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Anatomy of a Proactive Password Changer
Department of Mathematics and Computer Science 1. Introduction
The issue of poor user selection of passwords has been discussed in many papers [6][7] and need not be repeated here. Among the techniques used to overcome these problems are random generation of passwords[3], challenge-response techniques [5], key crunching [4], and the examination of user-selected passwordseither by cracking them or by analyzing them before allowing the password to be changed. In this paper welook at a program specifically designed to do the latter.
This paper will describe a new version of the UNIX password changing program called passwd+. This program provides extensions to both the password changing facility and the password checking facility. Theformer allows users to be given full responsibility for, and control over, accounts other than their own; thelatter allows the system administrators to constrain password selection so that users cannot install passwordsdeemed easily guessable.
2. Password Changing
The standard UNIX system password changing program passwd(1) [8] allows users to change one of three types of information: their password, their login shell, and their user information (called the “GECOSinformation” here). If password aging is enabled, a user may be unable to change his or her password; how-ever, any user may change his or her login shell or GECOS information at any time. Of course, the superusermay change any user’s information at any time.
Different vendors have extended this program in ways appropriate to their environment and configura- tion; for example, some versions have an option to print out the time until the user’s current passwordexpires. These extensions must be allowed for in any replacement program.
In what follows, we describe the relevant components of passwd+ from a system administrator’s point of view. We explain why each feature is present, how it is used, and give several examples.
When passwd+ starts, it obtains information about the user named on the command line (or the current user) from the password file. In what follows, this will be called the current information. All functions areperformed upon it, and this information resides in memory until it is written back to the password file.
2.1. User Interfaces
Three interfaces allow the system administrator to replace the current passwd, chsh(1), and chfn(1) pro- grams transparently to the user. The fourth interface provides an interactive editor for the password, loginshell, and GECOS information. In addition, the user can list the current information, obtain help, change theuser whose password file information is being edited, and obtain the type and name of the password filebeing edited.
The interactive interface begins by printing a “message of the day” and then places the user in an inter- active mode which has the following commands: exit without saving changes made to the current information The support of grant NAG 2-628 from the National Aeronautics and Space Administration to DartmouthCollege is gratefully acknowledged. Portions of this work were done while the author was visiting theDepartment of Computer Science at the University of California at Davis, CA.
change the current GECOS information; the user is prompted as necessary.
print a message naming the current password file, configuration file, and user: Current configuration file: ./Sample/Config print the current information in an easy-to-read format: change the current password; the user is prompted as necessary exit; if there are unsaved changes, this will give an error message shell change the current login shell; the user is prompted as necessary write save the current information in the password file.
Unknown arguments are passed to a system-dependent routine which can act accordingly. This allows extensions to be added on a per-system (and per-site, if appropriate) basis.
2.2. Configuration File
The actions of the password changing part of the program are controlled by a configuration file; this allows sites to alter the default behavior as desired without having to edit C code and/or recompile the pro-gram. Although we shall discuss the specific components of this file below, a few general comments are inorder.
The configuration file is read once, when passwd+ starts. Reading is done by lines, and if the line just read ends with a backslash, the next line is taken as a continuation and is appended. (Because the input isstored dynamically, lines may be as long as desired.) The line is examined to see if it applies to the currentuser (here, “current user” is the user with the real UID of the process). If not, it is discarded. Otherwise, it isprocessed.
Because some strings in the configuration file may have blanks or other separators in them, passwd+ observes two escape conventions. The first, a backslash followed by a character, interpolates the characterliterally, unless that character is a newline. The second, a sequence of characters not containing an unes-caped newline and surrounded by double quotation marks, is taken to be a single string. For example, thefollowing two sequences are both read as “my shell” with a blank between the “y” and the “s”: These conventions are followed whenever any reading of the configuration file is done.
An unescaped sharp sign “#” begins a comment which extends to the end of the line.
2.3. Permissions
The ability to change information is controlled on a per-user basis by the configuration file. There are two types of controls: validate allows a user to change a set of fields after supplying his or her password, and novalidate allows the changes without the user supplying a password. For example, the lines to pro-vide controls equivalent to those of the standard password changer are: The first line says that the superuser (root) can change any of the information fields for all users with- out supplying a password. The second line says that the user of the program (:self:) can change his or herpassword, but must supply his or her current password. The third line allows the user to change his or herlogin shell and GECOS information without supplying a password.
