uploading only changed file parts by turning off 7zip compression
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#182 uploading only changed file parts by turning off 7zip compression
dropbox is capable of uploading only parts of a file that has changed. This works well for me if uploading an aes encrypted truecrypt container file as, being a block cypher, parts of the file that were not changed are left alone. However when 7zip compresses my Truecrypt container file it seems that after making a small change, the whole file is changed by the compression. It think so because it seems to take a long time to upload. If I could tell cryptsync use 7zip in no-compression mode, it would only do the aes encryption so only the part of the file that changed will have to be uploaded. So the task is to change the interface adding an option for 7zip compression type or a checkbox for turning compression on or off.
Turning off compression wouldn't help. What most responsible for changing your whole file isn't compression, it's encryption.
With compression but without encryption, all the data blocks following the changed block would be changed, but blocks that come before stay the same.
With encryption, with or without compression, every block in your file would be changed, even if the original file stay exactly the same. Because 7zip uses AES with CBC mode, which uses an initialization vector (IV) that should be different each time you re-encrypt something. A different IV changes every block in your data.
I'm not sure why you'd want your TrueCrypt container to be again encrypted by CryptSync / 7zip. I don't think there's a meaningful increase in security.