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Password Security: A Case History Encryption
Computing
Robert Morris
Ken Thompson
ABSTRACT
This paper describes the history of the
design of the password security scheme on a
remotely accessed time-sharing system. The
present design was the result of countering
observed attempts to penetrate the system. The
result is a compromise between extreme security
and ease of use.
April 3, 1978
Password Security: A Case History Encryption
Computing
Robert Morris
Ken Thompson
INTRODUCTION
Password security on the UNIX time-sharing system [1]
is provided by a collection of programs whose elaborate and
strange design is the outgrowth of many years of experience
with earlier versions. To help develop a secure system, we
have had a continuing competition to devise new ways to
attack the security of the system (the bad guy) and, at the
same time, to devise new techniques to resist the new
attacks (the good guy). This competition has been in the
same vein as the competition of long standing between
manufacturers of armor plate and those of armor-piercing
shells. For this reason, the description that follows will
trace the history of the password system rather than simply
presenting the program in its current state. In this way,
the reasons for the design will be made clearer, as the
design cannot be understood without also understanding the
potential attacks.
An underlying goal has been to provide password secu-
rity at minimal inconvenience to the users of the system.
For example, those who want to run a completely open system
without passwords, or to have passwords only at the option
of the individual users, are able to do so, while those who
require all of their users to have passwords gain a high
degree of security against penetration of the system by
unauthorized users.
The password system must be able not only to prevent
any access to the system by unauthorized users (i.e. prevent
them from logging in at all), but it must also prevent users
who are already logged in from doing things that they are
not authorized to do. The so called ``super-user'' pass-
word, for example, is especially critical because the
super-user has all sorts of permissions and has essentially
unlimited access to all system resources.
_________________________
|- UNIX is a trademark of Bell Laboratories.
- 2 -
Password security is of course only one component of
overall system security, but it is an essential component.
Experience has shown that attempts to penetrate remote-
access systems have been astonishingly sophisticated.
Remote-access systems are peculiarly vulnerable to
penetration by outsiders as there are threats at the remote
terminal, along the communications link, as well as at the
computer itself. Although the security of a password
encryption algorithm is an interesting intellectual and
mathematical problem, it is only one tiny facet of a very
large problem. In practice, physical security of the com-
puter, communications security of the communications link,
and physical control of the computer itself loom as far more
important issues. Perhaps most important of all is control
over the actions of ex-employees, since they are not under
any direct control and they may have intimate knowledge
about the system, its resources, and methods of access.
Good system security involves realistic evaluation of the
risks not only of deliberate attacks but also of casual
unauthorized access and accidental disclosure.
PROLOGUE
The UNIX system was first implemented with a password
file that contained the actual passwords of all the users,
and for that reason the password file had to be heavily pro-
tected against being either read or written. Although his-
torically, this had been the technique used for remote-
access systems, it was completely unsatisfactory for several
reasons.
The technique is excessively vulnerable to lapses in
security. Temporary loss of protection can occur when the
password file is being edited or otherwise modified. There
is no way to prevent the making of copies by privileged
users. Experience with several earlier remote-access sys-
tems showed that such lapses occur with frightening fre-
quency. Perhaps the most memorable such occasion occurred
in the early 60's when a system administrator on the CTSS
system at MIT was editing the password file and another sys-
tem administrator was editing the daily message that is
printed on everyone's terminal on login. Due to a software
design error, the temporary editor files of the two users
were interchanged and thus, for a time, the password file
was printed on every terminal when it was logged in.
Once such a lapse in security has been discovered,
everyone's password must be changed, usually simultaneously,
at a considerable administrative cost. This is not a great
matter, but far more serious is the high probability of such
lapses going unnoticed by the system administrators.
Security against unauthorized disclosure of the
- 3 -
passwords was, in the last analysis, impossible with this
system because, for example, if the contents of the file
system are put on to magnetic tape for backup, as they must
be, then anyone who has physical access to the tape can read
anything on it with no restriction.
