They do use a KDF but only use the resulting key to encrypt the header. It's the approach used by most disk encryption implementations in order to allow the user to rotate the password without having to encrypt the whole drive again, only the header. Easy to make that atomic too.
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If you do that, rotation has weaker security properties. You really do need to reencrypt everything to rotate key without preserving any weakness.
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Sure, but the user wanting to change their passphrase is different than them wanting to rotate the underlying disk encryption key. It could require a lot of time and storage space or simply backing up, resetting and restoring from the backup.
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No, you can rekey incrementally in background by just having two active keys. Same way wireguard can rekey nondisruptively despite udp being out of order.
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How does it know which block is encrypted with a given key for block-based encryption? I can see how this could be implemented for higher-level filesystem-level encryption but I don't think either the ext4/f2fs approach or the ZFS approach provides incremental rekeying.
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Since dm-crypt is block layer encryption with 1:1 mapping between encrypted blocks and decrypted blocks, it isn't able to do it. It's possible if you actually have proper authenticated encryption but that requires having somewhere to store the extra metadata.
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A working block level FDE would address that before it got off the ground. Like by reserving some blocks out of band for tags.
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The ext2/f2fs encryption approach is essentially still block-based encryption by the way and they still need a global key for metadata. It's entirely possible to use with ONLY a boot passphrase and end result is essentially the same. They do encrypt every block when set up right.
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The filenames actually get encrypted 2x because they encrypt the metadata blocks with the global key and then also encrypt the filenames with the per-file keys.
They don't currently have proper authenticated encryption in the mainline Linux kernel implementation either though.
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dm-crypt can be layered on top of dm-integrity which provides the storage for tags. It's a somewhat weird leaky abstraction. It isn't really used that way in practice though, and the higher-level tools aren't doing it yet.
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Yeah I think it's a bad layering. Symmetric crypto just should not be used outside an AEAD.
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I think it can actually use AEAD because it's a lossy abstraction where dm-integrity is able to just provide the storage for tags. It's fairly weird and I don't really get why they designed it the way that they did instead of it just being the same thing.
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