As an example of how this mechanism can be used, suppose a professor with account name bishop is to be allowed to change the password and GECOS information for any accounts issued to his class. If the classaccounts are cs5801, cs5802, and cs5803, the following lines suffice: validate :password: bishop cs5801 cs5802 cs5803 novalidate :gecos: bishop cs5801 cs5802 cs5803 Note that bishop must enter his password to change any of the passwords of the class accounts, but need not do so to change the GECOS information. Further, as no permission is given to change the login shell ofthe class accounts, only the students (or the superuser) can do that.
The first word following the validate or novalidate is the type of information to which the remainder of the line applies; if this is omitted, the remainder applies to all types. Legal values are: (Incidentally, the semicolon was chosen as a delimiter because that character cannot be used in an accountname.) The next word is the user name; if it is not that of the current user, the line is ignored. If it matches that of the current user, the remainder of the line contains the account names for which the current user canchange the previously-indicated type of information. In addition to account names, the following specialnames have special effects: means no accounts; this is useful with accounts that no user should ever log into means all accounts for which the current user has not previously been given permissions the account belonging to the current user Following the principle of fail-safe defaults, unless access is granted by a validate or novalidate control line,the request to change information is denied.
As a final example, the following control lines allow the standard changing, except that the accounts ftp, uucp, and audit may never change their own passwords: Either validate or novalidate could be used in the last two lines.
As a safety and debugging measure, a list of distinguished user identification numbers can be made privileged at compile time. Any member of this set can change any information for any user, regardless of the settings in the configuration file. As distributed, this list is empty. However, as the superuser can edit apassword file directly, there seems little point in not adding the UID of 0 to this list.
2.4. Changing the Password
If a user is allowed to change his or her password (as controlled by the permissions described in the pre- vious section), passwd+ prompts him or her for the new password. It is then subjected to proactive analysis.
If a subprogram is to be used, it is named in the control line This program must expect as (standard) input the following information, separated by newlines: proposed new passwordold (current) password if available; this line is blank if notaccount namecurrent hashed passwordGECOS informationlogin shellhome directoryuser identification (UID)primary group identification (GID)time when password can next be changed (in seconds since the epoch)time when password expires (in seconds since the epoch) All output from the program is sent to passwd+’s standard output and standard error, except that any lines onthe standard output that begin with **LOG** are entered into passwd+’s log file. The exit status code of theanalysis program determines what passwd+ does next. If that code is 0, the password is accepted as hard toguess. If the exit status code is 1, the password is rejected as easy to guess. If the analysis program returns 2or 3, there was an error that prevented the analysis program from using all its test but the password wasdeemed hard to guess (code 2) or easy to guess (code 3) according to those tests completed. Any other exitstatus code is treated as a 3.
If the analysis program did not complete successfully, the control line controls what happens. The following settings for action cause the indicated response: reject the password (that is, exit status code 2 causes rejection) accept the password (that is, exit status codes 2, or 3 cause acceptance) accept the password if the exit status code is 2 and reject if it is 3 run passwd+’s internal tests and accept or reject based on their success Unless an onerror line appears, an exit code other than 1 or 2 causes the internal tests to be used.
If the analysis program fails, or none is named, or it cannot be executed for any reason, a series of inter- if the password is the login name or the login name reversed (regardless of capitalization), the passwordis rejected; if the password is under 6 characters, it is rejected; if the password has no nonalphanumeric characters, it is rejected.
While these tests are not very adequate, they are the minimum qualities a reasonable password should pos-
sess. Note that unlike most implementations of the standard password changing program, these cannot be
relaxed or disabled without modifying the code.
2.5. Changing the GECOS Information
Sites store information in the GECOS field using very different formats; further, the type of information stored at each site is different. For example, at the Research Institute for Advanced Computer Science, thetypical GECOS information was stored as a name, an office, and a telephone extension number: But in one department at Dartmouth College, faculty members have only their names stored: and in another department at the University of California at Davis, the system accounts have GECOS fieldsof one word and user accounts have the user name followed by office number, year of graduation (if any),phone number, home phone number, and faculty sponsor: Matt Bishop,4413 Chem Annex,27324,,Karl Levitt (The home telephone number is omitted here.) Given this variety even within sites, the password changershould parse the GECOS field, determine what format is being used, and either use the same one or, if pre-ferred, supply a new format. For this reason, control lines are needed to pick out the components of the field,tie prompts to the (so the user can be asked to update the information) and to reformat the fields.
