How to best update remote compressed, encrypted archives incrementally?

Steven D'Aprano steve at
Sat Mar 11 17:13:33 CET 2006

On Sat, 11 Mar 2006 16:09:22 +0100, robert wrote:

>> Lastly, have you considered that your attempted solution is completely the
>> wrong way to solve the problem? If you explain _what_ you are wanting to
>> do, rather than _how_ you want to do it, perhaps there is a better way.
> So, there seems to be a big barrier for that task, when encryption is on 
> the whole archive. A complex block navigation within a block cipher 
> would be required, and obviously there is no such (handy) code already 
> existing. Or is there a encryption/decryption method which you can can 
> use like a file pipe _and_ which supports 'seek'?


Let's try again: rather than you telling us what technology you want to
use, tell us what your aim is. I suspect you are too close to the trees to
see the forest -- you are focusing on the fine detail. Let's hear the big
picture: what is the problem you are trying to solve? Because, frankly, as
far as I can see, the solution you are looking for doesn't exist. But
maybe I'm too far from the forest to see the individual trees.

"I need encryption that supports seek" -- no, that's you telling us _how_
you want to solve your problem.

Perhaps you can tick some/all of the following requirements:

- low bandwidth usage when updating the remote site

- transmission needs to be secure

- data on the remote site needs to be secure in case of theft or break-ins

- remote site is under the control of untrusted parties; 
or remote site is trusted

- remote site is an old machine with limited processing power and very
small disk storage; 
or remote site can be any machine we choose

- local site needs to run Windows/Macintosh/Linux/BSD/all of the above

- remote site runs on Windows/Macintosh/Linux/BSD/anything we like

- we are updating text files/binary files 

- anything else you can tell us about the nature of your problem 


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