The recommendation is to not use QKD. This is the correct recommendation. QKD solves key agreement if you have an authenticated line. But authentication is the harder more crucial problem.
Here's an interesting related aside: the likely design of a practical quantum internet would make QKD totally trivial. What a quantum internet would do is deliver kinda-noisy entangled Bell pairs to endpoints that wanted to communicate. The endpoints would then purify [1] this kinda-noisy entanglement into actually-good entanglement (e.g. from 1% error to 0.0000000000001% error). The purified Bell pairs can then be consumed in order to transmit qubits [2]. However, because of the monogamy of entanglement [3], the purification process must detect and correct eavesdropping (or else fail to produce output). So, once you have a sufficiently purified Bell pair, it can be measured to get a bit that can be used as a one time pad. (That said, this does still assume you have an authenticated channel! Purification requires communication, because without authentication you can be man-in-the-middle'd.)
All national agencies I'm aware of do not support QKD except in "very specific cases" and instead recommend Post-Quantum Cryptography (PQC).
From the UK NCSC [1]:
> QKD does not provide authentication, nor do any other quantum techniques. Therefore, in practice, QKD must be combined with other cryptographic services to provide security against the threat from quantum computing, and therefore should not be relied on as a mechanism that provides substantial security value. [...] The NCSC will not support the use of QKD for government or military applications. PQC is the best mitigation to the threat to cryptography from quantum computers.
And the German BSI (and partners)[2]:
> Together with European partner agencies from France, the Netherlands and Sweden, the BSI has published a Position Paper on QKD. The paper concludes that QKD can only be used in niche use cases due to its technological limitations and that QKD is not yet sufficiently mature from a security perspective. Therefore, in light of the necessary migration to quantum-safe schemes, the clear priority should be the migration to post-quantum cryptography.
This is despite different choices for which PQC algorithms to use. E.g. NIST (and many others including the UK) have gone initially with ML-KEM for key exchange, while Germany/BSI have selected FrodoKEM and Classic McEliece.
1. There is a strong anti-QKD bias on HN or, at least, a very vocal few who reliably heckle anyone who discusses it. I get shouted at if I even mention it, and will likely get shouted at for saying this.
2. Should you trust the NSA's recommendations? This is a valid question, now more than ever.
there is a strong anti-QKD bias among experts who understand QKD. It is fun academic concept, but does not solve a real world problem, and does not use techniques available at remotely comparable costs to classical cryptography in the real world, and even if you pay the enormous costs for it, it is trivial for an attacker to completely disrupt your communication in a way that cannot be recovered from (without out-of-band communication, e.g. either sending a courier, or using computational cryptography).
If you hate the NSA that's fine. Nobody in the EU cried foul over the NSA's recommendations though (and the NIST-winning schemes are European). Chinese scholars submitted some fundamentally similar schemes, the Chinese Academy of Sciences have formally recommended lattice-based schemes. While the Chinese (government-run) standardization is only starting, it is a very good bet that they will use a lattice-based scheme.
So, unless you think all of the world's governments (again, including China) are in a massive cabal to allow the NSA specifically to spy on the entire world, #2 is not a particularly valid question.
You don’t have to trust the recommendations, you can analyze the reasoning behind their decisions and argue that. In this case the risk being at the engineering and hardware side and also denial of service. In addition to the trusted relays. Those are valid disputes.
You can argue these exhaustively. They have not done that here. Some of their arguments are complete bunk.
e.g. "Quantum key distribution requires special purpose equipment"
Yes, it requires special equipment. That hasn't deterred some from using it where the added expense is warranted. Commercial QKD systems have been in use for decades. The technology is not currently useful for credit card transactions from your living room, but that doesn't mean it has no applications.
"Since QKD is hardware-based it also lacks flexibility for upgrades or security patches."
This is like arguing that, because your internet connection runs on hardware, nothing can be done to upgrade it or fix security vulnerabilities. If your last-mile connection is copper, as it is for many, there have likely been massive upgrades to its bandwidth and security over the years in the form of changes to what's on either end of the copper. Fiber is the same way. A huge part of QKD protocols is software as well.
When I see points like these, I question the source. They appear to have an agenda, and they certainly have motive. Remember, this is an organization whose business has been spying on its own citizens for decades.
This page came about because of how long it took PQC to get standardized. This was a slow enough process that a whole slew of QKD vendors arose and sold a lot of products promising this as a solution to dealing with quantum computers and harvest now decrypt later attacks. Many of those products did not do a great job at actually preventing listening in on their lines since QKD is an ongoing field of research where new issues are routinely being discovered.
> QKD is an ongoing field of research where new issues are routinely being discovered.
This always bothers me a bit. QKD is on a very solid theoretical footing — if you have an authenticated classical communication channel and an actual quantum communication channel that sends actual qubits that are genuinely only in the basis you think they’re in, then it’s secure, full stop. It’s been proven for decades.