Many programs must get information of various kinds
about the users of the system, and these programs in general
should have no special permission to read the password file.
The information which should have been in the password file
actually was distributed (or replicated) into a number of
files, all of which had to be updated whenever a user was
added to or dropped from the system.
THE FIRST SCHEME
The obvious solution is to arrange that the passwords
not appear in the system at all, and it is not difficult to
decide that this can be done by encrypting each user's pass-
word, putting only the encrypted form in the password file,
and throwing away his original password (the one that he
typed in). When the user later tries to log in to the sys-
tem, the password that he types is encrypted and compared
with the encrypted version in the password file. If the two
match, his login attempt is accepted. Such a scheme was
first described in [3, p.91ff.]. It also seemed advisable
to devise a system in which neither the password file nor
the password program itself needed to be protected against
being read by anyone.
All that was needed to implement these ideas was to
find a means of encryption that was very difficult to
invert, even when the encryption program is available. Most
of the standard encryption methods used (in the past) for
encryption of messages are rather easy to invert. A con-
venient and rather good encryption program happened to exist
on the system at the time; it simulated the M-209 cipher
machine [4] used by the U.S. Army during World War II. It
turned out that the M-209 program was usable, but with a
given key, the ciphers produced by this program are trivial
to invert. It is a much more difficult matter to find out
the key given the cleartext input and the enciphered output
of the program. Therefore, the password was used not as the
text to be encrypted but as the key, and a constant was
encrypted using this key. The encrypted result was entered
into the password file.
ATTACKS ON THE FIRST APPROACH
Suppose that the bad guy has available the text of the
password encryption program and the complete password file.
Suppose also that he has substantial computing capacity at
his disposal.
- 4 -
One obvious approach to penetrating the password
mechanism is to attempt to find a general method of invert-
ing the encryption algorithm. Very possibly this can be
done, but few successful results have come to light, despite
substantial efforts extending over a period of more than
five years. The results have not proved to be very useful
in penetrating systems.
Another approach to penetration is simply to keep try-
ing potential passwords until one succeeds; this is a gen-
eral cryptanalytic approach called key search. Human beings
being what they are, there is a strong tendency for people
to choose relatively short and simple passwords that they
can remember. Given free choice, most people will choose
their passwords from a restricted character set (e.g. all
lower-case letters), and will often choose words or names.
This human habit makes the key search job a great deal
easier.
The critical factor involved in key search is the
amount of time needed to encrypt a potential password and to
check the result against an entry in the password file. The
running time to encrypt one trial password and check the
result turned out to be approximately 1.25 milliseconds on a
PDP-11/70 when the encryption algorithm was recoded for max-
imum speed. It is takes essentially no more time to test
the encrypted trial password against all the passwords in an
entire password file, or for that matter, against any col-
lection of encrypted passwords, perhaps collected from many
installations.
If we want to check all passwords of length n that con-
sist entirely of lower-case letters, the number of such
passwords is $26 sup n$. If we suppose that the password
consists of printable characters only, then the number of
possible passwords is somewhat less than $95 sup n$. (The
standard system ``character erase'' and ``line kill'' char-
acters are, for example, not prime candidates.) We can
immediately estimate the running time of a program that will
test every password of a given length with all of its char-
acters chosen from some set of characters. The following
table gives estimates of the running time required on a
PDP-11/70 to test all possible character strings of length
$n$ chosen from various sets of characters: namely, all
lower-case letters, all lower-case letters plus digits, all
alphanumeric characters, all 95 printable ASCII characters,
and finally all 128 ASCII characters.
- 5 -
One has to conclude that it is no great matter for someone
with access to a PDP-11 to test all lower-case alphabetic
strings up to length five and, given access to the machine
for, say, several weekends, to test all such strings up to
six characters in length. By using such a program against a
collection of actual encrypted passwords, a substantial
fraction of all the passwords will be found.
Another profitable approach for the bad guy is to use
the word list from a dictionary or to use a list of names.
