Re: [Encfs-users] syncing ciphertexts

[Matei D. <ma...@cs...>]

Can you look at the "001" line, at columns 2&4? So:

uniqueIV=0
chainedNameIV=0
externalIVChaining=1

In the plaintext folder view we have 4 identical files:
file-a
file-b
dir/file-a
dir/file-b
The all contain "hello\n".

In the ciphertext view, running md5sum on all 4 files I see:
3fee6e6aa7ad918709fac8ebb2ddaa3f  /tmp/.docs-1/dRh67KocD9v2gE-vcY5FjQHU
3fee6e6aa7ad918709fac8ebb2ddaa3f  /tmp/.docs-1/9khiedHAIWM,QyXyrau2qsLP
3fee6e6aa7ad918709fac8ebb2ddaa3f  /tmp/.docs-1/dJB18,Uy0wDV69ZbYwf6TGgf/dRh67KocD9v2gE-vcY5FjQHU
3fee6e6aa7ad918709fac8ebb2ddaa3f  /tmp/.docs-1/dJB18,Uy0wDV69ZbYwf6TGgf/9khiedHAIWM,QyXyrau2qsLP

So:
- The plain paths of the 2 files in the root dir are different:
  "file-a" and "file-b"
- The encoded paths of the 2 files in the root dir are different:
  "dRh67KocD9v2gE-vcY5FjQHU" and "9khiedHAIWM,QyXyrau2qsLP"
- Yet, the file data encoding of "file-a" and "file-b" is the same(!)

How does this fit in with the description of externalIVChaining? With
"001", I would expect to see 2 different md5sums. What am I missing?


On Fri, 21 Nov 2014 19:59:23 +0100
Jakob Unterwurzacher <jak...@gm...>
wrote:

> Hi Matei,
> have you taken a look at this?
> https://github.com/vgough/encfs/blob/master/encfs/FileUtils.cpp#L847
> On Nov 21, 2014 6:22 PM, "Matei David"
> <ma...@cs...> wrote:
> 
> > On Thu, 20 Nov 2014 17:47:59 -0800
> > "Mark R. Pariente"
> > <mar...@gm...> wrote:
> >
> > > On Thu, Nov 20, 2014 at 5:28 PM, Matei David
> > > <ma...@cs...> wrote:
> > > > Hi,
> > > >
> > > > I'm interested in keeping two encfs folders in sync. One option
> > > > is to run rsync/unison between the plaintext folders. I would
> > > > like to have the additional option of performing sync on the
> > > > ciphertexts.
> > > >
> > > > >From the tests that I ran, it seems to me that this is only
> > > > >possible if
> > > > uniqueIV is set to 0 in the configuration file. Whenever this is
> > > > set to 1, the ciphertexts of two identical plaintext folders
> > > > seem to be different. To clarify, my tests consisted of running
> > > > this script while tweaking various parameters inside encfs6.xml.
> > > >
> > > > #!/bin/bash -x
> > > > cat encfs6.xml
> > > > rm -rf /tmp/.docs-{1,2} /tmp/docs-{1,2}
> > > > mkdir -p /tmp/.docs-{1,2} /tmp/docs-{1,2}
> > > > echo password |
> > > > ENCFS6_CONFIG=encfs6.xml encfs -S /tmp/.docs-1 /tmp/docs-1
> > > > echo password |
> > > > ENCFS6_CONFIG=encfs6.xml encfs -S /tmp/.docs-2 /tmp/docs-2
> > > > echo "hello" >/tmp/docs-1/a-file
> > > > rsync -a /tmp/docs-1/ /tmp/docs-2/
> > > > md5sum /tmp/.docs-[12]/*
> > > > fusermount -u /tmp/docs-1
> > > > fusermount -u /tmp/docs-2
> > > >
> > > > My question is, are there security considerations why I would
> > > > want to keep uniqueIV set to 1? I checked the manual and the
> > > > guide here
> > > > http://www.ict.griffith.edu.au/anthony/info/crypto/encfs.hints
> > > > Neither mention unique IVs or what they are good for.
> > >
> > > UniqueIV generates a random IV value for each file - that is why
> > > you are seeing the same plaintext/path resulting in different
> > > ciphertext when copied.
> > >
> > > The reasoning for uniqueIV is to prevent statistical attacks -
> > > without it the same plaintext results in the same ciphertext so an
> > > observer can tell how many copies of a file you have, and this is
> > > considered information leakage.
> >
> > Thanks for the prompt reply.
> >
> > From the description in the manual, I would have thought that
> > externalIVChaining would prevent such an attack: I thought 2
> > identical plaintext files would encrypt in different ways depending
> > on their names (or paths, with chainedNameIV). But my understanding
> > is wrong, right? I think I'm mixing up 2 things that are being
> > encrypted: file names, and file contents.
> >
> > Can you explain what externalIVChaining does exactly? I tried to
> > figure it out but I don't see any effect. Here's what I tried:
> >
> > - for every triple of IV-related options:
> >   (uniqueIV, chainedNameIV, externalIVChaining)
> > - create 2 identical plaintext files (file-a, file-b)
> > - copy them to a  directory (dir/file-a dir/file-b)
> > - so in total, there are 4 identical files, with 2 different base
> > names
> > - sync the plaintexts between 2 mounts
> > - count:
> >   - unique file names in ciphertext of 1 mount
> >   - unique file names in ciphertext of both mounts
> >   - unique checksums in ciphertext of 1 mount
> >   - unique checksums in ciphertext of both mounts
> >
> > I'm attaching the script. The results I see are:
> > 000 2 2 1 1
> > 001 2 2 1 1
> > 010 4 4 1 1
> > 011 4 4 1 1
> > 100 2 2 4 8
> > 101 2 2 4 8
> > 110 4 4 4 8
> > 111 4 4 4 8
> >
> > Based on this, I infer that:
> > - uniqueIV affects only file content encryption, not file names
> > - chainedNameIV affects only file name encryption, not file contents
> >
> > What I don't understand is the effect of externalIVChaining. I don't
> > see anything chaning when it is enabled.
> >
> > Thanks,
> > M
> >
> >
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