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My information security model right now is "if you want the data that I am storing on the cloud, you can have it" and I would like to change that to "if you want the data and have a $5 wrench, you can have it." To that end I am considering using rsyncrypto to perform encrypted backups in a bandwidth efficient manner. The rsyncrypto program does this by changing

the encryption schema from plain CBC to a slightly modified version. This modification ensures that two almost identical files, such as the same file before an after a change, when encrypted using rsyncrypto and the same key, will produce almost identical encrypted files.

The wiki provides a little more information about the encryption algorithm

Rsyncrypto uses the standard CBC encryption mode, except every time the decision function triggers, it uses the original IV for the file instead of the previous cypher block.

The developer of rsyncrypto suggested that since the files are compressed before encryption, the issues raised in Wikipedia about CBC and IV reuse do not matter.

Would using rsyncrypto provide any appreciable security benefit over simply uploading unencrypted files to cloud based storage?

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  • As an alternative to rsyncypto, have you considered duplicity for encrypted bandwidth-efficient backups over rsync?
    – mti2935
    Jul 26, 2023 at 17:04
  • @mti2935 I have. I don't like the delta style backups duplicity does where you need all the steps in the chain to recover and the backup repository continually grows even if you delete things.
    – StrongBad
    Jul 26, 2023 at 17:12

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The design of rsyncrypto would be considered insecure by a cryptographer, for the reason mentioned by this comment you cited. Especially since the fix is obvious: increment the IV every time the “decision function” triggers. But this could dephase/offset the IV depending on the changes made to the file, so it’s not perfect.

However, the vulnerability would not be easy to exploit, and it would only leak that some data repeats, which might not be a threat in your context.

Using rsyncrypto would still provide a significant protection of the confidentiality of your data compared to unencrypted storage.

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  • I am pretty sure the IV reuse is critical to rsyncrypto. I am not worried about people finding out that some of my files have repeating text in them. Rsyncrypto uses a different key across files so I believe there is no across file leakage of information.
    – StrongBad
    Jul 26, 2023 at 17:16

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