Hi list,
This is a proposal to use quantum-safe hybrid handshake for Tor communications. Given NSA's recent announcement on moving towards quantum-safe cryptography, it would be nice to have a quantum-safe feature for Tor.
The idea of the quantum-safe hybrid handshake is to combine both classical key exchange and a key encapsulation mechanism (KEM) instantiated by a quantum safe encryption algorithm, so that the combination gives both (classical) authentication and quantum safety. In a bit more details, the client and the server agrees on a classic pre-master secret, $c$, using the ntor protocol. In parallel, client generates a public/private key pair of the quantum-safe encryption algorithm, and send the public key to the server. The server picks a random string, $q$, encrypts it with the public key and send the ciphertext back to the client. The final secret is the output of KDF(c|q).
This proposal defeats the harvest-then-decrypt attack with a minimum impact to the existing ntor protocol. An adversary needs to be able to break the quantum-safe encryption algorithm to learn q. On the other hand, if the quantum-safe encryption algorithm turns out to be not secure, the protocol is still as secure as ntor protocol. In other words, it will at least do no harm to the current security.
In addition, this is a modular design that allows us to use any quantum-safe cryptographic primitives. As a proof of concept, we instantiated the protocol with NTRUEncrypt lattice-based crypto. We implemented the the protocol with NTRU parameters that gives 128 bits security. The code is available at https://github.com/NTRUOpenSourceProject/ntru-tor
Please find the attachment for the request to change the feature. The proof of the protocol can be found at: https://eprint.iacr.org/2015/287.pdf
Some known issue: 1. cell size. As far as we know, all quantum-safe encryption algorithms have large key and/or ciphertext size that exceeds the cell size ~500. So this protocol needs to transmit multiple cells, no matter which quantum-safe encryption algorithm we chose. This is addressed by "Proposal 249: Allow CREATE cells with >505 bytes of handshake data".
2. quantum-safe authentication: there is no quantum-safe authentication in this protocol. We believe that authentication can wait, as future (quantum) adversary cannot come back to present time and break authentication. Hence, we use ntor authentication to keep the proposal compact and simple. It will be a future work after this proposal.
Thanks for your time, and happy holidays!
Zhenfei Zhang Security Innovation.