Chapter 6. Security

Table of Contents
6.1. DES, MD5, and Crypt
6.2. S/Key
6.3. Kerberos
6.4. Firewalls

6.1. DES, MD5, and Crypt

Contributed by Garrett Wollman 24 September 1995.

In order to protect the security of passwords on UN*X systems from being easily exposed, passwords have traditionally been scrambled in some way. Starting with Bell Labs' Seventh Edition Unix, passwords were encrypted using what the security people call a ``one-way hash function''. That is to say, the password is transformed in such a way that the original password cannot be regained except by brute-force searching the space of possible passwords. Unfortunately, the only secure method that was available to the AT&T researchers at the time was based on DES, the Data Encryption Standard. This causes only minimal difficulty for commercial vendors, but is a serious problem for an operating system like FreeBSD where all the source code is freely available, because national governments in many places like to place restrictions on cross-border transport of DES and other encryption software.

So, the FreeBSD team was faced with a dilemma: how could we provide compatibility with all those UNIX systems out there while still not running afoul of the law? We decided to take a dual-track approach: we would make distributions which contained only a non-regulated password scrambler, and then provide as a separate add-on library the DES-based password hash. The password-scrambling function was moved out of the C library to a separate library, called libcrypt because the name of the C function to implement it is crypt. In FreeBSD 1.x and some pre-release 2.0 snapshots, the non-regulated scrambler uses an insecure function written by Nate Williams; in subsequent releases this was replaced by a mechanism using the RSA Data Security, Inc., MD5 one-way hash function. Because neither of these functions involve encryption, they are believed to be exportable from the US and importable into many other countries.

Meanwhile, work was also underway on the DES-based password hash function. First, a version of the crypt function which was written outside the US was imported, thus synchronizing the US and non-US code. Then, the library was modified and split into two; the DES libcrypt contains only the code involved in performing the one-way password hash, and a separate libcipher was created with the entry points to actually perform encryption. The code was partitioned in this way to make it easier to get an export license for the compiled library.

6.1.1. Recognizing your crypt mechanism

It is fairly easy to recognize whether a particular password string was created using the DES- or MD5-based hash function. MD5 password strings always begin with the characters $1$. DES password strings do not have any particular identifying characteristics, but they are shorter than MD5 passwords, and are coded in a 64-character alphabet which does not include the $ character, so a relatively short string which doesn't begin with a dollar sign is very likely a DES password.

Determining which library is being used on your system is fairly easy for most programs, except for those like init which are statically linked. (For those programs, the only way is to try them on a known password and see if it works.) Programs which use crypt are linked against libcrypt, which for each type of library is a symbolic link to the appropriate implementation. For example, on a system using the DES versions:

    % ls -l /usr/lib/libcrypt*

    lrwxr-xr-x  1 root  wheel  13 Mar 19 06:56 libcrypt.a -> libdescrypt.a

    lrwxr-xr-x  1 root  wheel  18 Mar 19 06:56 ->

    lrwxr-xr-x  1 root  wheel  15 Mar 19 06:56 libcrypt_p.a -> libdescrypt_p.a

On a system using the MD5-based libraries, the same links will be present, but the target will be libscrypt rather than libdescrypt.