For example, a large commercial dictionary contains typi-
callly about 250,000 words; these words can be checked in
about five minutes. Again, a noticeable fraction of any
collection of passwords will be found. Improvements and
extensions will be (and have been) found by a determined bad
guy. Some ``good'' things to try are:
- The dictionary with the words spelled backwards.
- A list of first names (best obtained from some mailing
list). Last names, street names, and city names also
work well.
- The above with initial upper-case letters.
- All valid license plate numbers in your state. (This
takes about five hours in New Jersey.)
- Room numbers, social security numbers, telephone
numbers, and the like.
The authors have conducted experiments to try to deter-
mine typical users' habits in the choice of passwords when
no constraint is put on their choice. The results were
disappointing, except to the bad guy. In a collection of
3,289 passwords gathered from many users over a long period
of time;
15 were a single ASCII character;
72 were strings of two ASCII characters;
464 were strings of three ASCII characters;
477 were string of four alphamerics;
706 were five letters, all upper-case or all lower-
case;
- 6 -
605 were six letters, all lower-case.
An additional 492 passwords appeared in various available
dictionaries, name lists, and the like. A total of 2,831,
or 86% of this sample of passwords fell into one of these
classes.
There was, of course, considerable overlap between the
dictionary results and the character string searches. The
dictionary search alone, which required only five minutes to
run, produced about one third of the passwords.
Users could be urged (or forced) to use either longer
passwords or passwords chosen from a larger character set,
or the system could itself choose passwords for the users.
AN ANECDOTE
An entertaining and instructive example is the attempt
made at one installation to force users to use less predict-
able passwords. The users did not choose their own pass-
words; the system supplied them. The supplied passwords
were eight characters long and were taken from the character
set consisting of lower-case letters and digits. They were
generated by a pseudo-random number generator with only $2
sup 15$ starting values. The time required to search (again
on a PDP-11/70) through all character strings of length 8
from a 36-character alphabet is 112 years.
Unfortunately, only $2 sup 15$ of them need be looked
at, because that is the number of possible outputs of the
random number generator. The bad guy did, in fact, generate
and test each of these strings and found every one of the
system-generated passwords using a total of only about one
minute of machine time.
IMPROVEMENTS TO THE FIRST APPROACH
1. Slower Encryption
Obviously, the first algorithm used was far too fast.
The announcement of the DES encryption algorithm [2] by the
National Bureau of Standards was timely and fortunate. The
DES is, by design, hard to invert, but equally valuable is
the fact that it is extremely slow when implemented in
software. The DES was implemented and used in the following
way: The first eight characters of the user's password are
used as a key for the DES; then the algorithm is used to
encrypt a constant. Although this constant is zero at the
moment, it is easily accessible and can be made
installation-dependent. Then the DES algorithm is iterated
25 times and the resulting 64 bits are repacked to become a
string of 11 printable characters.
- 7 -
2. Less Predictable Passwords
The password entry program was modified so as to urge
the user to use more obscure passwords. If the user enters
an alphabetic password (all upper-case or all lower-case)
shorter than six characters, or a password from a larger
character set shorter than five characters, then the program
asks him to enter a longer password. This further reduces
the efficacy of key search.
These improvements make it exceedingly difficult to
find any individual password. The user is warned of the
risks and if he cooperates, he is very safe indeed. On the
other hand, he is not prevented from using his spouse's name
if he wants to.
3. Salted Passwords
The key search technique is still likely to turn up a
few passwords when it is used on a large collection of pass-
words, and it seemed wise to make this task as difficult as
possible. To this end, when a password is first entered,
the password program obtains a 12-bit random number (by
reading the real-time clock) and appends this to the pass-
word typed in by the user. The concatenated string is
encrypted and both the 12-bit random quantity (called the
$salt$) and the 64-bit result of the encryption are entered
into the password file.
