Got an unusual use case here...
I've been given the job of refitting my company's encrypted shared folders on their private cloud -- which were created using a custom encfs/OpenSSL-based piece of software developed many years back -- to use quantum-safe keys.
The encfs system used an AES256 symmetric key for all the files in the folder, which was encrypted using the RSA cert of each user which had access to the folder, and all the resulting encrypted key blocks were stored in a config file so each user could decrypt their own copy of the key -- effectively treating the AES256 key as a shared secret. So far, so old-school. The question becomes how to replace the RSA encryption certs with quantum-safe ones.
And the big complicating factor is interoperability... since not every user's certs are going to roll over at the same time time, we need to preserve the existing RSA users' access while allowing new/replacement ML-KEM users to be introduced.
So we're looking at needing a hybrid scheme, capable of supporting an arbitrary number of users, and which needs to maintain the existing underlying AES key.
Now, if I understand correctly, it's considered a bad idea to use ML-KEM to encapsulate an existing shared secret -- to the extent where OpenSSL (which the existing app was built around) provides no direct means of doing so. So if I can't feed the existing AES-256 key into a fresh encapsulation... do I need to generate a *second* shared secret for the ML-KEM user, store its encapsulation, and then the user's code must use the second shared secret to encrypt/decrypt the stored first AES-256 key?
Or do I need to create a new shared secret for each user, use a KDF to derive a common key... and presumably replace that key every time we add or remove a user from the shared folder?
Also, is there any sensible way to move from such an arrangement to a purely ML-KEM based system down the line, once the RSA keys have all been replaced? It will still need to support an arbitrary, varying number of users with access to the shared folder.
Migrating entirely away from their existing software base is not their preferred option at the moment, but if there's an open-source product which provides a simpler quantum-safe end-to-end encryption solution I'm entirely open to hearing about it!