UNIX Password Security
December 6, 1993
This document was written to make system administrators aware of the
importance of well-chosen passwords. Easy-to-guess passwords offer
hackers the possibility to enter a system. (`Crackers' is a better
term, because `hackers' is historically reserved for people who take
the most out of their computer because of their great knowledge of
soft- or hardware. I will continue to use the term `hacker' because
this is common in literature.) More and more computers are being
connected to the world-wide Internet (the latest estimations speak of
about 1.5 billion systems). This means there will be more and more
users, and therefore more and more hackers. By means of good
password-security, one can protect a system from newbie hackers.
There are many types of systems, and every one has a lot of security
aspects to it. I will restrict myself to the password-security of UNIX
systems. The reason being that UNIX systems are very popular,
especially in an educational environment, where one can expect an
increased concentration of hackers due to the openness that is
appreciated in such environments. This is in contrast to a commercial
environment, where data has to be protected. E.g. against competitors.
There are many ways to hack a UNIX system, and there are many programs
for finding a user's password. These programs can be used by people who
have little knowledge of UNIX. Choosing good passwords can therefore
help in keeping newbie hackers out. (`Advanced hackers' are often
capable of entering a system without using passwords. This implies
that the security of a system depends not solely on well-chosen
Besides the importance of good passwords (i.e. not easily guessable) we
will take a look at how they work. Next we will see an actual example
which shows how bad passwords are usually chosen. To conclude I will
give several methods for chosing good passwords.
2. The importance of good passwords
The goal of a hacker is usually obtaining the superuser-status (`root').
The normal strategy to do this is using badly installed software, bugs
in (system)software and human errors. There are several ways to hack a
computer, but most ways require extensive knowledge. A (relatively) easy
way is logging in as a normal user and searching the system for bugs to
become superuser. To do this, the hacker will have to have a valid
usercode/password combination to start with.
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It is of utmost importance that all (!) users on a system choose a
password that is not easy to guess. The security of each individual
user is closely related to the security of the whole system. Users
often have no idea how a multi-user system works and don't realise that
they, by chosing an easy to remember password, indirectly make it
possible for an outsider to manipulate the entire system. It is
essential to educate the users well to avoid attitudes like the one
described in [Muf]: "It doesn't matter what password I use on _MY_
account, after all, I only use it for laserprinting...". The users have
to get involved with the security of the system they are working on. In
[Pet] we read "Users have a responsibility to employ available security
mechanisms and procedures for protecting their own data. They also have
a responsibility for assisting in the protection of the systems they
use". It also says that it is important to notify the users of the
security guidelines. A solution might be giving new users a limited
course. Or at least make them understand why good passwords are
essential. This can be done e.g. when a user gets his or her initial
password from the system administrator.
3. How a hacker finds a password
Most UNIX systems don't use the so-called shadow passwordfiles which we
will look at in a moment. In most cases, the passwords are stored
encrypted in the file /etc/passwd, or, if the system is a client, on
the server. In the latter case, one can get the passwordfile by giving
the command `ypcat passwd'.
A line from the passwordfile looks like this:
account:coded password data:uid:gid:GCOS-field:homedir:shell
A user with account gigawalt, crypted password fURfuu4.4hY0U, userid
129 (a user with userid 0 (of which there can be more than one) is
superuser), groupid 129, information (GCOS) Walter Belgers,
homedirectory /home/gigawalt and shell /bin/csh will have an entry in
/etc/passwd like this:
Things are a bit different when a shadow passwordfile is being used.
All coded password data is replaced with a `*' in /etc/passwd, and a
second file (the shadowfile) is used to store the original data. This
file can only be read with sufficient privileges so that normal users
cannot get the crypted passwords.
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Passwords are being crypted using the DES algorithm. DES (`Data
Encryption Standard') is an American encryption standard since 1979.
With DES it is possible to encrypt and decrypt data using a key. UNIX
password encryption uses the DES algorithm 25 times in a row. The first
DES round uses 64 0-bits as input and encrypts them with the password
the user inputs, with a permutation taking place during the encryption
process. There are 4096 possible permutations. The permutation used is
randomly chosen for each user. The chosen permutation is coded into two
bytes called `salt'. The salt is stored in the passwordfile. The output
is used as input for the next DES round, which uses the same key and
permutation. This process repeats until there is a final output from
the 25th DES round. This output is coded into eleven bytes which are
put in the passwordfile. So the coded password data in the passwordfile
consists of thirteen bytes, first the salt and then the encrypted
This method of encryption is almost irreversible, which means that it
is easy to encrypt a string, but it is impossible to find the original
of a string encrypted in the way described above, except by
systematically trying out all possible keys and salts. (By impossible
we mean here that there is no known method that does this quicker.) It
is however possible to find the original of a string encrypted using
_single_ DES. For more information concerning single DES encryption see
[Til]. If it's impossible to decrypt the encrypted password, how can a
user log in? This works as follows: the user inputs his or her password
which is used as key to crypt 64 0-bits using the aforementioned
method, using the salt as it can be found in the passwordfile for that
particular user. If the output corresponds to the eleven bytes that
represent the crypted password in the passwordfile the password is
considered valid and the user will be permitted to access the system.
