Making changes to long files

Berqash

Please consider this scenario:
I back a long database file using BoxCryptor in dropbox.
I add a few records to the database.
Does BoxCryptor have to reupload the whole file or simply the changed blocks ?
Thanks for your answer.

Robert

Hi Berquash,

it depends on how the database application changes the files. If it simply adds the data to the end of the file, only the new data has to be uploaded. If the data is added within the file, the new data and all old data afterwards have to be reuploaded.

Generally, BoxCryptor encrypts blocks of data (normal block size is 4 KB) and the number of a block is taken into account when encrypting a block (so a block is encrypted differently depending on its position within a file). When a file is changed there are 3 scenarios:

a) Parts of a file are changed but the file size is exactly the same: Only the changed blocks have to be reencrypted and reuploaded.

b) Parts of a file are changed and the file size is changed: The changed blocks and all following blocks have to be reencrypted and reuploaded. (e.g. if data is inserted within the file)

c) Data is added/deleted from the end of a file: Only the added data has to be uploaded

Hope I could help you,
Robert

Berqash

Hi Robert
Thank you for your comprehensive answer.

I have a 125 MB database (MySQL) which changes daily and is "dropboxed" daily.
When I put it directly in DropBox it updates in 5-10 min (after the first full backup of ~3 hours). When I put it in BoxCryptor it updates in ~1 hour.

Is it possible to make BoxCryptor behave like the old RSyncrypto and backup only the changed blocks without block chaining. This would remove any overheads and make BoxCryptor essentially as efficient as DropBox.

Finally: Is BoxCryptor compatible DropBox Version 1.2.x?

Thanks
Aly

Robert

Using a cipher mode without block chaining can impose a serious security problem because data patterns can be detected even in the encrypted files. See http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation#Electronic_codebook_.28ECB.29 for more information. Additionally it would break the compatibility with EncFS.

But I understand that in some cases the efficiency benefit of not using block chaining might be more important than the security loss and we're investigating if and how we can better support these use cases.

Regarding the Dropbox compatibility: The automatic Dropbox detection does not work with new Dropbox verions which encrypt their configuration database, but everything else works just fine.

Berqash

Thank you...