Password Checkers/Crackers

The power of password crackers is over hyped. In real life unless you manage to steal the shadow file from some server, it's impossible to apply cracker to the system. But in order to read shadow file you usually need to be root or to have a physical access to the server (and a possibility to reboot it). In both cases it's unclear why one need to crack passwords on the server that as already penetrated, unless the goal is to reuse them for attacking other servers.

The primary legitimate purpose of Unix password crackers is to detect and eliminate weak user passwords but even this task is dubious: if system is configured to disable account, say for an hour after three unsuccessful attempts, then even weak password can withstand most of the attacks. But still due diligence is not a bad strategy.

It's important to understand that "realistic" cracking scenario requires access to the password hashes and a very fast computer. In most modern Unix systems (UP-UX is a notable exception) shadow files that contain password hashes are readable only by the root user. In a way introduction of shadow files devalued the importance of crackers. Still if hashes can be intercepted (for example if NIS is used) crackers can be used against them.

Length of password is very important: longer passwords usually cannot be cracked in a reasonable time. Brute force attack on password longer then 8 characters is rarely successful, especially if AOL scheme of generating long password using two short words with one of several delimiters between them. That's why it makes sense to enforce practices that help to create stronger passwords and first of all to limit the minimal password length to at least 8, This is especially important for root passwords.

For the same reason it is important to mix upper and lower cases in password: that's double alphabet size and for the 8 character passwords makes brute force attack approximately 256 times longer (2^8). That's why enforcing usage of at least one digit, mixed case and delimiters in the password is so important for root passwords. But again unless hashes are stolen brute force attack is impossible with modern limitation on the number of failed attempts. So it is an interesting theme for theoretical discussion not so much for practice.

But this terrain should be navigated with extreme caution. It is very easy to go overboard with this and excessive zeal backfires. Naive Unix administrators who after reading some superficial security book try to enforce a Draconian schemes of unsuspecting users (random passwords or too complex password schemes) usually weaken the security due to unanticipated side effects of such actions.

If you're looking for a way to recover or bypass a lost password, it's usually easier just change it or ask your system administrator to do so. If it's the administrator (root) password that is lost, there's almost always a way to bypass OS security by booting from CD and change it after mounting the filesystem (the exact details are depends on Unix flavor. For Solaris the procedure can be found at Solaris root password recovery.) The most complex situation is if the archive (zip or rar) or Microsoft document or spreadsheet is encrypted and password is lost. Here you need to work two ways:

Old Unixes use DEC-based hashing and password are limited to just 8 characters, which is a major disadvantage. Better, MD5-based hashes that permits passwords longer then 8 characters originated in FreeBSD are now also used in Linux and Solaris 10. In this case it is strongly recommended that users switched to "AOL scheme": two short words connected with some delimiter (:, -, /, etc). Like faKe43-secuR55.

Old News ;-)

The pam_cracklib Module The pam_cracklib module is intended to work only with the password module type. It's purpose is to check a password for strength and for length, both elements being configurable with arguments described below. This module functions only in a stack, since it has no updating capabilities. It requires the libcrack library and the cracklib_dict Crack dictionary, both of which are found in /usr/lib of the Red Hat 5.2/6.0 distribution. As you can see, this module depends heavily on elements of the Crack utility, which is discussed in Chapter 12.

The flexibility of PAM is evidenced by the fact that this is not the only password strength checking PAM module. Another is pam_passwd+, which is available at

http://www.us.kernel.org/pub/linux/libs/pam/modules.html

The arguments available to pam_cracklib are described in Table 5.5.

Table 5.5 Arguments for pam_cracklib

Argument Description

debug This argument writes additional module behavior information to syslog, but does not log passwords.

type=STRING This argument replaces the string UNIX with STRING in the messages it generates, such as New UNIX password:.

retry=n This is the number of retries this module allows a user when changing a password. The default is 1.

difok=n This represents the number of characters in the new password that must be different from the old password. The default is 10. Regardless of this limit, however, any new password that has at least half the characters different from the old will be accepted.

minlen=n This argument specifies the minimum password length + 1. By default it is set to 9 which means the minimum password length is actually 10. To further confuse the issue, this minimum length may actually be reduced depending upon the values specified for the *credit parameters listed below.

lcredit=n The value specified here is the number of characters by which the minlen value is reduced by virtue of having at least one lowercase character in the new password. The default is 1. Can be set to 0 to eliminate the credit.

ucredit=n The value specified here is the number of characters by which the minlen value is reduced by virtue of having at least one uppercase character in the new password. The default is 1. Can be set to 0 to eliminate the credit.

dcredit=n The value specified here is the number of characters by which the minlen value is reduced by virtue of having at least one numeric character in the new password. The default is 1. Can be set to 0 to eliminate the credit.

ocredit=n The value specified here is the number of characters by which the minlen value is reduced by virtue of having at least one nonalpha-numeric character in the new password. The default is 1. Can be set to 0 to eliminate the credit.

In addition to the configurable options in Table 5.5, the pam_cracklib checks the new password for strength by

Verifying that the new password is not the reverse of the old password.

Verifying that the new password is not a simple case change of some characters of the old password.

Checking if the new password is in the cracklib_dict. If it is, it warns the user but does not force another password choice.

The Effect of Stacking pam_pwdb and pam_cracklib For Module Type password In this section, we will take a look at how pam_cracklib interacts with pam_pwdb in the stack shown in Example 5-3 on page 86.