But this is hard (hint: a commercially useful quantum computer does not exist yet), so people fudge it with optical techniques that approximate, poorly, what is needed. And the result is not secure.
I wouldn't call it "solid theoretical footing". The rough sketch of QKD is
1. BB84 key exchange requires an authenticated channel. typically you do this with a
2. Carter-Wegman MAC, which is information-theoretically secure, but requires shared randomness that cannot be reused.
Successful protocol execution refreshes randomness (you can net gain from it), so you can communicate back and forth continuously when everything is working. An MiTM who simulates a network failure though can expend some of your pre-shared randomness (without it being refreshed). If they do this enough, they can exhaust your shared randomness, and bring down the link until you exchange more shared randomness somehow out of band. if you want to maintain information theoretic security, this might involve e.g. a courier with a USB or whatever (or a carrier pigeon, who knows).
This is still "secure", but is also a significant issue any QKD (even "real" QKD) has that classical cryptography does not have, and has always made me question the "solid" story for QKD.
QKD is interesting from the PoV of perfect secrecy. But AFAIK with e.g. BB84, the basis orientation communication (used to detect OTP delivery eavesdropping) is done with Wegman-Carter (unconditionally secure) authentication using... a pre-shared key.
So if you're only interested in computational security that is post-quantum, why not pre-share a symmetric key for some AEAD scheme? You'll get forward secrecy with hash ratchet and neither provides future secrecy in principle.
Neither solves the bootstrap and QKD requires a really, really expensive and complex infrastructure just to provide perfect secrecy which we're fine without.
In my opinion, QKD (implemented correctly) performs key exchange, basically like Diffie-Hellman except that it’s secure even against an adversary with unlimited computing power. If I had a quantum computer and a quantum network anyway, may I’d use it, but probably not with Wegman-Carter. If not, I wouldn’t.
(BB84 is from 1984. The terminology was different, and the understanding of what mattered in cryptography was different.)
BB84 (and QKD overall) requires authenticated channels. You have to get those somewhere. You can get them from an information-theoretically secure MAC, but it has significant downsides. You can get them with computationally secure primitives, but then there's no point in using QKD in the first place. You cannot instantiate QKD securely without one of those two choices though.
Basing modern cryptography on unsolved mathematics where one can say "this algorithm is resistant to quantum computers, but may still, though we BELIEVE unlikely, be vulnerable to a yet discovered classical algorithm" is dangerous.
you cannot be mathematically rigorous with computational lower bounds. It is not possible with current mathematics. No ones that are relevant to cryptography exist, so any computational cryptography must separate into
1. provable constructions based on "hard" problems, and
2. best-effort cryptanalysis of "hard" problems.
This is true of lattice-based problems. It is true of EC crypto. It is true of RSA. it is true of McCliece. It is true of AES. That is the nature of things, and there is no avoiding that.
Analysis of Kyber was honest and mathematically rigorous. It's also beside the point, as all of your criticisms hold for EC/AES as well (despite EC having some reasonable lower bounds, e.g. in (extensions of the) generic group model. These of course rely on the conjecture that EC groups are generic groups).
> We should be working on standardising and moving towards methods that are independent of, rather than rely on, unresolved questions in mathematics.
There are no known methods that are remotely economically viable. There is (completely seriously) a clearer path towards fixing climate change than what you say. There is also a clearer path towards fixing global hunger. It is a complete fantasy to want to solely rely on mathematically provable techniques in cryptography, and not one that it is worth engaging with.
Furthermore, it's completely pointless. We might as well frame your question as
> We cannot prove that AES is hard, so we should not use it.
Why? It would be cool to prove that AES is hard. Sounds fun. And practically, the hardness assumptions of deployed cryptography are almost never the cause of a security vulnerability. If we care about secure systems, proving AES is hard is so low down on the priority list that it is difficult to think of something less important. Again, completely seriously, we would have MUCH more secure systems if we paid each person in the country to use better passwords.
Given that this is the case, it seems unreasonable to suggest spending \Omega(billions) updating our network infrastructure to worse-performing links just to "fix" a problem that doesn't exist. I'm even speaking as a cryptographer who (unreasonably) dislikes heuristics in the field, and tries to replace them with provable alternatives. It is a fun academic exercise. But it is not a real world issue.
The recommendation is to not use QKD. This is the correct recommendation. QKD solves key agreement if you have an authenticated line. But authentication is the harder more crucial problem.
