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.
Password security on the UNIXtime-sharing system  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 security 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'' password, for example, is especially critical because the super-user has all sorts of permissions and has essentially unlimited access to all system resources.
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 computer, 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.
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 protected against being either read or written. Although historically, 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 systems showed that such lapses occur with frightening frequency. 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 system 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 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 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 password, 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 system, 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 convenient and rather good encryption program happened to exist on the system at the time; it simulated the M-209 cipher machine  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.
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.
One obvious approach to penetrating the password mechanism is to attempt to find a general method of inverting 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 trying potential passwords until one succeeds; this is a general 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 maximum 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 collection of encrypted passwords, perhaps collected from many installations.
If we want to check all passwords of length n that consist entirely of lower-case letters, the number of such passwords is . If we suppose that the password consists of printable characters only, then the number of possible passwords is somewhat less than . (The standard system ``character erase'' and ``line kill'' characters 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 characters 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 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.
26 lower-case 36 lower-case letters 62 alphanumeric 95 printable all 128 ASCII n letters and digits characters characters characters 1 30 msec. 40 msec. 80 msec. 120 msec. 160 msec. 2 800 msec. 2 sec. 5 sec. 11 sec. 20 sec. 3 22 sec. 58 sec. 5 min. 17 min. 43 min. 4 10 min. 35 min. 5 hrs. 28 hrs. 93 hrs. 5 4 hrs. 21 hrs. 318 hrs. 6 107 hrs.
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 typicallly 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 authors have conducted experiments to try to determine 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;
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 entertaining and instructive example is the attempt made at one installation to force users to use less predictable passwords. The users did not choose their own passwords; 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 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 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.