Re: Back Doors

From: John A. Malley (102667.2235_at_compuserve.com)
Date: 12/20/03 

Date: Sat, 20 Dec 2003 14:12:07 -0800

mike3 wrote:

> Hi.
>
> How can one implement "back doors" in an encryption algorithm that
> would allow someone to decode all messages encrypted with it provided
> they know some secret "magic" formula? Is this a very diffcult problem
> or an easy one?
>
> Just curious.
>

Here's a thought inspired by some recent reading on the connection
between steganography and secure computing:

Use a side channel to leak information about the key while the cipher is
in use. Deliberately design in (one or more) seemingly innocuous
behaviors that your implementation modulates to signal an external
receiver.

What's hard is selecting the behavior to modulate, determining how to
modulate the behavior in a way that won't attract attention
(dynamically, while running on user system, and statically, when subject
to code reviews with the safe assumption of open-source code
distribution), and how to measure the modulation remotely and without
attracting attention.

That's a bare-bones answer, there's a lot more to think through here. I
regret not giving a specific example, but in theory this should be
possible, and would be specific to an implementation.

John A. Malley
102667.2235@compuserve.com