3 ms·

But this isn't disk encryption! Filesystems have metadata: which is where a MAC can go, next to the filename. There's just no way this should be unauthenticate

by qrmn 11y ago

But this isn't disk encryption! Filesystems have metadata: which is where a MAC can go, next to the filename. There's just no way this should be unauthenticated XTS, this is simply the wrong mode to use, you should use an AEAD. ChaCha20_Poly1305 - RFC7539: https://datatracker.ietf.org/doc/rfc7539/ https://datatracker.ietf.org/doc/rfc7539/ - makes much more sense. Faster, too, on the target mobile devices which have poor crypt performance. I don't think this should be merged early or as-is. Unauthenticated encryption is against good advice and a bad idea. Standardising that is an even worse idea. Edit: I do see that they're intending this as a minimum viable patch for Android 'M' and they intend to add GCM support later (again, I'd point to CC20 as a newer, better alternative), they're just waiting on a transactional update first. I do question whether shipping it broadly with this design as-is should happen in the vanilla kernel, however: Android uses its own patches anyway. Maybe let this one mature a bit first before merging upstream?

fulafel 11y ago
> Filesystems have metadata: which is where a MAC can go, next to the filename. File-level MACs would require reading/writing the entire file in one go, andpreclude efficient in-place modification of files. The alternative, lots of per-block MACs stored in extended attributes, wouldn't work for ext4 because of the 4 kb extended attribute size limitation, and would also have problems that the grandparent listed (lots of seeks).
- qrmn 11y ago
  A fair point. btrfs would have a much easier time of it. We then have the interesting consequence that we can observe times and sizes of COW updates, but in truth SSDs have that anyway.
caf 11y ago
ChaCha20 may be better in the abstract, but as a practical matter AES-GCM can take advantage of AESNI. I don't think a patch adding support for ChaCha20 would be particularly controversial anyway.

by qrmn 11y ago

fulafel 11y ago

> Filesystems have metadata: which is where a MAC can go, next to the filename. File-level MACs would require reading/writing the entire file in one go, andpreclude efficient in-place modification of files. The alternative, lots of per-block MACs stored in extended attributes, wouldn't work for ext4 because of the 4 kb extended attribute size limitation, and would also have problems that the grandparent listed (lots of seeks).

qrmn 11y ago
A fair point. btrfs would have a much easier time of it. We then have the interesting consequence that we can observe times and sizes of COW updates, but in truth SSDs have that anyway.

caf 11y ago

ChaCha20 may be better in the abstract, but as a practical matter AES-GCM can take advantage of AESNI. I don't think a patch adding support for ChaCha20 would be particularly controversial anyway.