5 ms·

No, I mean why would ZFS have anything to do with encryption at all? It doesn't seem to be a filesystem issue, rather something you put underneath it (encrypti

No, I mean why would ZFS have anything to do with encryption at all? It doesn't seem to be a filesystem issue, rather something you put underneath it (encrypting the block device, no matter what file system is on top) or on top of it (encrypting separate files).

otterley 10y ago
It's a feature that trades off layering purity (which ZFS has never adhered to) for increased usability.
- black_knight 10y ago
  I see. When crypto is concerned, this kind of purity seems attractive though… Easier to reason about when using it. If I were to use ZFS’s native crypto I would have to look into the details of their implementation to understand what guarantees are provided, and where in the stack the crypto is applied etc…
  - tptacek 10y ago
    In fact, it's significantly harder to reason about encryption at the block layer, and not just because it's difficult to reason about the most popular construction (XTS). Among other things, you have to come up with a semantic for handling authentication failures at a sector level. What does it mean for one sector of a file to fail authentication? Have you lost the whole file? Do you fill with zeroes? How do you signal the precise error to the kernel or users?
    - black_knight 10y ago
      The semantics seem pretty strait forward. That sector is untrusted. You do not fill with zeroes(!), but fail as hard as you can. I understand that kernel currently may not have any semantics for files or sectors failing authentication, but that issue would doesn't seem to depend on the file/sector dilemma, and would have to be solved anyhow.
      - tptacek 10y ago
        To me, this is very simple: File-level encryption has total format flexibility. A filesystem can store arbitrary metadata. It turns out that strong encryption schemes want internal metadata. You can go through contortions --- contortions no block encryption scheme currently goes through --- to get some sort of space for metadata at the block layer, at significant expense. Or you can get it for free with files. File-level encryption is also message-aware. Encryption cares about the boundaries between messages (just for instance: because it leaves no ambiguity about the set of disk sectors that makes up any particular file at any particular moment in time). Block-level crypto can't provide this; it's by definition oblivious to messages. We have block-level crypto because it's a cheap easy win, for the minimal security it provides. If the FBI is interdicting you, block crypto isn't going to help you: they incurred weeks worth of planning before anyone with handcuffs approached you, and the extra planning it takes to make sure they grab you when your key is resident is a rounding error. But if you leave your laptop full of financial secrets in a taxi cab, block crypto will probably keep you from having to roll all your keys, change your account numbers, notifying your clients, etc.
        tedunangst 10y ago
        But won't the FBI also wait to make the grab when using filesystem encryption? We're not talking about some filesystem that uses unique passwords for every file.
        tptacek 10y ago
        Unlike block encryption, filesystem encryption doesn't have to be all-or-none; you can have lots of different keys, unlock on a directory-by-directory basis, whatever. There's room for arbitrary metadata. If you're using your computer at all, block encryption is unlikely to help you. I'm not saying any particular extant system does those things; I'm just saying they're a possibility for filesystems and foreclosed for device-level encryption.
        tedunangst 10y ago
vbit 10y ago
With this you can encrypt one dataset and leave another unencrypted, on the same block device, paying the cost only or the encrypted one.
- mrsteveman1 10y ago
  Which means you can also potentially use different keys for each encrypted filesystem as well, one for each user or various other scenarios
  - black_knight 10y ago
    That’s interesting, but it again seems like something which can be implemented independently of the particular choice of file system.
    - rincebrain 10y ago
      Only sort of? ZFS is both filesystem and logical volume abstraction, so to implement different keys on different "filesystems" on ZFS, you'd need to expose them as block devices, not filesystems, and then use your encryption du jour on them, then your filesystem atop that - which also kills most of your compression or encryption properties, since you're doing it before ZFS sees the data, so to speak.
      - black_knight 10y ago
        I guess I was thinking more like a 9P[0] approach where a file system can be given to a module (or any program) and based on it the module can export a related file system. For example the module would encrypt the files before storing it to the given file system. However, I realise this does not quite fit into the current kernel architectures. [0] https://en.wikipedia.org/wiki/9P_(protocol) https://en.wikipedia.org/wiki/9P_(protocol)
- black_knight 10y ago
  What is a dataset in this setting?
Titanous 10y ago
In order to do filesystem encryption properly, it needs to be done at the file layer, not the block layer. Block-level encryption is not authenticated because there are no extra bytes to add the authentication tag. If a ciphertext is not authenticated, it can be trivially tampered with. This means that someone with access to the encrypted drive could add backdoors or otherwise tamper with the executables and data even though it is encrypted.
- black_knight 10y ago
  You can perfectly fine divide the block device into a bit smaller chunks so that you can fit a MAC or similar at the end.
  - Titanous 10y ago
    My understanding is that in practice this is quite hard to do. As far as I know, none of the mainstream block-layer encryption systems (BitLocker, FileVault, dm-crypt) provide authenticated encryption. Here's a short thread about the problems with adding it to dm-crypt: http://comments.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt/6689 http://comments.gmane.org/gmane.linux.kernel.device-mapper.d... I'm sure you can find more threads if you look around.
    - black_knight 10y ago
      Thanks! That’s good to know. Indeed, encryption without authentication is poor security.
  - deleted 10y ago
    [deleted]
  - mioelnir 10y ago
    This is how FreeBSD's GELI (which has authenticated encryption for blockdevices) did it. For every 4k data block it presented up the stack, it consumed 9 512 sectors on disk. Each of them contained 480 bytes, the rest for MAC. With 4k native drives, this became completely impractical. To keep ratios similar, you would have to present 32k byte devices up the stack, which filesystems have troubles with. Or have 1 MAC sector per data sector or similar, cutting your storage in half.
anorwell 10y ago
Encrypting on top of ZFS negates ZFS' compression and de-duplication. My understanding is that this implementation preserves both of those. For example, ZFS can compress before encrypting.

