alt Hacker NewsAre 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.
Replies
> 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".
"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.