Recall that in Example 5-3 the two stacked entries appeared in /etc/ pam.d/passwd.
Password  required  /lib/security/pam_cracklib.so retry=3
Password  required  /lib/security/pam_pwdb.so use_authtok
The first entry invokes pam_cracklib and prompts the user for his or her new password (remember that the auth module type pam_pwdb entry is responsible for prompting the user for his or her old password, for authentication). After the user has supplied the new password, pam_cracklib requests that it be repeated for verification. Once completed, pam_cracklib performs its checks to see if the password is acceptable. If so, it passes the new password to pam_pwdb which has the use_authtok argument meaning it will accept this new password and request the pwdb library to update the appropriate database.

Let's take a look at the power and flexibility of these modules by considering an example. Suppose that we would like to use md5 instead of the standard UNIX crypt(3) mechanism for hashing purposes. This is a good idea, because popular password-guessing tools like Crack require significantly more CPU resources to guess passwords (see The White Hat Use of Crack on page 337). The major benefit of using md5 is that you can require longer passwords—20, 30, or even more characters. Let's look at an example requiring 20-character passwords. We'll also set the type argument to see if our users are paying attention. Example 5-5 shows what the stack might look like if we impose these changes in /etc/pam.d/passwd.

Example 5-5 Using md5 and minlen in /etc/pam.d/passwd
Password  required  /lib/security/pam_cracklib.so minlen=20\
     retry=3 type=SECRET
password  required  /lib/security/pam_pwdb.so md5 use_authtok
WARNING

If you make changes similar to what is shown in Example 5-5, you must also change all equivalent instances of pam_pwdb and pam_cracklib using module type password. In Red Hat 5.2, this would minimally include the files chfn, chsh, login, rlogin, su, and xdm in /etc/pam.d.

Now that we have made these changes, let's see what happens to the user, mary, when she tries to change her password in Example 5-6. She is offered three opportunities to select a password. This is due to the retry=3argument to pam_cracklib (see Example 5-5 on page 91). Actually, it appears that Mary is attempting to make good password choices. Unfortunately she doesn't know about the changes to PAM and therefore doesn't know that she needs to choose a longer password. So you, being the responsive administrator, inform her that she needs to use a 20-character password. "What?!" she replies. And you gently tell her that she can use a passphrase. Happy now, she goes about her task (Example 5-7).

Example 5-6 Unsuccessful Password Change
$passwd
Changing password for mary
(current)UNIX password:j3n#Ky
New SECRET password:Rt!72g
BAD PASSWORD:is too simple
New SECRET password:8x@$iI
BAD PASSWORD:is too simple
New SECRET password:P5-+yh
BAD PASSWORD:is too simple
New SECRET password:8x@$iI
passwd:Authentication token manipulation error
$
Example 5-7 Successful Password Change
$passwd
Changing password for mary
(current)UNIX password:j3n#Ky
New SECRET password:I need a #%$3+raise
Retype new SECRET password:I need a #%$3+raise
passwd:all authentication tokens updated successfully
$
Notice that the message displayed by pam_cracklib contains our type entry, New SECRET password:. This change does not appear in the message from pam_pwdb—(current) UNIX password:—because pam_pwdb does not support the type argument.

News	See Also	Recommended Links	Papers	Reference
npasswd	PAM-based checkers	Crack	John the Ripper	Humor	Etc

Password Checkers/Crackers

Old News ;-)

Table 5.5 Arguments for `pam_cracklib`

Example 5-5 Using `md5` and `minlen` in `/etc/pam.d/passwd`

Example 5-6 Unsuccessful Password Change

Example 5-7 Successful Password Change

Recommended Links

Npasswd

Crack

John the Ripper

Papers

Etc

Argument	Description
`debug`	This argument writes additional module behavior information to `syslog`, but does not log passwords.
`type=STRING`	This argument replaces the string `UNIX` with `STRING` in the messages it generates, such as `New UNIX password:`.
`retry=n`	This is the number of retries this module allows a user when changing a password. The default is 1.
`difok=n`	This represents the number of characters in the new password that must be different from the old password. The default is 10. Regardless of this limit, however, any new password that has at least half the characters different from the old will be accepted.
`minlen=n`	This argument specifies the minimum password length + 1. By default it is set to 9 which means the minimum password length is actually 10. To further confuse the issue, this minimum length may actually be reduced depending upon the values specified for the `*credit` parameters listed below.
`lcredit=n`	The value specified here is the number of characters by which the `minlen` value is reduced by virtue of having at least one lowercase character in the new password. The default is 1. Can be set to 0 to eliminate the credit.
`ucredit=n`	The value specified here is the number of characters by which the `minlen` value is reduced by virtue of having at least one uppercase character in the new password. The default is 1. Can be set to 0 to eliminate the credit.
`dcredit=n`	The value specified here is the number of characters by which the `minlen` value is reduced by virtue of having at least one numeric character in the new password. The default is 1. Can be set to 0 to eliminate the credit.
`ocredit=n`	The value specified here is the number of characters by which the `minlen` value is reduced by virtue of having at least one nonalpha-numeric character in the new password. The default is 1. Can be set to 0 to eliminate the credit.

Password Checkers/Crackers

Old News ;-)

Table 5.5 Arguments for pam_cracklib

Example 5-5 Using md5 and minlen in /etc/pam.d/passwd

Example 5-6 Unsuccessful Password Change

Example 5-7 Successful Password Change

Recommended Links

Npasswd

Crack

John the Ripper

Papers

Etc

Table 5.5 Arguments for `pam_cracklib`

Example 5-5 Using `md5` and `minlen` in `/etc/pam.d/passwd`