content: I guess I should have been more clear.  I was attempting to delineate where the insecurity lies.  That by discussing a specific algorithm, we can then move on to the real problems.  (Specifically, key management.)  If you put a high security lock on your apartment but have to give a key to the superintendent.  They are prevented by law from using it without a court order.  Is your new lock still secure?  Is your apartment secure?    As to your second point, that "backdoors simply can not be done at all with encryption", I just detailed one possible cryptographic system that includes a backdoor.  You can simply store which public/private key pair corresponds to which session key as part of the key pair.  If you want a real world example, Microsoft's EFS uses almost this exact algorithm to implement the "recovery agent."