Supersingular Isogeny Key Exchange is one that was invented to be quantum-safe but turned out to be unsafe at any speed, so hybrid encryption is still a good idea. You use both a quantum-safe algorithm and a classical algorithm, encrypting your data twice and remaining secure if either one is broken.
The controversy lately isn’t for encryption. We have a fine hybrid KEM, and it’s being standardized/deployed most places.
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.
No. "Post-quantum" is not a kind of cryptography; it's an attribute of many different kinds of cryptography. SIKE and modular lattices are completely unrelated. SIKE is moon math that genuinely was introduced to mainstream cryptographers as a post-quantum construction. Lattices have been carefully studied for decades; in the 1990s, it was a live discussion whether the successor to RSA was going to be elliptic curves or lattices.
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.
Worth mentioning the lattice KEM he backed (NTRU prime) is part of a class of lattice-based assumptions that admitted devastating attacks (though not in the parameter regime relevant to public-key cryptography applications). By this I mean the dense sub lattice attacks on NTRU.
He has also repeatedly pointed to (seemingly random) pieces of lattice cryptography and claimed that it is the cause for concern/plausibly where attacks may come from. Here, I mean the galois group structure, the whole “quotient vs product” stuff he was doing trying to pretend LWE is a variant of ntru (and less secure, which was explicitly wrong), and his “spherical models” claims. These last ones included an explicit claim of subexponential attacks to be presented later, which have been delayed for a number of years now.
In short, his fearmongering over lattices, while persistent, has never been right. He’s pointed fingers at things we have not found issues with, and either backed sides in debates which ended up being less secure (NTRU vs LWE), or completely missed other things (say the sPIP attacks a decade ago). He may plausibly be the least credible person to make predictions about lattices in the world.
This is ignoring all of his other explicitly embarrassing behavior, for example
1. Insinuating all lattice cryptographers are on the payroll of the NSA. The winning schemes were European teams predominantly.
2. Adding a license to all emails he sends in the IETF wg that is incompatible with the wg. This ends up with him getting censure, which he then argues is unjust.
3. Recently, finding a bug in a 2017 piece of software, and then fabricating 3 other bugs. He then wrote a 60 page paper on it, using it as justification to argue against lattices. All of the bugs would be caught by standard high quality testing procedures, eg mutation testing, which he appears unfamiliar with. I believe the “actual” bug (from the v1 reference impl a decade ago) is caught by current test vectors as well.
> You use both a quantum-safe algorithm and a classical algorithm, encrypting your data twice and remaining secure if either one is broken.
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.
https://durumcrustulum.com/2024/02/24/how-to-hold-kems/
> If you encrypt your data twice, and one of them is broken by a quantum computer, the adversary gets the plaintext anyway.
Is the idea here that "you broke quantum and quantum breaks classical, therefor layering is pointless"?
If you encrypt your data twice (taken very literally):
If we assume E1() is broken by a quantum computer, E2 doesn't matter to protect p.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.
This in practice looks like a KDF based on a hash function where the component shared secrets (and, depending on the underlying KEM's binding properties, underlying ciphertexts too) are concatenated.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.
It seems to me you assumed that the poster that replied to you meant encrypting in parallel, while it seems pretty clear to me what they meant was c = E1(E2(p, k2), k1).
both encrypting in parallel and encrypting in the second way you mentioned are bad ideas, and are far from being what is seriously being discussed when people talk about hybrid KEMs. Encrypting in parallel is explicitly IND-CPA insecure if one of the ciphers is broken. Your construction is IND-CPA secure, but quite inefficient, and would not fit into modern protocols.
If this was a typical cryptographic topic, this might be fine, and is how I would likely phrase things for an undergraduate cryptography course. Unfortunately, this is a topic that a certain cryptographer with a decently large public following has been spreading conspiracy theories (and slandering other cryptographers about) for a number of years now. So, discussions on this topic often come from a place where the audience is misinformed, and more care is required in grounding the discussing in what is actually being discussed/considered.
The thing is: Quantum computers don't break AES-GCM, ChaCha20-Poly1305, or any other modern authenticated cipher. Layering encryption or doing cipher cascades is pointless.
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.
