Re: How do I know if its encrypted?

At 12:40 AM 01/12/95, Dale Harrison (AEGIS wrote:
>Won't work!  You can always embed an encrypted message in what 'looks'
>like plaintext.  A trivial example: Encrypt a message with a caesar
>cypher, then build a story where the first char of each word maps to
>each subsequent char from the encrypted text.  At the cost of expanding
>the size of the message by a factor of 5 to 10 you've hidden the
>encrypted message in what looks like a letter to your mother (or a news
>story in the NY Times, etc.)  This is old technique.

The context this was being discussed in, was trying to make _plaintext_
look like _ciphertext_. The operator of a data haven or remailer might
hypothetically want to ensure that all text he dealt with was encrypted.
So your method wouldn't do anything in that area.  Unless you can think of
a way to embed plaintext in ciphertext in such a  way that it looks like
ciphertext, and my guess is that any method that did that well would be
sufficiently obscure as to be analagous to encryption for our purposes.
Really bad encryption, with very little point, but still hidden text. Which
is the real point, the operator doesn't want to deal with any text  that
isn't "hidden" in some way.

Of course, we're not just dealing with text.  So the scheme has got to be
changed a bit so as to be able to detect unencrypted GIFs, and mu-law
files, and as yet to be determined unknown files.  I don't know enough
about what's being talked about to know if this entropy detecting stuff
will generalize to non text files. Cause we want to catch unencrypted GIFs
too.  [And doesn't compression alone do similar things to the entropy as
encryption does, anyhow? If someone compresses their file with a good
compression algorithm, as I understand it the non-randomness left will be
pretty low. But it won't meet the needs we're discussing, I don't think.]

Of course, we're not just dealing with text.  So the scheme has got to be
   changed a bit so as to be able to detect unencrypted GIFs, and mu-law
   files, and as yet to be determined unknown files.  

Each of these data formats has it's own regex recognizers available.
Just apply them.

The point, though, is to enforce the presumption that the remailer
operator does not, in fact, look at the traffic in order to understand
the content.  You don't need a completely airtight algorithm in order
to do this.

Eric

On Thu, 12 Jan 1995, Jonathan Rochkind wrote:
 
> Of course, we're not just dealing with text.  So the scheme has got to be
> changed a bit so as to be able to detect unencrypted GIFs, and mu-law
> files, and as yet to be determined unknown files.  I don't know enough
> about what's being talked about to know if this entropy detecting stuff
> will generalize to non text files.

Graphics files are already compressed, so they pass the entropy 
test, but they start with a distinctive header.

The best way to stop graphics would be a volume limit per 
apparent source and per apparent destination.

To program that is a bit like hard work.

 ---------------------------------------------------------------------
We have the right to defend ourselves and our       
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are.  True law derives from this right, not from    James A. Donald
the arbitrary power of the omnipotent state.        jamesd@netcom.com

Jonathan Rochkind wrote:

> The context this was being discussed in, was trying to make _plaintext_
> look like _ciphertext_. The operator of a data haven or remailer might
> hypothetically want to ensure that all text he dealt with was encrypted.
> So your method wouldn't do anything in that area.  Unless you can think of
> a way to embed plaintext in ciphertext in such a  way that it looks like
> ciphertext, and my guess is that any method that did that well would be

As I said in a recent message, there is no general way to "prove" that
a file is encrypted, only to have pretty good confidence that it is.
(This is at the core of algorithmic information theory, a la
Kolmogorov and Chaitin, goes to the heart of what is meant by
"randomness," and is linked as well to the halting problem and other
such stuff.)

I had a need to actually do what Jonathan is talking about: make a
file look like it was encrypted but actually have a simple text
message in it.

My application was an experiment to bait the line for some thought
policemen who decided that they would decide which pictures were
approprate and which were not. So, I created a plausible-looking
PGP-like file and posted it to the new group
"alt.binaries.pictures.erotica.children" and announce that an
"interesting" picture existed there.

The squeals of the net.cops were impressive to behold! Demands to
Netcom that I be expelled from the Net, that the "Child Welfare
Agents" would soon be breaking down my doors, etc. 

But, given the climate of our time (this was in July 1993) and given
the potential failure of the "It's not a _real_ file" defense, I
protected myself in any easy way, but running an English message down
the diagonal, saying something like "This is not a real encrypted file," etc.
Even a lawyer would have to admit that no real encrypted file could
have English emerging randomly. Entropy, and all that.

So, this was an ostensibly encrypted file which contained unencrypted
text. It would very likely have passed any tests for "randomness"
and hence would have been passed by any "encrypted only" filters. (The
English text was a tiny fraction of the entire file, so the deviation
from near-maximal entropy would likely go undetected. Fluctuations
would be larger, depending on file size.)

Nobody spotted the message. After several days, I "let the truth be
told," which of course enraged others. 

I though this digression might be amusing to some.

--Tim May


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