Selection key crypto protocol trial balloon

[Hal]

rishab at dxm.ernet.in (Rishab Aiyer Ghosh) writes:

>wcs at anchor.ho.att.com writes:

>> I'm not sure I really believe it's workable, but what it does
>> sound a bit like is CDMA spread-spectrum, which lets a bunch of

>Um no. The main thing is that what is extracted with the selection key is
>_different_ from what was put in...

Let me get straight where we are.  Rishab's concrete proposal was not an
implementation, but rather a set of requirements.  There was no
suggestion about any specific algorithms that would meet those
requirements, right?  The question is whether any such algorithm could
exist.

It is hard for me to see how this could possibly work.  The message
receiver sends this "selection key" to the intermediary, and that somehow
pulls out the saved message, but in a form such that the intermediary
doesn't recognize it.  And the intermediary himself can't tell exactly
which message is produced.  But it is nevertheless exactly the message
which was meant for this particular receiver.

The thing is, the receiver does not have much more information than the
intermediary.  At best he knows a secret key which may help decrypt the
message in some way.  But I don't see how that can be used to pull out
the message data since it can't be revealed to the intermediary.

I can't really prove that this is impossible, but it certainly looks that
way.

Hal