For more information on the precise working of DES on UNIX see [Fel2].
Schematically it looks like this:
| 25x DES | <---------------vv
64 0-bits ----> | encryption | ---- fURfuu4.4hY0U ------
---------------- | (from /etc/passwd) |
^ | |
| (key) -------------> Rfuu4.4hY0U -----|
| (result) |
inputted password |
--> correspondance: password
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As I just pointed out it is not possible (in real life) to decrypt
passwords. However, it is possible to encrypt 64 0-bits with some
words and see if the result `incidentally' is the password. Then the
account is hacked. One could remark that it must be possible to check
all possible passwords this way. However, this would take the fastest
computer longer than the time the universe exists. By trying out only
passwords consisting of six lowercase characters, it becomes possible
to try out all combinations in reasonable time, provided one has an
extremely powerful computer. The latest record for passwords consisting
of six lowercase characters stands at one hour per user. Passwords of
accounts that are attractive to hackers (the ones with a lot of
privileges) should therefore never consist solely of lowercase
We see that the usual way to find passwords is by guessing them. So you
have to make sure users do not use easy-to-guess passwords, i.e.
passwords that can be found in lists (a dictionary, an encyclopedia,
files with astronomical terms, flora and fauna, etc.). It is very easy
to obtain such lists via the Internet.
Suppose a line from the passwordfile looks like this:
Then passwords that should NOT be used are e.g.:
. all English words and derivatives (so `laugh', `laughs', `laughing',
. all words from a foreign language (it is easy to get foreign
. words that can be found in the passwordfile itself like Walter,
Belgers, gigawalt, etc.
. patterns like 123456, qwerty, etc.
. geographical names
. words from an encyclopedia (`Socrates')
. the license plate of a car, the roomnumber, the phonenumber or other
things that have something to do with the owner of the account
. given names
. variations of these (walter, WALTER, retlaw, Walter, wAlter, walter0,
walt3r, Retlaw4,...) Also take into account doubling words or adding a
4. An actual example
To show how users choose their passwords badly, I used a password
guessing program on a passwordfile of a system in operation.
The program I used was Crack v4.1 with ufcrypt (ultra-fast crypt, a fast
implementation of the DES algorithm) on a network of SUN ELC computers.
The performance of these computers (20 MIPS) is comparable to that of a
modern PC. The program was stopped before it was finished after almost
60 hours. The passwords that were found were found within the first 25
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Type of machines: 11x SUN ELC
Total number of accounts: 521
Number of hacked accounts: 58 (11.1%) (with interactive shell 56 (10.7%))
Total time 59:13 (real time, not CPU time)
1 lists 42 (7.2%)
2 common names 1 (0.2%)
3 user/account name 5 (0.9%)
4 phrases and patterns 3 (0.5%)
5 women's names 2 (0.3%)
6 men's names 4 (0.7%)
7 cities 1 (0.2%)
. cyclades, paardens, fiesta, regen, gnosis, police, fuselier, ballon,
smaragd, marques, farao, kasteel, valent, adagio, clematis,
gehannes, koeien, gnomen, onderkin, zeilboot, druppel, fietsen,
testen, marathon, tamtam, global, vrijheid, wolf, kwiek, basket,
stones, klomp9, fiets9, Zoutje, Biefstuk, neenee, tnbrg (this is
`tonbrug' without vowels).
. guest had password guest. This is not a user's fault of course, but
the fault of the system administrator. It has to be seen whether or
not logging in as `guest' with password `guest' is criminal in the
. qwerty, unesco.
. heather, joanne.
. piet, atilla, Frans2, vatsug (this is `gustav' spelled backwards).
Some people have studied the amount of passwords that is easily
guessable in the past. In [Kle] Daniel Klein finds 21% of 15,000
passwords using one week CPU time. The first 2.7% was found within 15
minutes (people who used their account as password, e.g. account
gigawalt, with password gigawalt). The categories in which over 1% of
the 15,000 passwords were found are:
common names 4,0%
user/account name 2,7%
phrases and patterns 1,8%
women's names 1,2%
men's names 1,0%
Comparing these results to the ones above is of little use because of
the limited extent of this investigation.
