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With all the NSA stuff in the news, I've been thinking about how to ensure the "who's talking to who" is just as private as the contents of the messages. I had an idea on the subject, about which I've got two questions: the first, does this concept work (i.e. meet its stated objectives); and if so, what services employ this technique.

The problem: how to ensure that the "who" is just as private as the "what" of communication

The idea:

  • There exists a voluntary network of nodes that all have public key encryption systems, and published public keys.
  • The sender sends a message to the first intermediate node, encrypted with that intermediate node's public key.
  • The first intermediate node decrypts the message, which contains the second intermediate node's address and a payload that's encrypted with the second intermediate node's public key. The first intermediate note sends the payload to the second intermediate node.
  • The second intermediate node decrypts the message, which contains the third intermediate node's address and a payload that's encrypted with the third intermediate node's public key. The second intermediate note sends the payload to the third intermediate node.
  • The third intermediate node decrypts the message, which contains the final recipients node's address and a payload that's encrypted with the final recipients's public key. The third intermediate note sends the payload to the final recipient.
  • The final recipient decrypts the message.

Key points about this system:

  • the sender needs the public keys of all intermediate nodes (a published database would be needed of addresses and public keys)
  • I was originally thinking of this in respect of email, but it should work with any communication, e.g. snail mail, IP packets, etc
  • on a computer it should be fairly easy to automate the multiple layers of encryption
  • the number of nodes can be manually selected, or selected at random
  • the nodes themselves should be selected at random
  • the communication could contain multiple next-hop nodes with a priority order, in case nodes fail. The nodes could provide an acknowledgement message back to the node they received communication from
  • the messages could potentially be randomly padded to ensure that the reducing size of the message over time (as address headers are removed) does not give a clue to the communication direction
  • random communication could be added to the network to further confuse snoopers
  • to prove the source of the correspondence to the sender, the sender and recipient would need to pre-agree a secret that the sender would include in the message. Alternatively, the entire system could be set up such that each node gives a unique secret to each other node encrypted using that node's public key. This would establish encrypted relationships by default, without the need for pre-agreeing, and wouldn't allow anyone to infer relationships from the establishment of the original secret

Challenges:

  • as the network scales, the database of addresses and public keys becomes potentially too large
  • if the secret sharing mentioned in the key points is employed, the number of secrets each node would need to hold could get prohibitively large if the network gets large
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You appear to be reinventing the concept of onion routing. –  Stephen Touset Sep 30 '13 at 21:03
    
"The idea: ..." $\:=\:$ "use a mixnet" $\;\;$. $\;\;\;\;$ My first other key point: a time interval should be included in the plaintext of each encrypted layer, to allow for defending against replay attacks. $\;\;$ My second other key point: authenticity could be established by having the innermost plaintext indicate the identities of the sender and receiver, the sender's signature verification key, and the signed message. $\hspace{1.18 in}$ –  Ricky Demer Sep 30 '13 at 21:20
    
Yep, thanks. Have now read up on onion routing and mix networks. I've not really studied cryptography, but perhaps am suffering from cryptomnesia nonetheless. –  user8676 Sep 30 '13 at 21:27
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Like @StephenTouset said - what you are describing smells like onion routing. For a practical example, check Anonymity network TOR which enables online anonymity and directs internet traffic through a worldwide network consisting of thousands of relays to conceal a user's location… which would cover most of what you need. But there are other implementations out there too; most of them handling encrypted, anonymous data (voice, text, binary) transfers using random node allocations and bridging. Ricky Demer already commented — it is worth diving into mixnets too. –  e-sushi Sep 30 '13 at 22:36
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1 Answer

Yes, the above concept is broadly equivalent to onion routing and mix networks (although please refer to these links for a more accurate description).

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To close the question correctly, please also accept your answer by clicking the checkmark underneath the up/down voting buttons (it will turn into a green checkmark). Thanks. –  e-sushi Oct 7 '13 at 16:23
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