The mechanism chosen to parse the fields was the system pattern matcher (the obvious alternative, the formats used by the reading function scanf(3), was rejected as too cumbersome). Currently a public-domainimplementation of the Berkeley (UNIX System Version 7) pattern matcher, and the source code of the GNUemacs pattern matcher, are provided. As distributed, the Berkeley pattern matcher is the default. The rele-vant control line is with pattern_matcher being bsd for the Berkeley version and gnu for the GNU emacs version.
The pattern matcher is used to assign portions of the GECOS field to variables. For example, consider the line from the RIACS file above. In the following control line getgecos “^\([^,]*\),\([^,]*\),\(.*\)$” fullname office extension the pattern matches the format of the RIACS GECOS field. The first part of the pattern, Matt Bishop, isassigned as the value of the variable fullname; the second part, N238-102, is assigned as the value of office,and the third, 46124, as the value of extension. If the pattern does not match the GECOS field, the line isskipped. The first line with a match does the assignment, and the process stops.
Next, the output format must be selected. Output formats are defined in setgecos control lines; these use a printf(3) format rather than a pattern. An appropriate output format for the RIACS line would be setgecos “%s,%s,%s” fullname office extension But before output can be done, the user must be prompted to update the current information, so a label anddefault value must be associated with each variable. The associate control line does this. For example,reasonable associate control lines for RIACS would be: associate office “Office Number” none The variable name comes first, followed by the prompt and the default value. Note that if the prompt is morethan one word, any intervening white space must be escaped. If the default is omitted, the word “none” isused.
When the setgecos line is reached, the user will be prompted for a new value of each variable in the setgecos list. The prompt is followed by a default value, which is the current value (if any) or the defaultvalue named in the associate line. So, in this example the prompts would look line: Note that if nothing is typed, the default named in the prompt is used. To supply a blank entry (that ism, the null string, the word “none” must be typed. This is to conform to the standard UNIX change GECOSutility interface.
As a more complex example, the following control lines handle the two distinct formats described for the University of California at Davis GECOS fields: getgecos “^\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\(.*\)$” name room \ setgecos “%s,%s,%s,%s,%s” name room wphone hphone sponsor associate wphone “Office phone number” 27004 associate hphone “Home phone number” 5551212 associate sponsor “Faculty sponsor” none If Matt Bishop wished to change his office entry and home phone number, here is what the session would look like (the responses to the prompts are in oblique typeface): Office [default “4413 Chem Annex”]: 4414 Chem Annex Office phone number [default “27324”]: Home phone number [default “5551212”]: 5551313 Faculty sponsor [default “Karl Levitt”]: If the superuser wished to change his GECOS information, which is simply the string the second getgecos line would be used (as the pattern on the first line does not match the current con-tents) and the output format would be that of the next setgecos field. In this case the session would looklike: Name [default “Charlie the Root”]: The Root of All Evil because the variable name’s value is set to Charlie the Root.
2.6. Changing the Login Shell
These lines control which shells may be used; they do not control who may change a login shell (the permissions lines control that). A user may change his or her shell only to those shells allowed by controls inthis section. For example, the control allows a user to use the system’s shells (as listed in “/etc/shells” or, if that file does not exist, either “/bin/sh”or “/bin/csh”) as login shells. The control allows any user to use the file “/usr/bin/special_shell” as a login shell. If users are to be allowed to use anyfile as a login shell, the control should be listed; if this is to apply only to a set of users, the account names of those users may be listed afterthe anyshell. Finally, the control means that bishop’s shell can only be a file he owns; if the name is omitted, that restriction applies to allusers.
As an example, here is the configuration which acts in the same way as the standard UNIX password changing program, in which the superuser can use any shell, but users can only use their own programs orstandard system shells: Note that the shell need not be executable so that the user can disable his or her account if desired.
