Re:really undetectable crypto made somewhat practical

[Steve Reid <steve@edmweb.com>]

I'm on FCPUNX instead of regular Cypherpunks, so please excuse me if I'm a
little behind the thread. 

> New Scheme:  First, calculate the MD5 hash of all the words in the various  
> dictionary files used by the password cracker program and create a  
> database containing every word and the first 4 bits of its MD5 hash.   
> Given such a database, it would be possible to write a program that  
> accepts as input a block of cyphertext (the stego message, encrypted),  
> chunks it up in to groups of 4 bits and then, for each chunk, displays the  
> words that have hashes that start with those same four bits.  The person  
> running the program would select words that form meaningful sentences but  
> also produce hashes that combine into the encrypted stego message.  This  
> scheme would send 4 stego bits per word.

As a slight improvement, you could turn this into a complete stealth
encryption scheme, using only the one-way hash function operating as a
MAC. 

Instead of hashing just the word in an effort to get stego bits, you could
hash a key along with the word. In order to get the intended hash you
would need to know the key. Since you're probably hashing a whole block of
512 bits (or whatever's specified in the algorithm) appending a key should
not affect the speed of the system. I'm certain that this would increase
the security, possibly enough that you wouldn't need to use a regular
encryption algorithm (but I wouldn't bet on it). 

Crude example:
Assume that Alice can use the words "Greetings" and "Salutations"
interchangably without drawing suspicion. Also assume that "PASSWD" is a
secret known only to Alice and Bob, and that the stego software looks at
the low bit of an MD5 hash. 

MD5 ("GreetingsPASSWD")   = c7bf6e051731a0dcf52baa330c9d2e7d  <- low bit=1
MD5 ("SalutationsPASSWD") = 2dd2ba080b5feb060ffbc6d196fd1b34  <- low bit=0

If you say "Greetings" you're sending a 1, if you say "Salutations" 
you're sending a 0. Eve doesn't know about "PASSWD" so she can't do the
hash and figure the bit. Of course, if you're using this to send more
bits, you'll need something harder to guess than "PASSWD". 

The trick is in figuring out which words have stego bits and which don't. 
It might be better to stego bits into a whole line instead of a word, as
that would probably offer more flexibility. 


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