When the user later logs in to the system, the 12-bit
quantity is extracted from the password file and appended to
the typed password. The encrypted result is required, as
before, to be the same as the remaining 64 bits in the pass-
word file. This modification does not increase the task of
finding any individual password, starting from scratch, but
now the work of testing a given character string against a
large collection of encrypted passwords has been multiplied
by 4096 ($2 sup 12$). The reason for this is that there are
4096 encrypted versions of each password and one of them has
been picked more or less at random by the system.
With this modification, it is likely that the bad guy
can spend days of computer time trying to find a password on
a system with hundreds of passwords, and find none at all.
More important is the fact that it becomes impractical to
prepare an encrypted dictionary in advance. Such an
encrypted dictionary could be used to crack new passwords in
milliseconds when they appear.
There is a (not inadvertent) side effect of this modif-
ication. It becomes nearly impossible to find out whether a
person with passwords on two or more systems has used the
same password on all of them, unless you already know that.
- 8 -
4. The Threat of the DES Chip
Chips to perform the DES encryption are already commer-
cially available and they are very fast. The use of such a
chip speeds up the process of password hunting by three ord-
ers of magnitude. To avert this possibility, one of the
internal tables of the DES algorithm (in particular, the
so-called E-table) is changed in a way that depends on the
12-bit random number. The E-table is inseparably wired into
the DES chip, so that the commercial chip cannot be used.
Obviously, the bad guy could have his own chip designed and
built, but the cost would be unthinkable.
5. A Subtle Point
To login successfully on the UNIX system, it is neces-
sary after dialing in to type a valid user name, and then
the correct password for that user name. It is poor design
to write the login command in such a way that it tells an
interloper when he has typed in a invalid user name. The
response to an invalid name should be identical to that for
a valid name.
When the slow encryption algorithm was first imple-
mented, the encryption was done only if the user name was
valid, because otherwise there was no encrypted password to
compare with the supplied password. The result was that the
response was delayed by about one-half second if the name
was valid, but was immediate if invalid. The bad guy could
find out whether a particular user name was valid. The rou-
tine was modified to do the encryption in either case.
CONCLUSIONS
On the issue of password security, UNIX is probably
better than most systems. The use of encrypted passwords
appears reasonably secure in the absence of serious atten-
tion of experts in the field.
It is also worth some effort to conceal even the
encrypted passwords. Some UNIX systems have instituted what
is called an ``external security code'' that must be typed
when dialing into the system, but before logging in. If
this code is changed periodically, then someone with an old
password will likely be prevented from using it.
Whenever any security procedure is instituted that
attempts to deny access to unauthorized persons, it is wise
to keep a record of both successful and unsuccessful
attempts to get at the secured resource. Just as an out-
of-hours visitor to a computer center normally must not only
identify himself, but a record is usually also kept of his
entry. Just so, it is a wise precaution to make and keep a
record of all attempts to log into a remote-access time-
- 9 -
sharing system, and certainly all unsuccessful attempts.
Bad guys fall on a spectrum whose one end is someone
with ordinary access to a system and whose goal is to find
out a particular password (usually that of the super-user)
and, at the other end, someone who wishes to collect as much
password information as possible from as many systems as
possible. Most of the work reported here serves to frus-
trate the latter type; our experience indicates that the
former type of bad guy never was very successful.
We recognize that a time-sharing system must operate in
a hostile environment. We did not attempt to hide the secu-
rity aspects of the operating system, thereby playing the
customary make-believe game in which weaknesses of the sys-
tem are not discussed no matter how apparent. Rather we
advertised the password algorithm and invited attack in the
belief that this approach would minimize future trouble.
The approach has been successful.
References
[1] Ritchie, D.M. and Thompson, K. The UNIX Time-Sharing
System. Comm. ACM 17 (July 1974), pp. 365-375.
[2] Proposed Federal Information Processing Data Encryption
Standard. Federal Register (40FR12134), March 17, 1975
[3] Wilkes, M. V. Time-Sharing Computer Systems. American
Elsevier, New York, (1968).
[4] U. S. Patent Number 2,089,603.