RFC: end-to-end compare -aq (Was: Re: [Box Backup] Win32 native client service bbackupd.conf)

E.W. Peter Jalajas boxbackup@fluffy.co.uk
Thu, 29 Jun 2006 12:27:46 -0700 (PDT)


My simple mind is having trouble understanding this discussion.

1.  Help me understand what is the goal of this end-to-end compare RFC
that is not satisfied with: 
bbackupquery compare -aq  
Are you trying to compare the attributes (which are not compared with

BTW, are the attributes the only thing not compared with -aq?

2.  Can somone please explain how 
bbackupquery compare -aq 
works?  How does it do its job without downloading all of the
ciphertext from the server and decrypting on the client for comparison
with what's on the client?  (Or, does it encrypt the blocks on the
client, checksum those, and send those checksums to the server to
compare against the stored ciphertext and to return the compare
results--if so, see 3. below?)

3.  One of the tenets of Box Backup is that we only trust the server to
return the stored ciphertext when requested by the client.  Does this
RFC encroach on that?   (Re 2. above, do we also trust it to return
valid checksums during compares?)

(4.  Can the ssl layer give us any useful checksums?)

Sorry if these are dumb questions.


--- Gary <gniemcew@yahoo.com> wrote:

> >> "Allow for end-to-end server data verification with bbackupquery  
> >> compare -aq (quick compare),
> >> without having to re-download all data (as is the case with  
> >> bbackupquery compare -a). The feature
> >> would require bbstored (server) to read factual data blocks off
> its  
> >> storage media and compare
> >> those data blocks against checksums both stored locally and sent
> up  
> >> from bbackupquery (client).
> >> The only undetected failure condition in such a scenario would  
> >> require both a server data block
> >> and a server checksum getting corrupted in such a way as to match
> a  
> >> client checksum - the chances
> >> of this are beyond astronomical."
> > The stored files are split into blocks, and each block is encrypted
> > using a strong random initialisation vector (IV). This is to
> maintain  
> Ahhhhh, didn't think of this. That explains why a block checksum
> relates to a plaintext, not to a
> ciphertext.
> > Another thing to remember is that the server will reassemble blocks
> > if the client sends "changes only" for a file. So you can't just  
> > record the checksum of the last file you sent.
> So the only way out of this would be for a client to include an
> additional per-block ciphertext
> checksum before the a data block is sent up to the server (to account
> for the possibility that the
> network-streamed data is not saved properly on the server side in the
> first place). The server
> would have to keep that information in the repository (that implies a
> verification-enabled
> repository would not be backward-compatible) and use it for a two
> stage compare: a plaintext
> checksum to verify client/server content match, and a ciphertext
> checksum to verify server/storage
> media match, right?
> [... hmmmm, wonder if yet another checksum of a plaintext checksum/a
> ciphertext checksum
> combination would be useful here...]
> The only way for this scenario to fail would be a "matching"
> corruption of a ciphertext checksum
> and a factual data block, unless data handed off to a server by a
> client is nonsense in the first
> place (a meltdown of client/server function, logic errors), and a
> client cannot decrypt/reassemble
> its own blocks at all, even if handed the original data.
> > I didn't really consider it worth the effort.
> Well, I am sure the paranoid among us, already paying big bucks for
> RAID arrays of Raptor drives,
> would be delighted at this feature ;).
> G.
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