I am trying to invoke the SAP Application through Canvas to embed the SAP page in Salesforce. But when we are checking in SAP, the signed request is coming blank from Salesforce to SAP. Do anyone know the reason for this? Please help me.
I would point you towards Salesforce lightning connect. This will give you many benefits over canvas, including:
Stored Signon capabilities. HTTPS Communcation Ability to do much more within Salesforce because the ability to write Apex Queries against remote data sources such as SAP R/3 Tables (think VBAK, VBAP, KNA1, etc.).
Please use the following SF Link to get started.
Additionally I have developed a step by step blog to use as an example. Please try reading this http://sgcsoftware.com/blog.jsp?id=37