Public key Steganography?

[Ray Arachelian <sunder@amanda.dorsai.org>]

The following are bits and pieces of a message I've sent to someone 
inquiring about stego...  It brought back the old idea I had about 
expanding WNS to do public key....  Would something like this be 
feasable?  the beginning of this is a description of what WNS does, 
followed by the public key idea...

---------- Forwarded message ----------

<<I am researching Steganography and related topics of a security course at 
George Mason University.  As part of my research, I am reviewing PC 
products that provide such services.  White Noise Storm (tm) which you 
developed in one such product.  I am interested in the kind of research 
you did to develop it (publications, readings, inspiration).  If you 
could point me to any documents I would be grateful.>>

Hi there,

Anyway to answer your questions, the idea of WNS came to me from the idea of
spread spectrum technology and frequency hopping, however, I added several
twists to it in order to improve security.  Instead of just having X channels
of communication which are multiplexed or changed with a fixed formula and
passkey, I have eight channels which are spread within a number of 8 bits*W
bytes channels.  W here represents a random sized window of W bytes.  Each of
these eight channels represents one single bit out of a whole byte, so each
Window holds one byte plus a bunch of unused bits.  These channels rotate
amongs themselves, for instance bit 1 might be swapped with bit 7, or all the
bits may rotate positions at once.  Further, these bits change location
within the window on the byte level.  The rules for these swaps (similar to
the substitution boxes in other cyphers) are dictated not only by your
passphrase - which is unlimited in length, but also by the previous window's
random data.   

This means that if you encrypt message M with key K two times, you get two
different cyphertexts, C.  C1 and C2 may be different, however, decrypted
with key K, both will yield the original message M.  This makes cryptanalysis
very hard because the same message will give you different cyphertexts each
time, thus making known cyphertext atacks useless.  (Of course if you don't
use random numbers, but rather the LSB's in an image, the same passphrase and
the same window size, you'll get the same cyphertext each time you run it -
which is one of the reasons you should destroy the original picture after
injecting it.  More about this later.)

Since the window size changes with each window, the bits both rotate among
themselves, and within the data window, it is close to impossible for an
attacker to guess where there is cyphertext data versus where there is random
data, which bit in that cyphertext represents what bit value, and in which
byte it is located in. 

The unused bits are actually random data when WNS is used by itself generated
by either a hardware random number generator, or by your compiler's random
functions which addmitedly aren't all that great.  However, if you are using
WNS for steganography, that random data is actually the least significant
bits of your original file, be it sound, or a picture or whatever.  An
injector is included for the PCX format because that is a format I am most
familiar with, however you may write your own for other media.

The reason I'm using live data out of the file you want to hide a message in
is so that the cyphertext produced by WNS closely matches the natural, native
LSB's of that file.  Also, WNS, when used with a large enough maximum window
size will give you a very secure stego channel since it will be very hard to
dected that 1 in say, 500 bits has been altered.  That's a bit extreme and
will waste a lot of data, but for small messages stored in very large
complex, noisy files, will look very much like the original image.

Another thing to throw off the attacker is that statistical fixing.  If WNS
writes a 1 bit over a 0 bit of an LSB file, it will look for an unused 1 bit
to write a zero over.  This balances the distribution of 0 and 1 bits to
match that of the original file as closely as possible, so that you cannot
use statistical analysis to even detect the presence of steganography, much
less break it.

Additionally, WNS's schema of steganography does give you a very strong
advantage: your recipient does not need to have the original picture or sound
in order to extract the hidden message.  There is a huge concern about this
since if, as some other algorithms require, your recipient needed to have the
original picture, you can bet that the bad guys would also have it and
compare the two and also be able to extract the message.  It is a good idea
for nobody to have the original image, not even you.  That is, scan something
in, inject your message in it, and destroy the original.   If the bad guys
search your hard drive and find the original picture, they can compare it to
the one you've sent which contains the data, thus they are able to find out
which bits WNS has changed, and possibly analyze the algorithm to see which
passphrases would generate such bits, extract the passphrase and then the
message.

