TCP/IP Stego (was CU-SeeMe)

[JonWienke at aol.com]

>A tcp header contains quite a bit of useful information.. but most of it 
>wouldnt be easily manipulated (by me) to get a bit. You cant very well 
>alternate the port number you are using, or change the packet sequence 
>numbers. However, the tcp protocol forces each tcp header to contain a 
>'checksum' field.. which is (more or less) the sum of all the octets in 
>the packet. This is used as error correction for the protocol. It 
>shouldn't be too difficult to force TCP to fiddle with the data in the 
>packet a bit to force the checksum to be a particular sort of thing (i.e. 
>even or odd number) .. 0 for even and 1 for odd would get us our one 
>bit of data per packet. 

This is a bad idea, because in addition to the extra processor overhead, it
is an incredible waste of bandwidth.  For a 512 byte packet, you are only
getting .02% efficiency, because you wouldn't be able to use the actual data
in the packet; otherwise someone would probably notice the increased error
rate if you dink around with the checksum.  This does not provide adequate
plausible deniability.  Stegoing a 16 bit WAV file gives you 50% efficiency,
(you can replace the 8 least significant bits with stego data and still keep
10 bits worth of sound quality) and someone else would be unable to prove
there was any encrypted info in it if you did it right.

Jonathan Wienke