Here's an interesting related aside: the likely design of a practical quantum internet would make QKD totally trivial. What a quantum internet would do is deliver kinda-noisy entangled Bell pairs to endpoints that wanted to communicate. The endpoints would then purify [1] this kinda-noisy entanglement into actually-good entanglement (e.g. from 1% error to 0.0000000000001% error). The purified Bell pairs can then be consumed in order to transmit qubits [2]. However, because of the monogamy of entanglement [3], the purification process must detect and correct eavesdropping (or else fail to produce output). So, once you have a sufficiently purified Bell pair, it can be measured to get a bit that can be used as a one time pad. (That said, this does still assume you have an authenticated channel! Purification requires communication, because without authentication you can be man-in-the-middle'd.)
[1]: https://en.wikipedia.org/wiki/Entanglement_distillation
[2]: https://en.wikipedia.org/wiki/Quantum_teleportation
[3]: https://en.wikipedia.org/wiki/Monogamy_of_entanglement
All national agencies I'm aware of do not support QKD except in "very specific cases" and instead recommend Post-Quantum Cryptography (PQC).
From the UK NCSC [1]:
> QKD does not provide authentication, nor do any other quantum techniques. Therefore, in practice, QKD must be combined with other cryptographic services to provide security against the threat from quantum computing, and therefore should not be relied on as a mechanism that provides substantial security value. [...] The NCSC will not support the use of QKD for government or military applications. PQC is the best mitigation to the threat to cryptography from quantum computers.
And the German BSI (and partners)[2]:
> Together with European partner agencies from France, the Netherlands and Sweden, the BSI has published a Position Paper on QKD. The paper concludes that QKD can only be used in niche use cases due to its technological limitations and that QKD is not yet sufficiently mature from a security perspective. Therefore, in light of the necessary migration to quantum-safe schemes, the clear priority should be the migration to post-quantum cryptography.
This is despite different choices for which PQC algorithms to use. E.g. NIST (and many others including the UK) have gone initially with ML-KEM for key exchange, while Germany/BSI have selected FrodoKEM and Classic McEliece.
[1] https://www.ncsc.gov.uk/paper/quantum-networking-technologie... [2] https://www.bsi.bund.de/EN/Themen/Unternehmen-und-Organisati...
Two points:
1. There is a strong anti-QKD bias on HN or, at least, a very vocal few who reliably heckle anyone who discusses it. I get shouted at if I even mention it, and will likely get shouted at for saying this.
2. Should you trust the NSA's recommendations? This is a valid question, now more than ever.
there is a strong anti-QKD bias among experts who understand QKD. It is fun academic concept, but does not solve a real world problem, and does not use techniques available at remotely comparable costs to classical cryptography in the real world, and even if you pay the enormous costs for it, it is trivial for an attacker to completely disrupt your communication in a way that cannot be recovered from (without out-of-band communication, e.g. either sending a courier, or using computational cryptography).
If you hate the NSA that's fine. Nobody in the EU cried foul over the NSA's recommendations though (and the NIST-winning schemes are European). Chinese scholars submitted some fundamentally similar schemes, the Chinese Academy of Sciences have formally recommended lattice-based schemes. While the Chinese (government-run) standardization is only starting, it is a very good bet that they will use a lattice-based scheme.
So, unless you think all of the world's governments (again, including China) are in a massive cabal to allow the NSA specifically to spy on the entire world, #2 is not a particularly valid question.
You don’t have to trust the recommendations, you can analyze the reasoning behind their decisions and argue that. In this case the risk being at the engineering and hardware side and also denial of service. In addition to the trusted relays. Those are valid disputes.
You can argue these exhaustively. They have not done that here. Some of their arguments are complete bunk.
e.g. "Quantum key distribution requires special purpose equipment"
Yes, it requires special equipment. That hasn't deterred some from using it where the added expense is warranted. Commercial QKD systems have been in use for decades. The technology is not currently useful for credit card transactions from your living room, but that doesn't mean it has no applications.
"Since QKD is hardware-based it also lacks flexibility for upgrades or security patches."
This is like arguing that, because your internet connection runs on hardware, nothing can be done to upgrade it or fix security vulnerabilities. If your last-mile connection is copper, as it is for many, there have likely been massive upgrades to its bandwidth and security over the years in the form of changes to what's on either end of the copper. Fiber is the same way. A huge part of QKD protocols is software as well.
When I see points like these, I question the source. They appear to have an agenda, and they certainly have motive. Remember, this is an organization whose business has been spying on its own citizens for decades.
This page came about because of how long it took PQC to get standardized. This was a slow enough process that a whole slew of QKD vendors arose and sold a lot of products promising this as a solution to dealing with quantum computers and harvest now decrypt later attacks. Many of those products did not do a great job at actually preventing listening in on their lines since QKD is an ongoing field of research where new issues are routinely being discovered.
> QKD is an ongoing field of research where new issues are routinely being discovered.
This always bothers me a bit. QKD is on a very solid theoretical footing — if you have an authenticated classical communication channel and an actual quantum communication channel that sends actual qubits that are genuinely only in the basis you think they’re in, then it’s secure, full stop. It’s been proven for decades.