Whether or not this is a feature or a bug is a matter of taste; it is present because the standard shell changingprograms allow it.
2.7. Running Subprograms
Subprograms may be run at three points. First, if a user requests help, the password checking routine may be executed with a special flag. Second, if logging is done, the configuration file can cause log mes-sages to be passed as input to a subprogram. Third, when the password checker itself is run, it is run as a sub-process. Given that all of these will be run with the super-user’s effective UID, care must be taken inspawning them; the threats to the system by setuid-to-root programs have been discussed elsewhere.
The passwd+ program thoroughly sanitizes the subprogram’s execution environment before running the subprogram by taking the following precautions: the shell environment variables are deleted; the PATH environment variable is reset to a safe state (as distributed, “/bin:/usr/bin:/usr/ucb:/etc”);
the SHELL environment variable is reset to a known default (as distributed, “/bin/sh”);
the IFS environment variable is reset to the default value; and
the umask shell variable is reset to a default value (as distributed, 022).
Further, all subprograms are invoked as named in the configuration file, so if full path names are provided,
the PATH variable will not be used. This is highly recommended.
If desired, the above behavior can be modified by use of several controls in the configuration file. The will cause the umask to be reset to the named value, which is given in octal, rather than the default value.
The control
says to pass the SHELL environment variable passed to the subprocess, and to give it the value “/bin/csh”.
If the value is omitted, the variable has the null value; and if the variable itself is simply named, the value
from the environment in which passwd+ is run is inherited. So, if a user uses the Korn shell, and SHELL is
set to “/usr/bin/ksh”, the line
causes the subprocess to have the SHELL environment variable’s value set to “/usr/bin/ksh”.
As a final note, the help and password checking programs are invoked directly and not through an inter- mediate shell; so if a shell variable is to be used in the program name, the name must be passed to a shell. Asan example, if every user had a program checkme in his or her home directory, to force that to be invoked,the line would need to be present, and the command itself would be given as 2.8. Miscellaneous Configuration Control Commands
The configuration file has several miscellaneous control statement. Because passwd+ is designed to be portable, it uses library routines to access the password file. These routines, supplied with passwd+ or writ-ten specifically for the system in use, use a common interface that is used to determine the account name,hashed password, UID and GID, login shell, home directory, and the relevant aging information. Currently,support for Berkeley 4.3 and Ultrix V4.2A password files are supported; to use the former, give the control and for the latter, replace bsd4_3 with ultrix. Other types will be added soon, as need (and/or contribu-tions) permit.
Three types of help are available; each default message may be replaced by the contents of a file. The first (nicknamed the “message of the day” file) is printed whenever passwd+ is invoked, for all interfaces.
The control line which sets this file name is: The second file is printed whenever the option -help is given on a command line; it contains informa-
tion about how to run passwd+. The file is printed, and the program then exits. The relevant control line is: The third file applies only to the interactive interface, and is printed when the user requests help (with the help command). The relevant control line is: In addition, a special help command can be run; typically, this is the password analysis program with a special option to print messages suggesting guidelines for choosing a good password. The line controllingthis is: If any of the help files are not present, a default help message is printed.
Finally, an extensive logging ability allows the system administrator to log messages about syntax errors in the configuration file, system errors (such as inaccessible files), information on the program’s useand on the success or failure of the use. This information may be stored in a log file, given as input to a com-mand, written to a syslog(8) daemon, or printed on the standard error. For example, the line sends all syntax and system error messages to all members of the mail alias staff (the last string means thatthe log messages generated from this line are input to the mail program), and the line writes messages indicating all invocations and their results into the log file “/etc/passwd.log”.
2.9. Example Configuration File
An example configuration file is given in Figure 1. Note that the pattern matcher used is the GNU emacs pattern matcher; given the patterns in the getgecos statement, this could equally well have been the Berkeleypattern matcher. In that same block, note that there is a getgecos line with a pattern that matches anything.
Were this omitted, if none of the patterns matched the GECOS information field, the user would not beprompted for anything.
3. Password Checking
The heart of the password validation scheme is the program to verify that the password is in fact diffi- cult to guess. The principles behind this program, which is invoked by passwd+, have been described else-where [1]. This section discusses pwcheck, the password checking program which is intended to be thepassword analysis component used by passwd+. Like passwd+, it uses its own configuration file.