otterley 10y ago

It's a feature that trades off layering purity (which ZFS has never adhered to) for increased usability.

black_knight 10y ago
I see. When crypto is concerned, this kind of purity seems attractive though… Easier to reason about when using it. If I were to use ZFS’s native crypto I would have to look into the details of their implementation to understand what guarantees are provided, and where in the stack the crypto is applied etc…
- tptacek 10y ago
  In fact, it's significantly harder to reason about encryption at the block layer, and not just because it's difficult to reason about the most popular construction (XTS). Among other things, you have to come up with a semantic for handling authentication failures at a sector level. What does it mean for one sector of a file to fail authentication? Have you lost the whole file? Do you fill with zeroes? How do you signal the precise error to the kernel or users?
  - black_knight 10y ago
    The semantics seem pretty strait forward. That sector is untrusted. You do not fill with zeroes(!), but fail as hard as you can. I understand that kernel currently may not have any semantics for files or sectors failing authentication, but that issue would doesn't seem to depend on the file/sector dilemma, and would have to be solved anyhow.
    - tptacek 10y ago
      To me, this is very simple: File-level encryption has total format flexibility. A filesystem can store arbitrary metadata. It turns out that strong encryption schemes want internal metadata. You can go through contortions --- contortions no block encryption scheme currently goes through --- to get some sort of space for metadata at the block layer, at significant expense. Or you can get it for free with files. File-level encryption is also message-aware. Encryption cares about the boundaries between messages (just for instance: because it leaves no ambiguity about the set of disk sectors that makes up any particular file at any particular moment in time). Block-level crypto can't provide this; it's by definition oblivious to messages. We have block-level crypto because it's a cheap easy win, for the minimal security it provides. If the FBI is interdicting you, block crypto isn't going to help you: they incurred weeks worth of planning before anyone with handcuffs approached you, and the extra planning it takes to make sure they grab you when your key is resident is a rounding error. But if you leave your laptop full of financial secrets in a taxi cab, block crypto will probably keep you from having to roll all your keys, change your account numbers, notifying your clients, etc.
      - tedunangst 10y ago
        But won't the FBI also wait to make the grab when using filesystem encryption? We're not talking about some filesystem that uses unique passwords for every file.
        tptacek 10y ago
        Unlike block encryption, filesystem encryption doesn't have to be all-or-none; you can have lots of different keys, unlock on a directory-by-directory basis, whatever. There's room for arbitrary metadata. If you're using your computer at all, block encryption is unlikely to help you. I'm not saying any particular extant system does those things; I'm just saying they're a possibility for filesystems and foreclosed for device-level encryption.
        tedunangst 10y ago

vbit 10y ago

With this you can encrypt one dataset and leave another unencrypted, on the same block device, paying the cost only or the encrypted one.

mrsteveman1 10y ago
Which means you can also potentially use different keys for each encrypted filesystem as well, one for each user or various other scenarios
- black_knight 10y ago
  That’s interesting, but it again seems like something which can be implemented independently of the particular choice of file system.
  - rincebrain 10y ago
    Only sort of? ZFS is both filesystem and logical volume abstraction, so to implement different keys on different "filesystems" on ZFS, you'd need to expose them as block devices, not filesystems, and then use your encryption du jour on them, then your filesystem atop that - which also kills most of your compression or encryption properties, since you're doing it before ZFS sees the data, so to speak.
    - black_knight 10y ago
      I guess I was thinking more like a 9P[0] approach where a file system can be given to a module (or any program) and based on it the module can export a related file system. For example the module would encrypt the files before storing it to the given file system. However, I realise this does not quite fit into the current kernel architectures. [0] https://en.wikipedia.org/wiki/9P_(protocol) https://en.wikipedia.org/wiki/9P_(protocol)
black_knight 10y ago
What is a dataset in this setting?

Titanous 10y ago

In order to do filesystem encryption properly, it needs to be done at the file layer, not the block layer. Block-level encryption is not authenticated because there are no extra bytes to add the authentication tag. If a ciphertext is not authenticated, it can be trivially tampered with. This means that someone with access to the encrypted drive could add backdoors or otherwise tamper with the executables and data even though it is encrypted.

black_knight 10y ago
You can perfectly fine divide the block device into a bit smaller chunks so that you can fit a MAC or similar at the end.
- Titanous 10y ago
  My understanding is that in practice this is quite hard to do. As far as I know, none of the mainstream block-layer encryption systems (BitLocker, FileVault, dm-crypt) provide authenticated encryption. Here's a short thread about the problems with adding it to dm-crypt: http://comments.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt/6689 http://comments.gmane.org/gmane.linux.kernel.device-mapper.d... I'm sure you can find more threads if you look around.
  - black_knight 10y ago
    Thanks! That’s good to know. Indeed, encryption without authentication is poor security.
- deleted 10y ago
  [deleted]
- mioelnir 10y ago
  This is how FreeBSD's GELI (which has authenticated encryption for blockdevices) did it. For every 4k data block it presented up the stack, it consumed 9 512 sectors on disk. Each of them contained 480 bytes, the rest for MAC. With 4k native drives, this became completely impractical. To keep ratios similar, you would have to present 32k byte devices up the stack, which filesystems have troubles with. Or have 1 MAC sector per data sector or similar, cutting your storage in half.

anorwell 10y ago

Encrypting on top of ZFS negates ZFS' compression and de-duplication. My understanding is that this implementation preserves both of those. For example, ZFS can compress before encrypting.