But this is hard (hint: a commercially useful quantum computer does not exist yet), so people fudge it with optical techniques that approximate, poorly, what is needed. And the result is not secure.
I wouldn't call it "solid theoretical footing". The rough sketch of QKD is
1. BB84 key exchange requires an authenticated channel. typically you do this with a 2. Carter-Wegman MAC, which is information-theoretically secure, but requires shared randomness that cannot be reused.
Successful protocol execution refreshes randomness (you can net gain from it), so you can communicate back and forth continuously when everything is working. An MiTM who simulates a network failure though can expend some of your pre-shared randomness (without it being refreshed). If they do this enough, they can exhaust your shared randomness, and bring down the link until you exchange more shared randomness somehow out of band. if you want to maintain information theoretic security, this might involve e.g. a courier with a USB or whatever (or a carrier pigeon, who knows).
This is still "secure", but is also a significant issue any QKD (even "real" QKD) has that classical cryptography does not have, and has always made me question the "solid" story for QKD.
QKD is interesting from the PoV of perfect secrecy. But AFAIK with e.g. BB84, the basis orientation communication (used to detect OTP delivery eavesdropping) is done with Wegman-Carter (unconditionally secure) authentication using... a pre-shared key.
So if you're only interested in computational security that is post-quantum, why not pre-share a symmetric key for some AEAD scheme? You'll get forward secrecy with hash ratchet and neither provides future secrecy in principle.
Neither solves the bootstrap and QKD requires a really, really expensive and complex infrastructure just to provide perfect secrecy which we're fine without.
In my opinion, QKD (implemented correctly) performs key exchange, basically like Diffie-Hellman except that it’s secure even against an adversary with unlimited computing power. If I had a quantum computer and a quantum network anyway, may I’d use it, but probably not with Wegman-Carter. If not, I wouldn’t.
(BB84 is from 1984. The terminology was different, and the understanding of what mattered in cryptography was different.)
BB84 (and QKD overall) requires authenticated channels. You have to get those somewhere. You can get them from an information-theoretically secure MAC, but it has significant downsides. You can get them with computationally secure primitives, but then there's no point in using QKD in the first place. You cannot instantiate QKD securely without one of those two choices though.
Basing modern cryptography on unsolved mathematics where one can say "this algorithm is resistant to quantum computers, but may still, though we BELIEVE unlikely, be vulnerable to a yet discovered classical algorithm" is dangerous.
that's not what people say? and pre-quantum crypto is also vulnerable to yet discovered classical algorithms?
> that's not what people say?
Well, they should, if they want to be honest and mathematically rigorous.
For instance, in the case of NIST's proposed post quantum cryptography standard Kyber that relies on lattice based methods.
> and pre-quantum crypto is also vulnerable to yet discovered classical algorithms?
True, and also disconcerting; with the most reckless being allowing fungible currency reliant on such methods.
We should be working on standardising and moving towards methods that are independent of, rather than rely on, unresolved questions in mathematics.
you cannot be mathematically rigorous with computational lower bounds. It is not possible with current mathematics. No ones that are relevant to cryptography exist, so any computational cryptography must separate into
1. provable constructions based on "hard" problems, and 2. best-effort cryptanalysis of "hard" problems.
This is true of lattice-based problems. It is true of EC crypto. It is true of RSA. it is true of McCliece. It is true of AES. That is the nature of things, and there is no avoiding that.
Analysis of Kyber was honest and mathematically rigorous. It's also beside the point, as all of your criticisms hold for EC/AES as well (despite EC having some reasonable lower bounds, e.g. in (extensions of the) generic group model. These of course rely on the conjecture that EC groups are generic groups).
> We should be working on standardising and moving towards methods that are independent of, rather than rely on, unresolved questions in mathematics.
There are no known methods that are remotely economically viable. There is (completely seriously) a clearer path towards fixing climate change than what you say. There is also a clearer path towards fixing global hunger. It is a complete fantasy to want to solely rely on mathematically provable techniques in cryptography, and not one that it is worth engaging with.
Furthermore, it's completely pointless. We might as well frame your question as
> We cannot prove that AES is hard, so we should not use it.
Why? It would be cool to prove that AES is hard. Sounds fun. And practically, the hardness assumptions of deployed cryptography are almost never the cause of a security vulnerability. If we care about secure systems, proving AES is hard is so low down on the priority list that it is difficult to think of something less important. Again, completely seriously, we would have MUCH more secure systems if we paid each person in the country to use better passwords.
Given that this is the case, it seems unreasonable to suggest spending \Omega(billions) updating our network infrastructure to worse-performing links just to "fix" a problem that doesn't exist. I'm even speaking as a cryptographer who (unreasonably) dislikes heuristics in the field, and tries to replace them with provable alternatives. It is a fun academic exercise. But it is not a real world issue.