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> Quantum cryptography is secure against quantum computers and is also vastly more simple than existing crypto (no complicated algorithms to mess up). Can you

> Quantum cryptography is secure against quantum computers and is also vastly more simple than existing crypto (no complicated algorithms to mess up). Can you explain this statement? I understand that algorithms such as RSA might require particular padding or what have you to be secure in practice, but is quantum-resistant crypto much different?

charlieflowers 9y ago
It's completely different. The underlying mechanism is the fact that observing the state on one member of an entangled pair changes the state of the other member.
- OscarCunningham 9y ago
  More specifically, the important fact is that it's impossible for three particles to be (maximally) entangled simultaneously. So you can verify that no one eavesdropped when you agreed your one-time-pad.
  - Kemschumam 9y ago
    Have you seen this recent result http://news.mit.edu/2018/physicists-create-new-form-light-0215 http://news.mit.edu/2018/physicists-create-new-form-light-02.... Does it change your thoughts on the utility of quantum cryptography?
    - OscarCunningham 9y ago
      They get three photons to be entangled, but not "maximally entangled" in the (impossible) way that would be needed for someone to listen to a communication without being detected.
OscarCunningham 9y ago
The phrase "quantum-resistant crypto" usually means classical algorithms (e.g. those involving elliptic curves) that are resistant to quantum computers. What I'm talking about is "quantum cryptography", in which qubits are actually used in the protocol. Quantum cryptography is also known to be secure against quantum computers (indeed it's secure against arbitrary amounts of computing resources, unless our theories of physics are wrong). It's also simpler than RSA or elliptic curves, so I hope that (after the kinks are worked out) it will also be less susceptible to bad implementations and side-channel attacks.
- Moodles 9y ago
  Apologies as I misread "quantum" as "quantum-resistant" above. However, I would still contest that quantum key exchange is "simpler". I mean, you need a quantum channel and to accurately exchange qubits without disturbance. Not to mention a small initial secret. It doesn't really work too well with our current infrastructure.
  - OscarCunningham 9y ago
    Agree with most of your points. But I suspect building a quantum infrastructure will be easier (and therefore occur earlier) than building quantum computers. >Not to mention a small initial secret This is interesting, what are you referring to here?
    - mannykannot 9y ago
      Encryption has become pervasive in modern communication, yet it is clear that we still need more to properly protect the things we are doing. Does the quantum infrastructure that you are proposing not imply an almost complete replacement of the current global communications network with quantum channels, or at least to shadow it with a co-extensive one for key distribution? Is that close to being feasible for mobile and other radio-connected devices?
    - PeterisP 9y ago
      Doesn't quantum transmission infrastructure require direct connections from A to B? I.e. you could use it for a single uninterrupted fiber cable, or for the channel between your antenna and a satellite, but not with our common fiber infrastructure that relies on repeaters / re-transmitters. As soon as there's any device between you and the recipient that breaks the entanglement, all the guarantees of quantum encryption go out of the window, and it's possible to attack the comms at that retransmission point.
      - OscarCunningham 9y ago
        >Doesn't quantum transmission infrastructure require direct connections from A to B? No, the retransmitters can preserve the entanglement (and A and B can verify that this has been done). In fact (providing qubits can be stored) the transmission can be done indirectly and in advance. The telecom company produces lots of entangled qubits and gives a bunch to each customer, keeping half of each pair for itself. Then when Alice wants to communicate securely with Bob they ask the telecom company to take the corresponding qubits and perform a joint measurement. This entangles Alice's qubits with Bob's (like making a connection at a telephone exchange). Then Alice and Bob can measure their qubits (in various bases) to create a one-time-pad. The clever thing is that Alice and Bob can (by checking that on a portion of the qubits their results always matched when they used the same basis) verify that they did indeed have maximally entangled qubits and therefore no one was listening-in.

charlieflowers 9y ago

It's completely different. The underlying mechanism is the fact that observing the state on one member of an entangled pair changes the state of the other member.

OscarCunningham 9y ago
More specifically, the important fact is that it's impossible for three particles to be (maximally) entangled simultaneously. So you can verify that no one eavesdropped when you agreed your one-time-pad.
- Kemschumam 9y ago
  Have you seen this recent result http://news.mit.edu/2018/physicists-create-new-form-light-0215 http://news.mit.edu/2018/physicists-create-new-form-light-02.... Does it change your thoughts on the utility of quantum cryptography?
  - OscarCunningham 9y ago
    They get three photons to be entangled, but not "maximally entangled" in the (impossible) way that would be needed for someone to listen to a communication without being detected.

OscarCunningham 9y ago

The phrase "quantum-resistant crypto" usually means classical algorithms (e.g. those involving elliptic curves) that are resistant to quantum computers. What I'm talking about is "quantum cryptography", in which qubits are actually used in the protocol. Quantum cryptography is also known to be secure against quantum computers (indeed it's secure against arbitrary amounts of computing resources, unless our theories of physics are wrong). It's also simpler than RSA or elliptic curves, so I hope that (after the kinks are worked out) it will also be less susceptible to bad implementations and side-channel attacks.

Moodles 9y ago
Apologies as I misread "quantum" as "quantum-resistant" above. However, I would still contest that quantum key exchange is "simpler". I mean, you need a quantum channel and to accurately exchange qubits without disturbance. Not to mention a small initial secret. It doesn't really work too well with our current infrastructure.
- OscarCunningham 9y ago
  Agree with most of your points. But I suspect building a quantum infrastructure will be easier (and therefore occur earlier) than building quantum computers. >Not to mention a small initial secret This is interesting, what are you referring to here?
  - mannykannot 9y ago
    Encryption has become pervasive in modern communication, yet it is clear that we still need more to properly protect the things we are doing. Does the quantum infrastructure that you are proposing not imply an almost complete replacement of the current global communications network with quantum channels, or at least to shadow it with a co-extensive one for key distribution? Is that close to being feasible for mobile and other radio-connected devices?
  - PeterisP 9y ago
    Doesn't quantum transmission infrastructure require direct connections from A to B? I.e. you could use it for a single uninterrupted fiber cable, or for the channel between your antenna and a satellite, but not with our common fiber infrastructure that relies on repeaters / re-transmitters. As soon as there's any device between you and the recipient that breaks the entanglement, all the guarantees of quantum encryption go out of the window, and it's possible to attack the comms at that retransmission point.
    - OscarCunningham 9y ago
      >Doesn't quantum transmission infrastructure require direct connections from A to B? No, the retransmitters can preserve the entanglement (and A and B can verify that this has been done). In fact (providing qubits can be stored) the transmission can be done indirectly and in advance. The telecom company produces lots of entangled qubits and gives a bunch to each customer, keeping half of each pair for itself. Then when Alice wants to communicate securely with Bob they ask the telecom company to take the corresponding qubits and perform a joint measurement. This entangles Alice's qubits with Bob's (like making a connection at a telephone exchange). Then Alice and Bob can measure their qubits (in various bases) to create a one-time-pad. The clever thing is that Alice and Bob can (by checking that on a portion of the qubits their results always matched when they used the same basis) verify that they did indeed have maximally entangled qubits and therefore no one was listening-in.