hide4pgp.doc

把信息隐藏到图片中

as if you would copy <file 1> to <file 2>. Likewise, use

  UnHide <file 1> <file 2> [options ...]

to extract (the secret data in) <file 1> to <file 2>. Any options MUST
follow behind the filenames (unlike with Hide4PGP itself, where options
may appear anywhere on the command line).


3 - Technical

Hide4PGP works only on real data in the data file and leaves unused
space untouched. This allows the data file to be converted into other
picture or sound formats and leave the data intact.

Hide4PGP now spreads the secret data evenly over the whole multimedia
file. With real quantitative data it is able to use more than one bit
per data point. It has built-in limits how many bits it may use: with
palettized SVGA BMP files 1 bit, with true grey-scale BMP files 2 bits
and with true color BMP files 6 bits per pixel (2 bits per channel),
with VOC and 8 bit WAV files 1 bit and with 12 or 16 bit WAV files 4
bit per sample. The limits should guarantee that no change of the
multimedia file contents will be evident to the normal viewer or
listener. The new format is incompatible to the versions before 2.0:
you need version 1.1 to recover hidden data stored with Hide4PGP 1.x.

256 colors bitmaps often have only 100 or less colors. The remaining
palette entries are still present but unused. Hide4PGP may use some
of them as partners for used colors whose first assigned partner
lacks enough similarity. For this purpose the color in question is
copied to the unused entry, resulting in two (nearly) identical
entries; only 'nearly' because partner colors always differ in the
least significant bit of one channel. Graphic programs or converters
that rearrange the palette will respect the small difference, but
scanning programs may search for those nearly identical entries.
Hide4PGP uses unused entries only on rare occasions, but this also may
be suppressed with the '-nu' switch.

Finding a suitable partner for each of the entries of a 256 color
palette needs a measure of similarity. As distances in the RGB
colorspace do not correlate very good with the human perception I
decided to convert the RGB coordinates into the CIE L*a*b* coordinates
as their distances give a much better correlation. Conversion formulas
for the proposed sRGB standard by HP/Microsoft are used, taking also
in account the nonlinearity of the typical monitor.

The DOS executable has been compiled using the Microsoft Quick C 2.5
Compiler in the compact model (code near, data far), the 32 bit OS/2
code is produced by IBM's C Set with a single thread and static
linkage for the 386 target processor, and the Win32 console
application was built using the lcc-win32/wedit C IDE by Jacob Navia,
available at http://www.cs.virginia.edu/~lcc-win32. The Linux
executable has been compiled with gcc (egcs-2.91.66 and glibc 2.0/2.1)
with help of Matthias Dort <[e-mail]> and a Makefile
provided mainly by Thomas Hertweck <[e-mail]>. If you
wonder why the Win32 executable is so small compared to the DOS and
OS/2 ones: the reason is that the Win32 systems have a standard C
library already included: crtdll.dll holds all standard c functions
(and some non standard ones too), so practically no libraries have to
be statically linked.


4 - Source Code

This program is written in ANSI C with the following exceptions:

On non UNIX systems the stdin/stdout streams are usually working in
text mode, i.e. they translate 0x0A to 0x0D 0x0A and vice versa. This
destroys the encrypted message when Hide4PGP reads and writes through
these channels. In ANSI C there is no way to switch the mode of the
already redirected stdin or stdout to binary operation without
loosing the redirection. The SAA 2 extension allows the use of the
freopen() function with an empty string as the first parameter
(redirection target). This worked with the IBM C Set Compiler, but not
with the MS Quick C I used for the DOS version or lcc for Win32. The
documentation of the Quick C recommends the use of the low level
setmode() function, which in turn doesn't work with the ICC. The
functionality is essential in the Hide4PGP.C module, so I decided to
implement both possibilities and use a preprocessor switch to activate
one of them. You can activate the freopen() method by defining the
macro USE_FREOPEN, and the setmode() alternative likewise by defining
USE_SETMODE. By default no constant is defined and the compiler will
abort with an error message on non UNIX systems. The macro definition
should be added to the compiler's options (e.g. /DUSE_FREOPEN). So
take a look at your compiler's specification if you can use one of
these possibilities. If not, you would have to substitute this line
with own code (in the middle of main() at the bottom of Hide4PGP.C).
On UNIX systems the UNIX macro has to be defined to avoid the error
abort.