With the original pre-WNS210 version, it was possible to break an encrypted
message by decoding it repeatedly with the same password while changing one
bit at a time in a window so as to find out which bits WNS wrote and which
were random, however this brute force method is a lot worse than most brute
force methods because given a stream of bits, and an unlimited password size
you have numbits^numpasswords combinations to work with - a very huge space
size for bruting, even for small messages.

However, this version uses not only the previous window's plain-text, but the
current window's unused random data bits to form the next window, that is in
a sense the algorithm is recursive.  (Not that the encryption function calls
itself, but rather it uses the previous window to form the next.)

Keep in mind that no-one, to my knowledge has done a differential
cryptanalysis or any other formal attempt to break this code.  It is possible
that the boys at the NSA would attempt this if they thought it was a threat
to them, but I haven't seen anyone report any weaknesses in this algorithm.
 This is why I suggest that whatever you send with WNS be compressed and then
encrypted with something like PGP first.  That extra bit of entropy generated
by first compressing, and then by encrypting with RSA and IDEA will increase
the entropy in your cyphertext even if the LSB's from the host picture aren't
all that random.

Related subjects from which I was inspired to write WNS: random numbers, the
sliding window protocols such as Zmodem which change their window size
depending on how noisy the channel is - WNS changes the window size to
improve security, not to help against noise - in fact, if you change a single
byte in a WNS cyphertext, the rest of the message gets garbled completly.
 Spread spectrum and channel hopping radio communications, finite state
machine automatons, steganography and stealth in general, etc.

At the time I wrote this, I did not have any papers I used to write this
code, only my own knowledge of the above topics, things I learned in school
about automatons and spread spectrum, the Zmodem protocol description of DSZ,
etc.

I would like to sometime in the future expand WNS to use a public key system
instead of a symmetric key, but I haven't yet found a feasable way to do
this.  As is, you'd have to know your recipient's private key in order to
encrypt something to send them.  If you do, it beats the whole point of
public key encryption.  If you use their public key, everyone has access to
it, so it's a kind of a catch 22 here.

Using their public key as the inital WNS key, then using an RSA encrypted
block to send the session key for WNS will produce the RSA encrypted block -
while this is fairly strong, RSA might have some tell-tale signs that would
give away the presence of steganography in the message?  I'm not sure how
insecure or secure this would be, but so far it is the only idea I have.

This would basically work like this:

  1. pick a randomly generated session key for the IDEA cypher, call this K1
  2. pick a randomly generated session key for the 2nd part of the WNS
session, call this K2.
  3. Use the recipient's public key Pub for an RSA or Diffie-Hillman session
key exchange.
  4. Encrypt Sessionkey=RSA(K1+K2,Pub)
  5. Encrypt Sessionblock=WNS(Sessionkey,randomnumbers,Pub) and merge with
random numbers generating a random sized block.
  6. First encrypt blocks of your plaintext with idea using
cyphertext1=IDEA(plaintextblock,K1)
  7. Then encrypt and write cyphertext2=WNS(cyphertext1,randomnumbers,K2) to
the output file.
  8. Go to 6 until no more data to write.

To decrypt ( you are the recipient):

  1.  Set the session key to your public key Pub.
  2.  Decrypt the start of the stegoed picture using WNS and your public key
to get K1 and K2:
       K1+K2=WNSDecrypt(cyphertext2,Pub) -> here cyphertext2 comes from the
file
  3.  Decrypt next block of code using cyphertext1=WNSDecrypt(cyphertext2,K2)
  4.  Decrypt the cyphertext1 with IDEA
  plaintext=IDEADecrypt(cyphertext1,K1)
  5.  Goto 3 until no more data.

Problem is, how much does it weaken WNS to use your recipient's RSA's public
key as the initial WNS key not only in terms of security, but also in terms
of allowing the attacker to detect the presence of stego'ed data in your host
image?