SPECIFICATION

SPECIFICATION file for stes
===========================

By Philip Hunt, last altered 21-Jul-2000.

This file describes some of the design goals behind the stes encryption
system.

Stes is a steganographic encryption system designed to be used at the
Unix prompt (it could be ported to other OSes without much difficulty).
The best way to explain what it will do is through examples.

Stes will enable a particular cyphertext to be decrypted into two or 
more different plaintexts, depending on which key is used. To be more 
precise:

   $ stes --create ctf hello myletter.txt goodbye apicture.jpg

This creates cypher text file <ctf> which contains an encrypted form of
the data in the plaintext files <myletter.txt> and <apicture.jpg>. The
data in <myletter.txt> is accessible by the key "hello", the data in
<apicture.jpg> by the key "goodbye".

Note that if you have the file <ctf>, there is no way of knowing how
many different files are encrypted inside it; Cyphertext files will
contain 'padding' so you can't infer contents by length, either.

   $ stes --decode ctf goodbye xxx.jpg

This looks in the file <ctf> and finds out whether there is any data
in it using the key "goodbye". if there is data, it is put into the
file <xxx.jpg>. (in this example, there is data, i.e. the contents of 
file <apicture.jpg>. (Note that stes doesn't know what a jpg file is, 
or anything else about file types))

   $ stes --alter ctf hello anotherfile.doc

This looks in the file <ctf> and finds if there is any data for the
key "hello". In this example, there is, so the old data stored under
the key "hello" is lost, and replaced with the new data from
<anotherfile.doc>. Note that if <ctf> didn't already have the key 
"hello", it could not be added; keys can only be put into a cyphertext 
file when it is created.
          
If <anotherfile.doc> is too big for the room stes has set aside for
the key "hello", then it is tough luck because stes cannot store
it in <ctf>. 

However, stes has a way round that: basically you will be able to 
designate one or more keys to hold mostly empty space, then you can use 
the "alter" function to reallocate that space to other keys.

I.e:

   $ stes --create ctf k1 pt1 k2 pt2 k3 pt3 -empty 100k k4 pt4 -empty 500k

Creates <ctf> with 4 keys (k1..k4). k3 holds <pt3> plus at least 100k
of spare room. k4 holds <pt4> plus at least 500k of spare room.

Now,

   $ stes --alter ctf k2 newpt2

will throw out <pt2> from <ctf>, and attempt to replace it with <newpt2>.
If <newpt2> is too big, this operation will fail. (k2) is able to hold at
least the size of <pt2>; it might be able to hold more.

But, when we created <ctf>, we ensured that it had at least 600KB of
empty space. So we can say:

   $ stes --altermk ctf k2 newpt2 k3

"--altermk" means alter using multiple keys.

Here, stes knows about k3's blocks, and can use k3's free space to put
part of <newpt2> in. So if size(newpt2) <= size(pt2)+100KB, the operation
will defintaly succeed.

Want more space? Just do:

   $ stes --altermk ctf k2 newpt2 k3 k4
            
and all of k3 and k4's reserve space (>= 600KB) can be used to store 
newpt2. 



;end.