Accessing canvas signed request while using VF page as Canvas app

Canvas_Vote.page

<apex:page showHeader="false" sidebar="false" controller="CanvasVoteController">
<apex:form >
	<apex:vote objectId="{!ideaId}"></apex:vote>
</apex:form>
</apex:page>

CanvasVoteController.cls

public with sharing class CanvasVoteController {

    public String ideaId {get;set;}
    public CanvasVoteController() {
            System.debug(ApexPages.CurrentPage().getParameters());
            String signed_request = ApexPages.CurrentPage().getParameters().get('signed_request');
            system.debug(signed_request);
            if(String.isNotBlank(signed_request)){
            String jsonData = EncodingUtil.base64Decode(signed_request.replace('.',':').split(':')[1]).toString();
            system.debug(jsonData);
            SignedRequestWrapper requestObj = (SignedRequestWrapper) System.JSON.deserialize(jsonData,SignedRequestWrapper.class);
            system.debug(requestObj);
            cls_parameters params = requestObj.context.environment.parameters;
            ideaId = params.Id;
            system.debug(ideaId);
            }

    }

    public class SignedRequestWrapper{
        public String algorithm;    //HMACSHA256
        public Integer issuedAt;    //1402819425
        public String userId;   //00590000000tQBwAAM
        public cls_client client;
        public cls_context context;
    }

    class cls_client {
        public cls_refreshToken refreshToken;
        public String instanceId;   //lk_main:Idea_Vote:0D59000001lbhaX
        public String targetOrigin; //https://ap1.salesforce.com
        public String instanceUrl;  //https://ap1.salesforce.com
        public String oauthToken;   //00D90000000azEO!ARYAQICDEbmnSC2XNqFFJd4F2lg6CM3u0DF2XHlQgQaUHnfOPSvDCUWeP3.T1PfDtjCt7zJssbofHuDF4w_L2aUCXwO9VnXo
    }
    class cls_refreshToken {
    }
    class cls_context {
        public cls_user user;
        public cls_links links;
        public cls_application application;
        public cls_organization organization;
        public cls_environment environment;
    }
    class cls_user {
        public String userId;   //00590000000tQBwAAM
        public String userName; //lakshay.katney@main.com
        public String firstName;    //Lakshay
        public String lastName; //Katney
        public String email;    //lakshay.katney@gmail.com
        public String fullName; //Lakshay Katney
        public String locale;   //en_US
        public String language; //en_US
        public String timeZone; //America/Los_Angeles
        public String profileId;    //00e90000000sbeO
        public String roleId;   //00E90000000oicG
        public String userType; //STANDARD
        public String currencyISOCode;  //USD
        public String profilePhotoUrl;  //https://c.ap1.content.force.com/profilephoto/729900000005sLc/F
        public String profileThumbnailUrl;  //https://c.ap1.content.force.com/profilephoto/729900000005sLc/T
        public cls_siteUrl siteUrl;
        public cls_siteUrlPrefix siteUrlPrefix;
        public cls_networkId networkId;
        public boolean accessibilityModeEnabled;
        public boolean isDefaultNetwork;
    }
    class cls_siteUrl {
    }
    class cls_siteUrlPrefix {
    }
    class cls_networkId {
    }
    class cls_links {
        public String loginUrl; //https://login.salesforce.com/
        public String enterpriseUrl;    ///services/Soap/c/34.0/00D90000000azEO
        public String metadataUrl;  ///services/Soap/m/34.0/00D90000000azEO
        public String partnerUrl;   ///services/Soap/u/34.0/00D90000000azEO
        public String restUrl;  ///services/data/v34.0/
        public String sobjectUrl;   ///services/data/v34.0/sobjects/
        public String searchUrl;    ///services/data/v34.0/search/
        public String queryUrl; ///services/data/v34.0/query/
        public String recentItemsUrl;   ///services/data/v34.0/recent/
        public String chatterFeedsUrl;  ///services/data/v31.0/chatter/feeds
        public String chatterGroupsUrl; ///services/data/v34.0/chatter/groups
        public String chatterUsersUrl;  ///services/data/v34.0/chatter/users
        public String chatterFeedItemsUrl;  ///services/data/v31.0/chatter/feed-items
        public String userUrl;  ///00590000000tQBwAAM
    }
    class cls_application {
        public String name; //Idea Vote
        public String canvasUrl;    //https://lk-main.ap1.visual.force.com/apex/Canvas_Vote
        public String applicationId;    //06P90000000VAyj
        public String version;  //1.0
        public String authType; //SIGNED_REQUEST
        public String referenceId;  //09H90000000TcYE
        public cls_options[] options;
        public String samlInitiationMethod; //None
        public String developerName;    //Idea_Vote
        public boolean isInstalledPersonalApp;
        public String namespace;    //lk_main
    }
    class cls_options {
    }
    class cls_organization {
        public String organizationId;   //00D90000000azEOEAY
        public String name; //Unneeded value
        public boolean multicurrencyEnabled;
        public String namespacePrefix;  //lk_main
        public String currencyIsoCode;  //USD
    }
    class cls_environment {
        public String locationUrl;  //https://ap1.salesforce.com/_ui/core/chatter/ui/ChatterPage
        public String displayLocation;  //ChatterFeed
        public cls_sublocation sublocation;
        public String uiTheme;  //Theme3
        public cls_dimensions dimensions;
        public cls_parameters parameters;
        public cls_record record;
        public cls_version version;
    }
    class cls_sublocation {
    }
    class cls_dimensions {
        public String width;    //100%
        public String height;   //250px
        public String maxWidth; //final
        public String maxHeight;    //1000px
        public String clientWidth;  //0px
        public String clientHeight; //0px
    }
    class cls_parameters {
        public String id;   //087900000008Zli
    }
    class cls_record {
        public String Id;   //0D59000001lbhaXCAQ
        public cls_attributes attributes;
    }
    class cls_attributes {
        public String type; //FeedItem
        public String url;  ///services/data/v34.0/sobjects/FeedItem/0D59000001lbhaXCAQ
    }
    class cls_version {
        public String season;   //SUMMER
        public String api;  //34.0
    }

}