That's a good point, I wasn't very specific about this (trying to keep it concise). To be specific here, the con is that there isn't what the industry calls Cryptographic Agility. https://en.wikipedia.org/wiki/Cryptographic_agility.
It is not true this con applies to all cryptography (e.g. look at TLS). It has more to do with how cryptography is configured, parameters are negotiated and keys are managed, than with point-in-time choices about algorithms. The con here is that unlike other deployments of cryptography, this one doesn't have parameter negotiation and key management - and therefore doesn't have cryptographic agility.
Re: "I'd wager that... AES..." is a also a good point. Modern cryptography has shown to be robust for decades and past their deprecation point. However, as you said, it IS a wager. There have been catastrophic failures of cryptographic primitives in the past. The con of this system is you will need to make a wager and tie yourself to the fate - you can't mitigate the risk if the catastrophic event comes or appears to be coming to pass.
Agility does nothing to stop the "ciphertext from the past is broken due to crypto improvements". It's just a means to shorten response time to crypto breaks for _new_ traffic.
This project is plenty agile, upgrading the primitive is very easy as it's not a communication protocol but a data at rest protocol.
> this one doesn't have parameter negotiation and key management
You're thinking in terms of live communication protocols between endpoints. There is no "parameter negotiation" when you are both the sender and receiver of a static ciphertext.
There is no existing remediation for protecting your existing ciphertexts against future cryptanalysis.
I really do hear what you're saying and think you're making a great point.
The part that I think applies is that this is a "data at rest protocol". Communication protocols are assumed (but maybe shouldn't be - a la PRISM disclosures?) ephemeral.
As an attacker, I need to have been in the middle for that specific instance of the communication, and save it for decades, to attack it. Crypto agility shortens the window from a break or weakness to a fix, forcing any adversary who has not already recorded communication traffic to do so in a hurry.
In this setting as a "data at rest" protocol, the work to persist the ciphertext has been done for the attacker. If there's a weakness or break it's up the defender to clean up all copies of the old secret ciphertext that's out there and publish new ones. In cases where the secret has been cached (e.g. Wayback Machine) that may not be possible.
I hope you agree with this nuance that there's something the defender needs to consider. I agree with you that not all defenders will find this consideration will be decisive in their decision to use this method or not.
I just think it's a misleading analysis to say "this system is vulnerable to XYZ" without including the fact that ALL systems in this class are equally vulnerable. Crypto agility is not a thing that can be applied to encryption at rest.
It's similar to criticizing an alcoholic drink by saying "this drink will cause liver damage" as opposed to saying "this drink, like all alcoholic drinks, will cause liver damage"
Without that caveat people will see that criticism as evidence that other alcoholic drinks do not cause liver damage. The absence of words can convey the wrong impression.
At this point I can not edit the top comment. I would have edited with something like: "this property is not an implementation bug but a design outcome shared with any deployment of cryptography that persists ciphertext data in public."
Of course in future I will endeavour for more clarity and hope others read into this thread.
Contrary to what Wikipedia says, I don't think agility is considered a desirable property by most cryptographers: you still have the "attacker stored encrypted material" problem, and now you have to worry about downgrade attacks.
Many of the most interesting/effective attacks on SSL/TLS have been downgrade attacks that stem directly from the protocol's (historically) agile design.
I disagree with your speculation as to what most cryptographers think. Are you basing this on any data you can share?
Please also see the other thread about how this secret storage system is different from a communication protocol. Namely, communication protocols have a two step attack: first attacker must MITM and record ciphertext, then they must wait. This secret storage method is different (one step attack): attacker looks for ciphertexts on either targeted or non-targeted basis that use old standard. Persistence, caches and publication of these secrets has been done for them.
It's a good point about downgrade attacks. They have been brutal for TLS to deal with.
> I disagree with your speculation as to what most cryptographers think. Are you basing this on any data you can share?
No, just conversations. I'll admit it's just speculation.
Maybe I should qualify: there's "cryptographic agility" in the protocol sense (a single version of a protocol can accept a wide range of primitives, with the idea being that users can upgrade their primitives as old ones become insecure), and there's "cryptographic agility" in a more abstract design sense: wire formats, etc. should be devolved from the primitives in such a way that the protocol can be switched to secure primitives without requiring unrelated changes, and in a way that doesn't surface any differences to the user.
To my understanding, the first sense of "cryptographic agility" is widely discouraged: we've yet to figure out a really reliable way to provide backwards compatibility without enabling malicious downgrades. The second sense is not something I've heard people use the phrase "cryptographic agility" for, but that's possibly just ignorance on my part. If that is indeed another form of agility, I believe that's widely considered to be good design (and this design is "agile" in that sense, since existing messages do not compromise the security of an upgraded scheme).
It is not true this con applies to all cryptography (e.g. look at TLS). It has more to do with how cryptography is configured, parameters are negotiated and keys are managed, than with point-in-time choices about algorithms. The con here is that unlike other deployments of cryptography, this one doesn't have parameter negotiation and key management - and therefore doesn't have cryptographic agility.
Re: "I'd wager that... AES..." is a also a good point. Modern cryptography has shown to be robust for decades and past their deprecation point. However, as you said, it IS a wager. There have been catastrophic failures of cryptographic primitives in the past. The con of this system is you will need to make a wager and tie yourself to the fate - you can't mitigate the risk if the catastrophic event comes or appears to be coming to pass.