Is it possible to secure a communications channel against both passive (sniffing

Question

Is it possible to secure a communications channel against both passive (sniffing) and active (injecting / MitM) attackers without either legitimate party knowing any pre-shared information?

I know that this isn't possible using "traditional" asymmetric crypto, since an active attacker could create its own public keys and relay the information on both sides. A trusted third party implies pre-shared information, since at least one party must provide their public key to the third party.

Are there any schemes that make this viable, or is it completely impossible?

Explanation / Answer

Yes. Simply send the data in the clear.

Passive attacks are not possible. For a passive attack to work, the data must be intercepted by someone other than the intended recipient. But by your definition of "pre-shared information" the existence of an intended recipient would count as "pre-shared information" (since both sides would know this). So anyone who receives the traffic is just as much the recipient as anyone else.

Active attacks are not possible. Active attacks involve someone other than the person the other side expects to be sending the data to be able to influence the data or receive the data. Since neither side has any expectations as to who is originating or receiving the data (such an expectation would be "pre-shared information" since both sides would need to have it), such an attack cannot, by definition, exist.

The idea of a "secure channel" to nobody in particular simply isn't coherent. And if both sides knew who they wanted to speak to or hear from, that would be "pre-shared information" by your expansive definition.

So this is not a coherent thing to want.

Consider two people considering such a scheme, Alice and Bill. If Bill knows who Bill is, Alice cannot know who Bill is as that would be pre-shared information. If Alice knows who Alice is, then Bill cannot know who Alice is, as that would be pre-shared information. Thus Bill could not distinguish a secure link to Alice from a secure link to Fred. To Bill, either is just as good. So it matters not if Fred intercepts or distorts the data. Fred is no less the intended recipient.

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