
		Stealth V1.1 by Henry Hastur
		----------------------------

Stealth is a simple filter for PGP which strips off all identifying header
information to leave only the encrypted data in a format suitable for
steganographic use. That is, the data can be hidden in images, audio
files, text files, CAD files, and/or any other file type that may contain
random data, then sent to another person who can retrieve the data from 
the file, attach headers, and PGP decrypt it.

Stealth is not intended to replace the standardised methods of using
encryption (e.g. ASCII-armoured PGP email) ; in an ideal world we would
all be able to send openly encrypted mail or files to each other with no 
fear of reprisals, however there are often cases when this is not possible,
either because the local government does not approve of encrypted 
communication, or perhaps because you are working for a company that
does not allow encrypted email but doesn't care about Mandelbrot
GIFs. This is where Stealth and steganography can come into play.


Compiling
---------

Stealth has currently only been tested on BSD and SVR4 Unix (and as
such should work with most varieties of Unix), with both non-ANSI
compilers and ANSI compilers with 'minimal ANSI' flags. In order to
compile the program, you should just be able to extract the files
from the tar file provided, then type 'make'. If that fails you may
need to change the definition of CC and CFLAGS in the makefile to
get it to compile. On machines with gcc, the GNU C compiler, Stealth
can be compiled by simply changing the 'CC=cc' line in makefile to
'CC=gcc'.

Stealth can be compiled on MS-DOS using the Microsoft C compiler with
the following command line :

cl /DDOS stealth.c

That's all there is to it - the compiler will output a STEALTH.EXE
ready for use. Other compilers should work, but you may need to change
header files and the code at the beginning of main() which sets the
mode of stdin and stdout to binary.

Usage
-----

Stealth always reads from its standard input and writes to the standard
output, though when adding headers to data the data has to be stored in a
temporary file (see Security Concerns below).

Command line arguments :

	-c 	Conventional encryption used rather than public key
	-a	Add headers (defaults to strip headers)
	-v	Verbose output.

The -a argument takes a string specifying the key id to put into the
header. This can be specified either as an identifying name (e.g.
foo.bar@snafu.com), or as a 24-bit key id as given by the pgp -kv
command, prefixed by '0x' (See examples below). The latter has been
added for MS-DOS users who can only pass a single word to the program
for a key name.

Stealth needs to be able to find your pubring.pgp file, which it does
by first checking in the directory pointed to by $PGPPATH, then the
current directory.
 

Examples
--------

To encrypt a file with PGP and store it in the file pgp.stl prior to sending :

pgp -ef < secrets.dat | stealth > pgp.stl

To encrypt a file with conventional (IDEA) encryption, and pass to a
steganography program called steg_program :

pgp -fec < secrets.dat | stealth -c | steg_program

To take the output from a steganographic extraction tool, add headers
for key "Your Id", and decrypt :

steg_program | stealth -a "Your Id" | pgp -f > secrets.dat

[ Note : this use of " marks will only work on Unix, not MS-DOS. If
  you have to specify a key with more than one word, you will probably
  have to specify the key id instead ]

To take the conventionally encrypted output from a steg program, attach
headers and decrypt :

steg_program | stealth -ac | pgp -f > secrets.dat

To take the output from a steganography program, add  the headers
for a key whose 24-bit id is 23ffff, and decrypt :

steg_program | stealth -a 0x23ffff | pgp -f > secrets.dat


Limitations
-----------

Files can be signed, but can only be encrypted to one recipient - extra
RSA headers for all but the first recipient will be stripped from the
file. In addition, if you specify conventional encryption but pass an
RSA-encrypted file into the filter the RSA-block will be stripped. In
either case, stealth will print out warnings to inform you of this.

Stealth provides no support for ASCII-armoured PGP messages - it will
only work with the binary output format, and the output will have to
be converted to a useable form after processing, either with a
steganography program or a standard utility such as uuencode.

Finally, for technical reasons there are potential problems with public
keys of size (typically) 8*n + 1 or 8*n + 2 (e.g. 513 or 1026). If you
are encrypting to a key of a peculiar size, it's possible that the algorithm
used to add headers could fail, but fortunately this can be detected while
stripping the headers, and a warning will be printed. If this warning appears,
you will probably want to encrypt the data again until a suitably sized
RSA-block is created.

It is NOT neccesary to remove garbage data that the steganography program
may have added to the end of the PGP-encrypted data. PGP output contains
an encrypted end-of-file mark that allows the program to decrypt correctly
and ignore any trailing garbage.


Security Concerns
-----------------

After passing through the stealth filter, the PGP-encrypted data is
essentially white noise, with no identifying marks, and whilst it may
well have enough peculiarities for an expert cryptanalyst to recognize
it as encrypted data, the probability is much less than would be the
case with a PGP header identifying the recipient attached.

One other concern is that stealth has to create a temporary file when
reading in data to attach headers, and depending on the build options
chosen the program will store it in either $PGPPATH, the current directory
or /tmp. On Unix machines, the file will be deleted as soon as it is opened,
making it difficult to capture, but on other operating systems the file
will only be deleted when it has been used. (In either case the file will
be zeroed before being closed).

In addition, some operating systems will use temporary files on your disk
to emulate unix pipes (e.g. MS-DOS) - these files will not be zeroed when
finished with !


Export Restrictions
-------------------

Stealth is probably not covered by current export restrictions under the
US ITAR regs, but I'm not a lawyer, so if in doubt check it out yourself.
It was written outside the US and imported, so should soon be available
on some European ftp sites as well as US sites.


						Henry Hastur


