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<title>MD5: Command Line Message Digest Utility</title>
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<h1><img src="md5.png" width="208" height="109" alt="MD5" /></h1>
<h2>
Command Line Message Digest Utility
</h2>
</center>
<hr />
<p class="j">
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>
<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>&hellip; ]
<h3>DESCRIPTION</h3>
<p class="j">
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>
<p class="j">
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 class="j">
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 <a href="rfc1321.html">RFC 1321</a> in April 1992, and was
placed into the public domain at that time. Message
digest algorithms such as MD5 are not deemed
&ldquo;encryption technology&rdquo; 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 class="j">
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://java.sun.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>
<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.</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.</dd>
<dt><b>-l</b></dt> <dd>Use lower case letters for hexadecimal
digits &ldquo;a&rdquo; through &ldquo;f&rdquo;. 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.</dd>
<dt><b>-n</b></dt> <dd>Suppress printing the file name (or
&ldquo;<tt>-</tt>&rdquo; for standard input) after the hexadecimal
signature.</dd>
<dt><b>-o</b><i>fname</i></dt> <dd>Write output to <i>fname</i>.
If <i>fname</i> is &ldquo;<tt>-</tt>&rdquo;, output is written
to standard output, which is the default is no
<b>-o</b> option is specified.</dd>
<dt><b>-u</b></dt> <dd>Print how-to-call information.</dd>
<dt><b>-v</b></dt> <dd>Print version information.</dd>
</dl>
<h3>FILES</h3>
<p class="j">
If no <em>infile</em> or <b>-d</b> option is specified or
<em>infile</em> is a single &ldquo;<tt>-</tt>&rdquo;, <b>md5</b>
reads from standard input. A single &ldquo;<tt>-</tt>&rdquo; on
the command line causes all subsequent arguments to be treated as
file names even if they begin with &ldquo;<tt>-</tt>&rdquo;. If
no <b>-o</b> option is specified or the <em>fname</em> is a
single &ldquo;<tt>-</tt>&rdquo;, output is sent to standard
output. Input and output are processed strictly serially;
consequently <b>md5</b> may be used in pipelines.
</p>
<h3>BUGS</h3>
<p class="j">
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>
<p class="j">
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>
<h2><img src="/images/icons/file.png" alt="" align="middle" width="40" height="40" />
Download <a href="md5.zip">md5.zip</a> (Zipped)
or <a href="md5.tar.gz">md5.tar.gz</a> (tar/gzip)</h2>
<p class="j">
The program is provided as either <a href="md5.zip">md5.zip</a>, a
<a href="http://www.info-zip.org/" target="_blank">Zipped</a> archive, or
<a href="md5.tar.gz">md5.tar.gz</a>, a <tt>gzip</tt>ped
<tt>tar</tt> archive. The two archive formats have identical
contents; both include a
ready-to-run Win32 command-line executable program, <code>md5.exe</code>
(compiled using Microsoft Visual C++ .NET),
and source code along with a
<code>Makefile</code> to build the program under Unix.
</p>
<h3>SEE ALSO</h3>
<b>sum</b>(1)
<h3>EXIT STATUS</h3>
<p class="j">
<b>md5</b> returns status 0 if processing was
completed without errors, 1 if the <b>-c</b> option was
specified and the given signature does not match that of the input,
and 2 if processing could not be performed at
all due, for example, to a nonexistent input file.
</p>
<h3>COPYING</h3>
<blockquote>
<p class="j">
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 &ldquo;as is&rdquo;
without express or implied warranty.
</p>
</blockquote>
<h3>ACKNOWLEDGEMENTS</h3>
<p class="j">
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.
</p>
<h3><a href="/">Fourmilab home page</a></h3>
<hr />
<address>
<a href="/">by John Walker</a><br />
January 14th, 2008
</address>
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