81 lines
4.1 KiB
Plaintext
81 lines
4.1 KiB
Plaintext
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<!-- $Id$ -->
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<!-- The FreeBSD Documentation Project -->
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<sect><heading>DES, MD5, and Crypt<label id="crypt"></heading>
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<p><em>Contributed by &a.wollman;<newline>24 September 1995.</em>
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<p><bf>History</bf>
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<p>In order to protect the security of passwords on UN*X systems from
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being easily exposed, passwords have traditionally been scrambled in
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some way. Starting with Bell Labs' Seventh Edition Unix, passwords
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were encrypted using what the security people call a ``one-way hash
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function''. That is to say, the password is transformed in such a way
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that the original password cannot be regained except by brute-force
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searching the space of possible passwords. Unfortunately, the only
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secure method that was available to the AT&T researchers at the
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time was based on DES, the Data Encryption Standard. This causes only
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minimal difficulty for commercial vendors, but is a serious problem
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for an operating system like FreeBSD where all the source code is
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freely available, because national governments in many places like to
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place restrictions on cross-border transport of DES and other
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encryption software.
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<p>So, the FreeBSD team was faced with a dilemma: how could we provide
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compatibility with all those UNIX systems out there while still not
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running afoul of the law? We decided to take a dual-track approach:
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we would make distributions which contained only a non-regulated
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password scrambler, and then provide as a separate add-on library the
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DES-based password hash. The password-scrambling function was moved
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out of the C library to a separate library, called `<tt>libcrypt</tt>'
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because the name of the C function to implement it is
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`<tt>crypt</tt>'. In FreeBSD 1.x and some pre-release 2.0 snapshots,
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the non-regulated scrambler uses an insecure function written by Nate
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Williams; in subsequent releases this was replaced by a mechanism
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using the RSA Data Security, Inc., MD5 one-way hash function. Because
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neither of these functions involve encryption, they are believed to be
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exportable from the US and importable into many other countries.
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<p>Meanwhile, work was also underway on the DES-based password hash
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function. First, a version of the `<tt>crypt</tt>' function which was
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written outside the US was imported, thus synchronizing the US and
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non-US code. Then, the library was modified and split into two; the
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DES `<tt>libcrypt</tt>' contains only the code involved in performing
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the one-way password hash, and a separate `<tt>libcipher</tt>' was
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created with the entry points to actually perform encryption. The
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code was partitioned in this way to make it easier to get an export
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license for the compiled library.
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<p><bf>Recognizing your `<tt>crypt</tt>' mechanism</bf>
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<p>It is fairly easy to recognize whether a particular password
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string was created using the DES- or MD5-based hash function.
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MD5 password strings always begin with the characters
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`<tt>$1$</tt>'. DES password strings do not have
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any particular identifying characteristics, but they are shorter
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than MD5 passwords, and are coded in a 64-character alphabet
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which does not include the `<tt>$</tt>' character, so a
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relatively short string which doesn't begin with a dollar sign is
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very likely a DES password.
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<p>Determining which library is being used on your system is fairly
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easy for most programs, except for those like `<tt>init</tt>' which
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are statically linked. (For those programs, the only way is to try
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them on a known password and see if it works.) Programs which use
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`<tt>crypt</tt>' are linked against `<tt>libcrypt</tt>', which for
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each type of library is a symbolic link to the appropriate
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implementation. For example, on a system using the DES versions:
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<tscreen><verb>
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$ cd /usr/lib
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$ ls -l /usr/lib/libcrypt*
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lrwxr-xr-x 1 bin bin 13 Sep 5 12:50 libcrypt.a -> libdescrypt.a
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lrwxr-xr-x 1 bin bin 18 Sep 5 12:50 libcrypt.so.2.0 -> libdescrypt.so.2.0
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lrwxr-xr-x 1 bin bin 15 Sep 5 12:50 libcrypt_p.a -> libdescrypt_p.a
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</verb></tscreen>
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On a system using the MD5-based libraries, the same links will be
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present, but the target will be `<tt>libscrypt</tt>' rather than
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`<tt>libdescrypt</tt>'.
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