The low level fstat and utime are used to reset the file modification
time to the pre-write state. It is assumed that the struct utimbuf is
defined and has the same types in the same order as the components
st_atime and st_mtime of struct stat. Otherwise additionally a
variable of type struct utimbuf would have to be defined and the
values assigned for each component.

The header file Hide4PGP.H defines types for several integer sizes.
They should work for most compilers provided that:
   char is 8 bit
   short int is 16 bit
   long int is 32 bit
If this is not true for you, you only will have to replace the defini-
tions in the header file with appropriate types for your compiler.

The program assumes that all integers are stored in the little-endian
format, which is true for all intel prozessors. For big-endian
machines there may be a compiler switch to simulate little-endian
behaviour. If your system only writes or reads 16 or 32 bit values in
the order MSB first, you will have to change all appearances of
'fread' and 'fwrite' with more than 8 bit datasize too.

The Makefiles provided are for MS Quick C 2.5 for DOS, for the IBM
C Set 2.1 for OS/2, for Jacob Navia's lcc-win32 version 3.1 and for
gcc under Linux and may have to be be changed for other compilers.
Generally spoken you need all *.c and the *.h file to compile to *.o
or similar and to be linked to one executable Hide4PGP(.EXE).

Please send me an email if you successfully ported this program to
another operating system. My email address is [e-mail].
This is also the address for bug reports, recommendations or improve-
ments!


5 - Other sources

Information about the structure of WAV and VOC files I found in an
article by Kai Schwirtzke in c't 1/93, p. 213. The BMP Windows format
was described in a file named BMP.DOC in one CompuServe graphics forum
- thanks to the unknown author. Infos about the OS/2 BMP structures I
found in the OS/2 header files of the compiler - and by looking at the
BMP files themselves. Information about the CIE colorspaces and
conversion formulas I found in the Color spaces FAQ by David Bourgin
(attention: the conversion formula XYZ -> Lab for L* is faulty!)  and
an article by Michael Haas and Todd Newman of the International Color
Consortium, 'Color Management: Current Practice and The Adoption of a
New Standard'. Some of the algorithms are modified from Robert
Sedgewick's 'Algorithms.'


6 - History

v1.0  first public release (Dec 20, 1996)

v1.1  second public release (May 22, 1997)
      - allows now any number of bits from 1 to 8
      - added OS/2 32 bit executable and makefile
      - increased portability with much more strict typing
      - minor change in split_pair routine (BMP.C)
      - fixed: out of memory error handler
      - removed a former second pairing method
      - removed the former -d '=' modifier
      - changed the former -v- option to -q (Quiet)

v2.0  third public release (Feb 10, 2000)
      - new: 'spyder'-derived method for palettized bitmaps
      - removed: former duplicating and pairing method
      - removed: -d and -p options
      - new: -u/-nu option (unused palette entries)
      - new: -nc option (suppress color adjustments)
      - removed: -# (number of bits) option
      - new: spreads secret data over whole multimedia file
      - new: saves and restores exact length
      - new: scrambles data before hiding
      - changed: completely new format, incompatible to versions 1.x
      - new: resets modification timestamp of multimedia file
      - new: -nf option (suppress timestamp faking)
      - new: optional filename for secret data instead of streaming
      - changed: return codes (now all defined in Hide4PGP.H)


7 - Legal Stuff

You may freely use this software in any environment. You may make
copies and freely distribute this software provided the archive
contains all files of the original distribution and is not modified in
any way. You may distribute it in any Freeware / Shareware / PD
collection provided the charge for the data carrier is only nominal
and covers only the media and copying costs. You may not sell this
program or include it into a for profit product without written
consent of the author. You may compile the source on any other
computer system. You may change the source code and distribute the
modified version only if you clearly state all differences, leave the
copyright notice intact and inform the author.

THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT WARRANTY OF ANY KIND.
UNDER NO CIRCUMSTANCES SHALL THE AUTHOR BE LIABLE FOR ANY INCIDENTAL,
SPECIAL OR CONSEQUENTIAL DAMAGES (INCLUDING DAMAGES FOR LOSS OF
BUSINESS PROFITS, BUSINESS INTERRUPTION, LOSS OF BUSINESS INFORMATION
AND THE LIKE) ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE
OR ITS DOCUMENTATION.