P1: JDW
WL040C-01 WL040/Bidgoli-Vol III-Ch-01 June 24, 2003 10:39 Char Count= 0
PASSWORDLENGTH ANDHUMANMEMORY 9every short alphanumeric password in a few minutes and
every possible keyboard password (except for special ALT-
characters) within a few days. Some security administra-
tors have dealt with this problem by requiring stronger
and stronger passwords; however, this comes at a cost (see
An Argument for Simplified Passwords).
In addition to implementing policies that require users
to choose strong passwords, the CERT Coordination
Center provides guidelines for securing passwords on
Windows systems (CERT, 2002):Using SYSKEY enables the private password data stored
in the registry to be encrypted using a 128-bit crypto-
graphic key. This is a unique key for each system.
By default, the administrator account is never locked out;
so it is generally a target for brute force logon attempts
of intruders. It is possible to rename the account in
User Manager, but it may be desirable to lock out the
administrator account after a set number of failed at-
tempts over the network. The NT Resource Kit provides
an application calledpassprop.exethat enables Ad-
ministrator account lockout except for interactive lo-
gons on a domain controller.
Another alternative that avoids all accounts belonging to
the Administrator group being locked over the network
is to create a local account that belongs to the Admin-
istrator group, but is not allowed to log on over the
network. This account may then be used at the console
to unlock the other accounts.
The Guest account should be disabled. If this account is
enabled, anonymous connections can be made to NT
computers.
The Emergency Repair Disk should be secured, as it con-
tains a copy of the entire SAM database. If a malicious
user has access to the disk, he or she may be able to
launch a crack attack against it.Password-Cracking Times
Let us start with a typical password of six characters.
When this password is entered into a system’s authen-
tication mechanism, the system hashes it and stores the
hashed value. The hash, a fixed-sized string derived from
some arbitrarily long string of text, is generated by a for-
mula in such a way that it is extremely unlikely that other
texts will produce the same hash value—unlikely, but not
impossible. Because passwords are not arbitrarily long—
they are generally 4 to 12 characters—this reduces the
search space for finding a matching hash. In other words,
an attacker’s password-cracking program does not need
to calculate every possible combination of six-character
passwords. It only needs to find a hash of a six-character
ASCII-printable password that matches the hash stored
in the password file or sniffed off the network.
Because an attacker cannot try to guess passwords at
a high rate through the standard user interface (as men-
tioned earlier, the time to enter them is prohibitive, and
most systems can be configured to lock the user out af-
ter too many wrong attempts), one may assume that the
attacker will get them either by capturing the system pass-
word file or by sniffing (monitoring communications) ona network segment. Each character in a password is a
byte. One does not typically need to consider characters
with a leading zero in the highest-order bit, because print-
able ASCII characters are in codes 32 through 126. ASCII
codes 0–31 and 127 are unprintable characters, and 128–
255 are special ALT-characters that are not generally used
for passwords. This leaves 95 printable ASCII characters.
If there are 95 possible choices for each of the six
password characters, this makes the password space
956 =735, 091, 890, 625 combinations. Modern comput-
ers are capable of making more than 10 billion calcula-
tions per second. It has been conjectured that agencies
such as the NSA have password-cracking machines (or
several machines working in parallel) that could hash and
check passwords at a rate of 1 billion per second. How fast
could an attacker check every possible combination of
six-character passwords? 735,091,890,625/1,000,000,000
=about 12 minutes (see Table 2).
What if the system forces everyone to use a seven-
character password? Then it would take the attacker 19
hours to brute-force every possible password. Many Win-
dows networks fall under this category. Due to the LAN
Manager issue, passwords on these systems cannot be
much stronger than seven characters. Thus, it can be as-
sumed that any password sent on a Windows system us-
ing LAN Manager can be cracked within a day. What if
the system enforces eight-character passwords? Then it
would take 77 days to brute-force them all. If a system’s
standard policy is to require users to change passwords
every 90 days, this may not be sufficient.PASSWORD LENGTH AND
HUMAN MEMORY
Choosing a longer password does not help much on sys-
tems with limitations such as the LAN Manager hash is-
sue. It also does not help if a password is susceptible to a
dictionary or hybrid attack. It only works if the password
appears to be a random string of symbols, but that can
be difficult to remember. A classic study by psychologist
George Miller showed that humans work best with the
magic number 7 (plus or minus 2). So it stands to reason
that once a password exceeds nine characters, the user is
going to have a hard time remembering it (Miller, 1956).
Here is one idea for remembering a longer password.
Security professionals generally advise people never to
write down their passwords. But the user could write
down half of it—the part that looks like random letters
and numbers—and keep it in a wallet or desk drawer. The
other part could be memorized—perhaps it could be a
misspelled dictionary word or the initials for an acquain-
tance, or something similarly memorable. When concate-
nated together, the resulting password could be much
longer than nine characters, and therefore presumably
stronger.
Some researchers have asserted that the brain remem-
bers images more easily than letters or numbers. Thus,
some new schemes use sequences of graphical symbols
for passwords. For example, a system called PassFace, de-
veloped by RealUser, replaces the letters and numbers in
passwords with sequences or groups of human faces. It