
                logger.debug("Authentication verifed.");
            }
            requestData.put(SalesforceConstants.SIGNED_REQUEST_JSON, signedRequestJSON);

            //TODO pull from signed request
            requestData.put(SalesforceConstants.LOGIN_ID, "JWALKER");
            requestData.put(SalesforceConstants.ACCOUNT, "CURRENCY");
        }
        catch(SecurityException se)
        {
            throw new GatewayException(SalesforceConstants.UNVERIFIED_REQUEST, "Signed request was not verified.", se);
        }
    }

Canvas App Configuration with Custom Attribute (I'm trying to read "cp_config_id" in my Canvas App):

Answer 1

When you retrieve your body in the canvas app it will have 2 elements, 0 will be the secret, and one will be the encoded envelope. The parameters will be in this object. If you post your code I would be happy to take a look.

EDIT: A sample of a VF page and Controller

<apex:page ="canvasCON" >
<apex:canvasApp applicationName="Appname" canvasId="appname" parameters="{!jParameters}"  />  

public with sharing class canvasCON{
private user u;
public canvasCON(){
    this.u=[select id,acme_configuration__c from user where id=:userinfo.getuserid() LIMIT 1];
}    
public string getjParameters(){
    string json;
    String jsonout = '{'+
    '  \"cp_config_id\": \"'+User.acme_configuration__c+'\",'+              
    '}';
    return json;
 }
}