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I'm building a cloud storage application that will store files on my servers.

I want these files to be encrypted with AES in CTR mode and the key will be always the same.

I'm wondering what would be the best way to derive an IV.

The encryption will happen on many servers so I can't use progressive values (e.g. incrementing first 8 bytes for every different encrypted file) since I can't keep count across servers.

Numbers of stored files can reach tens of billions.

What's the best practise in this scenario to derive the IV (or maybe a Key + IV)?

  1. Should I use only one key that never changes?
  2. Should I derive a new key for every encrypted file (from the master key)?

Edit:

If key mustn't be the same, would it be secure to do the following:

  1. A new key (256-bit) is randomly-generated for every file.
  2. This key is encrypted with the masterkey in ECB mode (since it's highly entropy data, do we need CBC?)
  3. The file key is appended at the beginning of the file.
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    $\begingroup$ If you must use the same key for every file then use SIV mode. If the "one key for all files" constraint is your own imposition then drop that right now and don't be afraid of using many keys! $\endgroup$ – Thomas M. DuBuisson Nov 5 '15 at 17:54
  • $\begingroup$ Furthermore, from the sound of your question it looks like you might be encrypting these files "because security", without actually considering what type of attack/attacker you're actually defending against. And your edited second approach is functionally no different than simply using a single key. $\endgroup$ – Stephen Touset Nov 5 '15 at 19:26
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    $\begingroup$ To explain my previous comment, a cloud storage application that will store tens of billions of files almost certainly will need stronger security requirements than "everything is encrypted" — if that's truly the only thing, to check a box, then full-disk encryption is probably good enough. If not, the solution will almost certainly require more thought and design than simply encrypting everything with a single key. $\endgroup$ – Stephen Touset Nov 5 '15 at 19:32
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You may want to rethink your whole setup. Like mentioned in the comments, there is a lot more to securely storing files than throwing encryption on them. In particular, you need to be concerned with authentication (CTR without authentication is malleable). I would also look into client side encryption in anything that puts user data in the cloud.

Still, there are probably some situations where just using CTR to encrypt a bunch of files is the right thing to do, so I will answer your questions below.

What's the best practise in this scenario to derive the IV (or maybe a Key + IV)?

Usually in CTR mode you have a nonce concatenated with a counter. If you want to support arbitrary file sizes, you probably want the counter to be 64 bits. However, that does not leave enough space for a nonce, since 64-bit nonces will collide after a few billion files. If e.g. your API restricts you to this setup, you would be better off with per-file keys.

If you can use random 128-bit nonces, with e.g. the lower half incremented as the counter, that should be fine. You would avoid the need for key setup, which would be an advantage especially with small files.

If key mustn't be the same, would it be secure to do the following:

  1. A new key (256-bit) is randomly-generated for every file.
  2. This key is encrypted with the masterkey in ECB mode (since it's highly entropy data, do we need CBC?)
  3. The file key is appended at the beginning of the file.

ECB would be acceptable for encrypting random data, but concerns about authentication remain. Alternatively you could use key derivation. That way you can use e.g. a random 128-bit salt for each file even with 256-bit keys.

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  • $\begingroup$ Hi otus, thank you very much for your answer. I tought to use random 128-bit nonces with the lower half incremented as the counter. In this case, would it be safe to don't worry about IV collisions? $\endgroup$ – user28875 Nov 6 '15 at 14:13
  • $\begingroup$ @user28875, yes, that would not collide with any practical data sizes. For a look at the math you can see this answer. $\endgroup$ – otus Nov 6 '15 at 14:35

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