A Post-Quantum Future for Let's Encrypt(letsencrypt.org) |
A Post-Quantum Future for Let's Encrypt(letsencrypt.org) |
Still better than the alternatives that would saddle us with worse performance for ~ever.
In Vernor Vinge's novel "A Fire Upon the Deep" one of the most valuable commodities were one time pads that are physically transported to communication nodes to enable unbreakable communication. The pads are split into three pieces that are XORed to create the actual pad to reduce risk of compromise.
I did read the books 20 years ago and forgot this aspect of the story
I am obviously not in the field, but as far as I know, no QC is close of working for a practical purpose(aside quantum research), but to make it practical, it needs a groundbraking brakethrough of some sort. But if a brakethrough happens, can we really estimate the consequences?
(Of course, basically all encryption, especially asymmetric encryption, is predicated on there not being some as-yet-undiscovered exploitable structure to the mathematics on which it is built. Modern cryptography, AFAIK, tends to have some decent arguments for why this is not expected to be the case, but it's never completely proven top-to-bottom outside of fairly niche/trivial cases. It's always in principle possible that someone discovers an attack on these new algorithms, classical or quantum)
However, just like for RSA we know that the problem of efficient integer factoring has been worked on for a long time with no progress, the same is true for quantum computing. We have been trying to figure out quantum algorithms for a great number of problems that are hard for classical computers for a long time now, and we haven't been able to, except for the ones that we have. Mathematicians have also developed certain intuitions for which problems have characteristics that make them potentially easier to solve on a QC and which don't.
In general, just like with P=NP?, we haven't proven yet if BQP, roughly the class of problems which have efficient QC versions, is equal or not to P, the class of problems that can be efficiently solved on a classical computer; and we also don't know if BQP=NP.
So yes, there is at least a theoretical possibility that the problems used for creating post-quantum encryption will turn out to be in BQP, will turn out to have an efficient quantum algorithm that solves them. But that would come from mathematical research, it is entirely unrelated to creating and tinkering with actual quantum computers. The math of quantum algorithms is currently far ahead of the engineering and physics on building the actual computers.
The relevant property here is known as "information-theoretic security", and I'm not sure if one-time pads are the only way to achieve it, e.g. Shamir's secret sharing also has this property (although the use case is slightly different): https://en.wikipedia.org/wiki/Information-theoretic_security
I think there is a sense in which we have a historical accident that has make quantum computers sound bigger than they are, in that we ended up with "factoring prime numbers" being the first thing we had to make practical encryption out of, and by what is from a human perspective mostly a coincidence, it so happens that quantum computers may be really good at that. But the problem is that quantum computers happen to be good at factorizing that is the problem, not that quantum computers are somehow "good at breaking encryption". It seams to me that in some sense "post-quantum computing" is actually "all practical encryption schemes except those based on factoring large numbers". Breaking large prime number-based schemes is the exception that QC happens to be good at, not the rule.
I think it's very funny to consider that if you were a time traveler tasked with making sure that humanity had the economic incentive to develop quantum computers, the most efficient way to ensure that in a single stroke would be by suggesting the use of prime factorization as a trapdoor function to Rivest, Shamir, or Adleman.
And even if there was only a 10% chance of QC breaking crypto, the community is not comfortable with a 10% chance of such a catastrophic scenario.
This is part of my day job, so here's another interesting fact: for migrating encryption use cases, you have to consider that attackers can capture your encrypted data today to break in the future. So, as a rule of thumb, your migration timeline is much shorter for encryption than for signatures.
> Post-quantum authentication is no longer a problem the Web PKI ecosystem should defer. Long-lived keys (root certificate authorities, code-signing keys, identity systems) are particularly valuable targets, and new technology takes years to gain broad adoption, so the work has to start early.
This is a problem that I have met so many times talking with people: they parrot the "Harvest-Now-Decrypt-Later is the only urgent problem, signatures can wait" mantra, and this piece of misinformation has spread so much that even AI repeats it (because it has been trained on open data, where the overwhelming sentiment has been following this trend), thereby reinforcing the problem. Ask Claude/ChatGPT/Gemini about the problem, and they will invariably tell you that signatures are less urgent because theyr are not subjective to retroactive compromise.
There are two problems here.
The first one is included by the Letsencrypt announcement: the migration path for signatures/certificates is typically longer and more complex than encryption: long-lived certificates, firmware update keys, secure boot certificates, these are all objects that are painful to migrate.
The second one, even more serious in my opinion, is: "retroactive" in respect to what? "Retroactive" presupposes you can observe the trigger (the arrival of a cryptanalytically-relevant quantum computer), but this is precisely the kind of capability an adversary keeps secret, and a quantum forgery is operationally indistinguishable from, e.g., key exfiltration, a library bug, or a classical break. You may see a forged signature, a drained wallet, a failing certificate, and have no way to attribute it to quantum cryptanalysis. The threat is dark: reactive migration against an unobservable trigger is structurally impossible.