A somewhat more extensive research (see [Far]) was concerned with
passwordfiles of several .COM systems (computers owned by US
companies). One would expect companies to have good security
measurements, but the passwords kept coming in, with the first
root-password (!) after little more than an hour. (There was a total of
1594 passwords of which 50 had been guessed within 15 minutes, and 90
after 35 minutes.)
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5. Picking good passwords
The above illustrates the importance of a good password for every
user. We will look at some methods for choosing good passwords. A good
password consists of 8 characters (a UNIX password can be up to eight
characters, any extra characters will be discarded, making the
passwords `Still won't talk, eh, Spiff?' and `Still wo' mutually
interchangable). It has to be hard to guess but easy to remember,
because otherwise, users will be tempted to write down their password
which totally takes away the function of a it.
Pick a password that not only consists of just upper- or lowercase
characters, or only one capital (`seCret' is thus a bad password). It
is preferable to use a non-alfanumeric character in the password
(%,=,*, etc.). The use of control characters is possible, but not all
control characters can be used, and it can give rise to problems with
some networking protocols.
A few methods:
. Concatenate two words that together consist of seven characters and
that have no connection to eachother. Concatenate them with a
punctuation mark in the middle and convert some characters to
uppercase. Examples: `Pit+idEa', `plOVer#me'.
. Use the first characters of the words of a certain (not too common)
sentence. When we use the sentence `My goldfish are called
Justerini and Brooks!' as example, we would get the password
`MgacJaB!'. (Also in this case make sure you use an eight-character
password with uppercase characters and/or punctuation marks.)
. Alternately pick a consonant and one or two vowels resulting in a
pronouncable (and therefore easy to remember) word. Examples:
6. Reducing break-in possibilities
It is important for users to have hard to guess but in the meantime easy
to remember passwords. There are methods for generating such passwords.
System operators should inform the users about the importance of good
To reduce the risk of a break-in there are several possibilities:
. Make sure the users know why a good password is important and how
they can choose one.
. Install a new /bin/passwd (or yppasswd) that checks whether the
password is not too obvious (by checking if it contains punctuation
marks, or by investigating if the password can be found in standard
. Install a shadow passwordfile (this involves changing some software).
. Let passwords expire, for example after three months for regular
users, after a month for users with extra privileges. The timespan a
password lives should not be chosen too small. What will still exist
is the danger of users using series of passwords, like `Secret1',
`Secret2',... making it easy for a hacker to, once he has obtained
a password, guess the successor.
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. Use a program that hacks passwords to check if some users have
guessable passwords. Let those users visit you personally to inform
them about the fact that a good password is everyone's concern.
. Switch to single-use passwords (this is radical and requires some
investments, see [Ven]).
. Use passwords of accounts with privileges like that of root on the
console only to avoid eavesdropping the network. When impossible,
try to avoid logging in on such accounts from computers or terminals
that are connected to a LAN segment on which people can easily
and/or anonymously wiretap the network, like classrooms.
. Keep in mind that total system security is as weak as the weakest
chain. Also keep in mind that a system with good passwords alone is
not yet a secure system.
[Bel] Walter Belgers, Password Security - A Case Study (in Dutch),
TimeWasters Online Magazine #5, march 9, 1993, can be obtained by
sending email with Subject `TOM5' to
[Cur] David A. Curry, UNIX System Security, Addison-Wesley, 1992.
[Far] Dan Farmer, Wietse Venema, Improving the Security of Your Site by
Breaking Into it, USENET newsgroup comp.security.unix, can be
obtained via ftp from ftp.win.tue.nl as
[Fel1] David C. Feldmeier, A High-Speed Software DES Implementation, can
be obtained by anonymous ftp from thumper.bellcore.com as
[Fel2] David C. Feldmeier, Philip R. Karn, UNIX Password Security - Ten
Years Later, Proceedings of Advances in Cryptology - CRYPTO '89,
[Kle] Daniel V. Klein, ``Foiling the Cracker'': A Survey of, and
Improvements to, Password Security (revised paper), Proceedings
of the USENIX Security Workshop, summer 1990.
[Muf] Alec E. Muffet, Almost Everything You Wanted To Know About
Security (but were afraid to ask!), USENET newsgroup
[Pet] R. Pethia, S. Crocker, B. Fraser, RFC1281: Guidelines for the
Secure Operation of the Internet, november 1991.
[Til] Henk C.A. van Tilborg, An Introduction to Cryptology, Kluwer
Academic Publishers, 1988.
[Ven] Wietse Venema, Using SecurID tokens in an open multi-host UNIX
environment, can be obtained by anonymous ftp from
ftp.nic.surfnet.nl as /surfnet/net-security/docs/securid.ps,
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