Are you saying that a "hybrid KEM" is different in theoretical risk from chaining two KEMs? The change of jargon from "encryption" to "KEM" doesn't mean anything to most people talking about this post-quantum risk. To the extent we know what KEM is, we think it is just encrypting the key used for the rest of the bulk encryption.
Whether or not people understand the nuance of encrypting the block cipher keys or encrypting the blocks themselves, I think we all mean to stack the two encryption methods for defense-in-depth protection. They intuit having to open two locks in series to get to the valuable stuff, not adding two different access paths that each suffice for access.
"Intuition" about how cryptography works is notoriously bad. Many intuitive things about cryptography are false, and many true things about cryptography are non-intuitive. For this reason it is difficult to seriously discuss cryptography when people are vaguely referring to what they intuitively hope to achieve, framed in terms of concrete constructions that are not secure.
This is also completely ignoring that designing secure systems is about MUCH more than selecting the right "hard problem". Concretely
> They intuit having to open two locks in series to get to the valuable stuff, not adding two different access paths that each suffice for access.
might mean requiring a much more complicated lock that, in its ideal implementation has the properties you want, but practically is easier to implement incorrectly, yielding a less secure scheme. Considerations of this form almost never appear, despite being very relevant to the end goal of protecting users.
Similarly, this "defense in depth" intuition is currently not particularly controversial for hybrid KEMs. it is currently quite controversial for hybrid signatures though. The intuitive story would work perfectly well for signatures though. So this intuition does not end up being particularly useful for understanding the actual discussion.
I don't disagree, but I think the folks who know this ought to remember the lay person perspective and try to address it more concretely.
Rather than rejecting the framing because they (we) aren't fluent in your jargon, provide a more constructive hint... E.g. "You may be thinking the symmetric cipher key is simply encrypted with the asymmetric cipher and concatenated to the bulk message. But, to mitigate known cryptographic system risks, modern solutions use specialized key encapsulation or key exchange methods (KEM) which are not directly encrypted messages containing key material."
> I think the folks who know this ought to remember the lay person perspective
That's fair. I hold Hacker News to a higher bar of technical proficiency than a general audience. My hope with insisting on correct framing is to nudge other experts in adjacent fields to teach your more general audiences how to think about these topics more correctly so it's more approachable to the general public.
> Are you saying that a "hybrid KEM" is different in theoretical risk from chaining two KEMs?
No, I'm saying that "hybrid KEM" or "chaining two KEMs" is very distinct from "encrypt twice". Confuse the two at your own peril.
> To the extent we know what KEM is, we think it is just encrypting the key used for the rest of the bulk encryption.
Encryption is reversible. If you have the key, you can decrypt. It's not encryption if you can't decrypt.
KEMs are their own class of algorithms. They combine an asymmetric encryption scheme with an all-or-nothing one-way transform (usually a key derivation function built on hash functions). It's the safest way to hold asymmetric encryption in practice (even not considering PQ; RSA-KEM beats RSA-OAEP in implementation safety).
Calling KEMs "encryption" is misleading to the point of malpractice. I will push back on conflating the two.
> Whether or not people understand the nuance of encrypting the block cipher keys or encrypting the blocks themselves, I think we all mean to stack the two encryption methods for defense-in-depth protection.
Your only defense-in-depth should be in delivering a strong pseudorandom ephemeral key over an untrusted network, and then using the tried-and-true AEAD constructions that we're already using today. Encrypt once. Do whatever you need to do to get the key exchanged securely.
I write a blog that very regularly covers applied cryptography. I deal with newbie confusion all the time. It's very important that we talk about these things correctly on forums like Hacker News comment threads so that the people learning from us won't get more confused.
Please don't call KEMs "encryption".
The idea would be:
I think this is different from what you wrote but I can't really tell.FWIW I am not advocating for "encrypt twice" at all, I'm just trying to understand.
A better idea is to do this:
This sounds more complex, but I'm just filling in the details implied by your pseudocode and making it at least 2x as fast.On the opposite side, their code looks like this:
If you mean "doing two different KEMs and then securely combining them", then just say that. "Hybrid KEM" is short enough and distinct from other verbage."Encrypt" means something specific, not just the vague use of cryptography.