This is not to say that Harvest-Now-Decrypt-Later is a less urgent threat, but it's not so asymmetric as people have been believing so far. Glad to see things are changing!
These upsides seem extremely promising, but I'm curious to know if there are any notable downsides as well.
> This is a problem that I have met so many times talking with people: they parrot the "Harvest-Now-Decrypt-Later is the only urgent problem, signatures can wait" mantra, and this piece of misinformation has spread so much that even AI repeats it (because it has been trained on open data, where the overwhelming sentiment has been following this trend), thereby reinforcing the problem. Ask Claude/ChatGPT/Gemini about the problem, and they will invariably tell you that signatures are less urgent because theyr are not subjective to retroactive compromise.
I can't speak to public sentiment, but the stance I've held for years was roughly:
HNDL is more urgent because people are already encrypting messages today that could be decrypted in the future if a quantum computer is ever built in the foreseeable future, and that harms their privacy for the entirety of human history until PQC is rolled out.
That's not the same as "authentication doesn't matter at all". It was, if you must pick a problem to solve today, this one will stop the bleeding sooner.
But they were always both important to solve. The question was whether we could delay PQ auth until better signature algorithms were deployed. The Google/Cloudflare 2029 decision signaled to the rest of us: "No, we need to start the migration now."
There were 5 levels being considered for each submission.
Level 1 - at least as difficult to attack as AES-128 (block cipher)
Level 2 - at least as difficult to attack as SHA-256 (hash function)
Level 3 - at least as difficult to attack as AES-192 (block cipher)
Level 4 - at least as difficult to attack as SHA-384 (hash function)
Level 5 - at least as difficult to attack as AES-256 (block cipher)
The security of attacking an N-bit block cipher is morally congruent to a birthday collision against a {2N}-bit hash function. With some caveats: https://soatok.blog/2024/07/01/blowing-out-the-candles-on-th...
ML-DSA-44 (smallest parameter set) targets Level 2 for signatures.
ML-KEM-768 targets Level 3 for KEMs.
The issue instead is for signatures. We don’t have a fine hybrid signature. Concretely, our current hybrid signatures achieve security in a weaker model (they do not achieve BUFF security) than what our PQ signatures achieve.
So the question is if we want explicitly weaker security to provide assurance against possible security issues in the PQ hardness assumption. Or we could delay standardization longer while people search for better ways of making hybrid signatures. Both seem stupid, especially as obtaining cryptographically relevant quantum computers recently seems less like “if” than “when”. Note that when cryptographically relevant quantum computers appear, we will NEED to have a PQ secure component. The main “pro hybrid” cryptographer (Bernstein) has himself predicted classical (public key) cryptography will likely be broken by 2032. Things must transition now.
People bring up SIKE/SIDH in these discussions because Daniel Bernstein has used it as innuendo in his arguments against the MLKEM standard (always left out of those discussions: Bernstein himself backed a lattice KEM in the same competition). It's aggravating because its very clear that he's succeeded in getting people to believe that SIDH somehow reflects on lattice cryptography. That's not a problem because it's persuasive (no cryptographer would take that argument seriously) but rather because he's succeeded in making people say dumb things.
No. Don't do that.
If you encrypt your data twice, and one of them is broken by a quantum computer, the adversary gets the plaintext anyway.
You want a Hybrid KEM, not encrypting twice. The nuance matters.
Is the idea here that "you broke quantum and quantum breaks classical, therefor layering is pointless"?
c1 = E1(p, k1)
c2 = E2(p, k2)
What you do instead is to use multiple KEMs and combine them securely (see the blog post I linked) in such a way that the confidentiality of your shared secret (i.e., the key you actually use for encryption) is preserved if any of the underlying KEMs is unbroken.
ss1, ct1 = KEM1(pk1)
ss2, ct2 = KEM2(pk2)
secret = Combiner(ss1, ss2, [ct1, [ct2]])
This is very different than merely "encrypt your data twice". You only encrypt your data once. The KEY YOU ENCRYPT WITH is, instead, the result of multiple asymmetric operations.
I cannot stress enough how different these proposition are. It's like suggesting someone swim downstream in electric current. The words might make logical sense to a non-expert, but it's utterly unsafe taken literally.
key = get_key()
aes_key = derive_key(key, "domain-aes")
qc_key = derive_key(key, "domain-qc")
ciphertext_a = aes_gcm(plaintext, aes_key)
ciphertext_b = qc_encrypt(ciphertext_a, qc_key)
The thing a cryptography-relevant quantum computer does is break RSA and elliptic curve cryptography, so that the underlying key (k1 or k2) is recoverable from its corresponding public component.
Hybrid KEMs, such as mlkem768x25519 (a.k.a. X-Wing) is a simple abstraction with security proofs that does both classical (X25519 is elliptic curve) and post-quantum (ML-KEM-768 is lattice-based) cryptography and combines them securely into a single key agreement.
"Encrypt twice" is bad advice. Even if you get the same approximate security, you're giving up a lot of performance.
Encrypt once, but encrypt with a key you can be confident in the secrecy of.