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<H1>The SNOW steganography program</H1>
<P>
<H3>Synopsis</H3>
<DL>
<DD> <B>snow</B> [ <B>-CQS</B> ] [ <B>-p</B> <I>passwd</I> ]
[ <B>-l</B> <I>line-len</I> ] [ <B>-f</B> <I>file</I> | <B>-m</B>
<I>message</I> ] [ <I>infile</I> [ <I>outfile</I> ]]
</DL>
<P>
<H3>Description</H3>
<B>snow</B> is a program for concealing messages in text files by
appending tabs and spaces on the end of lines, and for extracting
messages from files containing hidden messages. Tabs and spaces are
invisible to most text viewers, hence the steganographic nature of this
encoding scheme.
<P>
The data is concealed in the text file by appending sequences of up
to 7 spaces, interspersed with tabs. This usually allows 3 bits
to be stored every 8 columns. An alternative encoding scheme, using
alternating spaces and tabs to represent zeroes and ones, was rejected
because, although it used fewer bytes, it required more columns per bit
(4.5 vs 2.67).
<P>
The start of the data is indicated by an appended tab character, which
allows the insertion of mail and news headers without corrupting
the data.
<P>
<B>snow</B> provides rudimentary compression, using Huffman tables
optimised for English text. However, if the data is not text, or if
there is a lot of data, the use of the built-in compression is not
recommended, since an external compression program such as
<B>compress</B> or <B>gzip</B> will do a much better job.
<P>
Encryption is also provided, using the
<A HREF="../ice/Welcome.html">ICE</A> encryption algorithm
in 1-bit cipher-feedback (CFB) mode. Because of ICE's arbitrary
key size, passwords of any length up to 1170 characters are supported
(since only 7 bits of each character are used, keys up to 1024-bytes
are supported).
<P>
If a message string or message file are specified on the command-line,
<B>snow</B> will attempt to conceal the message in the file <I>infile</I>
if specified, or standard input otherwise. The resulting file will be
written to <I>outfile</I> if specified, or standard output if not.
<P>
If no message string is provided, <B>snow</B> attempts to extract a
message from the input file. The result is written to the output file
or standard output.
<P>
<H3>Options</H3>
<DL>
<DT> <B>-C</B>
<DD> Compress the data if concealing, or uncompress it if extracting.
<P>
<DT> <B>-Q</B>
<DD> Quiet mode. If not set, the program reports statistics such as
compression percentages and amount of available storage space used.
<P>
<DT> <B>-S</B>
<DD> Report on the approximate amount of space available for hidden
message in the text file. Line length is taken into account, but
other options are ignored.
<P>
<DT> <B>-p</B> <I>password</I>
<DD> If this is set, the data will be encrypted with this password during
concealment, or decrypted during extraction.
<P>
<DT> <B>-l</B> <I>line-length</I>
<DD> When appending whitespace, <B>snow</B> will always produce lines
shorter than this value. By default it is set to 80.
<P>
<DT> <B>-f</B> <I>message-file</I>
<DD> The contents of this file will be concealed in the input text file.
<P>
<DT> <B>-m</B> <I>message-string</I>
<DD> The contents of this string will be concealed in the input text file.
Note that, unless a newline is somehow included in the string, a newline
will not be printed when the message is extracted.
</DL>
<P>
<H3>Examples</H3>
The following command will conceal the message "I am lying" in the
file <I>infile</I>, with compression, and encrypted with the password
"hello world". The resulting text will be stored in <I>outfile</I>.
<P>
<DL>
<DD> <CODE> snow -C -m "I am lying" -p "hello world" infile outfile </CODE>
</DL>
<P>
To extract the message, the command would be
<P>
<DL>
<DD> <CODE> snow -C -p "hello world" outfile </CODE>
</DL>
<P>
Note that the resulting message will not be terminated by a newline.
<P>
To prevent line wrap if text with concealed whitespace is likely to
be indented by mail or news readers, a line length of 72 or less can
be used.
<P>
<DL>
<DD> <CODE> snow -C -l 72 -m "I am lying" infile outfile </CODE>
</DL>
<P>
The approximate storage capacity of a file can be determined with
the <B>-S</B> option.
<P>
<DL>
<DD> <CODE> snow -S -l 72 infile </CODE>
</DL>
<P>
<HR>
<FONT SIZE=-1>
Document written by Matthew Kwan, 29 December 1996 <BR>
Please send any comments or corrections to
<A HREF="mailto:mkwan@cs.mu.oz.au">mkwan@cs.mu.oz.au</A>
</FONT>
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