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The reason I am asking this is the following:

My boss wants me to sell a backup solution where encrypted backups are to be stored at our site. But we shouldn't have the key to decrypt them (in case we are hacked). The backups should also only be tested on the client's side, not on ours. Because according to him, this wouldn't be necessary.

So here is one scenario where I think that kind of assumption would be problematic.

Let's say the ransomware is able to fully control the backup software. It will show all tests as ok, but secretly it encrypts all backups with two public keys. One key pair is generated on the machine itself (the private key will later be securely erased). When the user tries to restore a backup, the ransomware will actually decrypt it correctly. It will also create modified rescue media. At a certain date, the ransomware will erase the private key and decrypt all live data on the network.

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  • Or it simply encrypts the data as it is backed up so that all test report as passing since the initial and backed up files are identical.
    – schroeder ♦
    Apr 15, 2021 at 14:55
  • Related: security.stackexchange.com/questions/222082/… and security.stackexchange.com/questions/241804/…
    – mti2935
    Apr 15, 2021 at 15:04
  • @schroeder It also needs to decrypt the data once the user wants to restore something.
    – CCK1979
    Apr 15, 2021 at 15:10
  • Yes, that's possible. If the backups are encrypted, and you don't have a key, then you can't check what's inside the backups. Do you want to check what's inside the backups? Maybe, maybe not. That's a business decision. If you can see inside them, you could notice they're not backing up the right stuff. But if you can see inside them, you could also steal trade secrets.
    – user253751
    Apr 15, 2021 at 16:49
  • @CCK1979 why would that be a requirement? What threat are you concerned about? Your scenario describes a specific way one type of threat might materialise, but what's the underlying threat you are worried about? What is the functional difference between 1) not encrypting at all, and 2) encrypting but decrypting transparently upon access?
    – schroeder ♦
    Apr 16, 2021 at 7:12

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