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<title>MD5: Command Line Message Digest Utility</title>
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<h1><img src="md5.gif" width="208" height="109" alt="MD5"></h1>
<h2> Command Line Message Digest Utility </h2>
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<p> </p>
<div class="bodycopy"> This page describes <b>md5</b>, a command line
utility usable on either Unix or MS-DOS/Windows, which generates and
verifies message digests (digital signatures) using the MD5 algorithm.
This program can be useful when developing shell scripts or Perl
programs for software installation, file comparison, and detection of
file corruption and tampering.
<p> </p>
<h3>NAME</h3>
<b>md5</b> - generate / check MD5 message digest
<h3>SYNOPSIS</h3>
<b>md5</b> [ <b>-c</b><i>signature</i> ] [ <b>-l</b> ] [ <b>-n</b>
] [ <b>-u</b> ] [ <b>-v</b> ] [ <b>-d</b><i>input_text</i>
| <em>infile</em>... ]
<h3>DESCRIPTION</h3>
A <em>message digest</em> is a compact digital signature for an
arbitrarily long stream of binary data. An ideal message digest
algorithm would never generate the same signature for two different sets
of input, but achieving such theoretical perfection would require a
message digest as long as the input file. Practical message digest
algorithms compromise in favour of a digital signature of modest size
created with an algorithm designed to make preparation of input text
with a given signature computationally infeasible. Message digest
algorithms have much in common with techniques used in encryption, but
to a different end; verification that data have not been altered since
the signature was published.
<p> Many older programs requiring digital signatures employed 16 or 32
bit <em>cyclical redundancy codes</em> (CRC) originally developed to
verify correct transmission in data communication protocols, but these
short codes, while adequate to detect the kind of transmission errors
for which they were intended, are insufficiently secure for
applications such as electronic commerce and verification of security
related software distributions. </p>
<p> The most commonly used present-day message digest algorithm is the
128 bit MD5 algorithm, developed by Ron Rivest of the <a
href="http://web.mit.edu/" target="_blank">MIT</a> <a
href="http://www.lcs.mit.edu/" target="_blank">Laboratory for Computer
Science</a> and <a href="http://www.rsa.com/" target="_blank">RSA Data
Security, Inc.</a> The algorithm, with a reference implementation, was
published as Internet RFC 1321 in April 1992, and was placed into the
public domain at that time. Message digest algorithms such as MD5 are
not deemed "encryption technology" and are not subject to the export
controls some governments impose on other data security products.
(Obviously, the responsibility for obeying the laws in the jurisdiction
in which you reside is entirely your own, but many common Web and Mail
utilities use MD5, and I am unaware of any restrictions on their
distribution and use.) </p>
<p> The MD5 algorithm has been implemented in numerous computer
languages including C, <a href="http://www.perl.org/" target="_blank">Perl</a>,
and <a href="http://www.javasoft.com/" target="_blank">Java</a>; if
you're writing a program in such a language, track down a suitable
subroutine and incorporate it into your program. The program described
on this page is a <em>command line</em> implementation of MD5, intended
for use in shell scripts and Perl programs (it is much faster than
computing an MD5 signature directly in Perl). This <b>md5</b> program
was originally developed as part of a suite of tools intended to
monitor large collections of files (for example, the contents of a Web
site) to detect corruption of files and inadvertent (or perhaps
malicious) changes. That task is now best accomplished with more
comprehensive packages such as <a href="http://www.tripwire.com/"
target="_blank">Tripwire</a>, but the command line <b>md5</b>
component continues to prove useful for verifying correct delivery and
installation of software packages, comparing the contents of two
different systems, and checking for changes in specific files. </p>
<h3>OPTIONS</h3>
<dl compact="compact">
<dt><b>-c</b><i>signature</i> </dt>
<dd>Computes the signature of the specified <em>infile</em> or
the string supplied by the <b>-d</b> option and compares it against the
specified <i>signature</i>. If the two signatures match, the exit
status will be zero, otherwise the exit status will be 1. No
signature is written; only the exit status is set. The
signature to be checked must be specified as 32 hexadecimal digits.
<p> </p>
</dd>
<dt><b>-d</b><i>input_text</i> </dt>
<dd>A signature is computed for the given <i>input_text</i>
(which must be quoted if it contains white space characters) instead
of input from <i>infile</i> or standard input. If input is
specified with the <b>-d</b> option, no <i>infile</i> should be
specified.
<p> </p>
</dd>
<dt><b>-l</b> </dt>
<dd>Use lower case letters for hexadecimal digits "a" through
"f". By default, upper case letters are used. Note that the <i>signature</i>
argument to the <b>-c</b> option may use upper or lower case
hexadecimal digits (or a mix) regardless of the setting of this
option.
<p> </p>
</dd>
<dt><b>-n</b> </dt>
<dd>Suppress printing the file name (or "<tt>-</tt>" for standard
input) after the hexadecimal signature.
<p> </p>
</dd>
<dt><b>-o</b><i>fname</i> </dt>
<dd>Write output to <i>fname</i>. If <i>fname</i> is "<tt>-</tt>",
output is written to standard output, which is the default is no <b>-o</b>
option is specified.
<p> </p>
</dd>
<dt><b>-u</b> </dt>
<dd>Print how-to-call information.
<p> </p>
</dd>
<dt><b>-v</b> </dt>
<dd>Print version information. </dd>
</dl>
<h3>FILES</h3>
If no <em>infile</em> or <b>-d</b> option is specified or <em>infile</em>
is a single "-", <b>md5</b> reads from standard input. A single
"-" on the command line causes all subsequent arguments to be
treated as file names even if they begin with "-". If no <b>-o</b>
option is specified or the <em>fname</em> is a single "-", output
is sent to standard output. Input and output are processed
strictly serially; consequently <b>md5</b> may be used in
pipelines.
<h3>BUGS</h3>
The mechanism used to set standard input to binary mode may be specific
to Microsoft C; if you rebuild the DOS/Windows version of the program
from source using another compiler, be sure to verify binary files work
properly when read via redirection or a pipe.
<p> This program has not been tested on a machine on which <tt>int</tt>
and/or <tt>long</tt> are longer than 32 bits. </p>
<h3>COPYING</h3>
<blockquote> This software is in the public domain. Permission to use,
copy, modify, and distribute this software and its documentation
for any purpose and without fee is hereby granted, without any
conditions or restrictions. This software is provided "as is"
without express or implied warranty. </blockquote>
<h3>ACKNOWLEDGEMENTS</h3>
The MD5 algorithm was developed by Ron Rivest. The public domain C
language implementation used in this program was written by Colin Plumb
in 1993. </div>
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