3.1. Configuration File
As with passwd+, pwcheck uses a configuration file to enable system administrators to define the notion of an easy to guess password without writing or altering a program. When pwcheck begins, it reads its con-figuration file (or the standard input); when it reaches the end of the configuration file (or of standard input),it terminates.
The configuration file consists of control lines and test lines. The control lines set internal variables, interpolate files, and so forth; the test lines define tests and messages.
The basic unit of the little language is the string, which is defined as: a maximal sequence of alphanumeric characters and underscores “_”; a double quote ‘”’, left brace “[“, left curly brace “{“, dollar sign “$”, or at sign “@” followed by anynumber of characters (except a newline) terminated by the first unescaped double quote, right brace “]”,right curly brace “}”, dollar sign, or at sign, respectively; or # who can do what and to whomnovalidate root :all: # root can change anything without validation # don’t validate user to change shell or gecos fields # what shell for root? anything she wants .
# users can make their own poison (anything they own) # GECOS information -- two forms allowed:pattern gnu name,office,work phone,home phone,sponsor getgecos “^\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\(.*\)$” name office home_phone work_phone sponsorsetgecos “%s,%s,%s,%s,%s” name office home_phone work_phone sponsor# form #2: name,contact (contact is who is responsible for that account) getgecos “^\([^,]*\),\(.*\)$” name contact# form #3 not allowed; this is anything else, and we want it put into form #2 getgecos “^\(.*\)$” namesetgecos “%s,%s” name contact # now the prompts for all these billions and billions of variables (well, all 6, anyway .)associate name “Name” none associate office “Office” “300 Bradley Hall” # the office (default is department office) associate work_phone “Office phone number” “(603) 646-2415” # office phone (default is dept number)associate home_phone “Home phone number” unlisted # home phone (unlisted by default)associate sponsor “Faculty sponsor” none associate contact “Contact person” none # subprograms; we’re very restrictive here (guess why?)umask 077 # they got the Bourne shell, like it or not environ PATH=”/usr/bin:/bin:/usr/ucb” # it’s an ultrix system, so .
pwfunc ultrix # how and what do we loglog syntax system “|mail staff” # bad errors get mailed to anyone who can fix them # log use and success/failure to syslog daemon Figure 1. A sample passwd+ configuration file.
any single character except alphanumerics, underscore, left and right braces, left and right curly braces,double quotes, dollar signs, and at signs.
Let us now look at how these strings may be used.
3.2. Setting and Referencing Variables
Unlike the previous version of passwd+, no variables are assigned values automatically. Instead, these values must be set by using explicit control lines. Also unlike the previous version, variable names may havemore than one character in them.
% [ : ] [ _ ] [ +- ] [ m.n ] [ ^*|#] variable: insert \ characters as necessary to ensure the characters are interpreted as characters and not asmetacharacters.
treat the value as an integer and subtract that value from the maximum password length. If thevalue is not an integer, 0 is interpolated.
if “-”, reverse the value; if “+” or omitted, do not reverse the value.
The characters beginning at position m (positions begin with 1) and through position n inclusive areinterpolated. If “.” is present, m and n are position 1 and the last position in the string, if omitted; if“.” is not present, the first m characters are interpolated.
^ makes all alphabetic characters in the value upper-case* makes all alphabetic charactes in the value lower-case| makes the first character in the value upper-case if it is alphabetic; no effect if not# interpolates the length of the string Figure 2. The escape sequence components for accessing a variable value. All are applied right to left, andare applied before the result is interpolated.
sets the internal variable p to the (string) value “bishop”. Similarly, variable values may be assigned usingpatterns; for example, the control line setpat “Matt Bishop* wrote 6 39” “^\(.*\) wrote \(.*\)$” who what x would assign the value “Matt Bishop*” to the variable who, “6” to the variable what, and “39” to thevariable x. As with getgecos, the interpretation of the pattern is controlled by (for the Berkeley pattern matcher; to get the GNU emacs version, replace bsd with gnu).
Variable values are accessed and manipulated using an escape facility reminiscent of that of printf(3); see figure 2. For example, suppose p, who, what, and x have the values shown above. Examples of how thevalues may be accessed in different ways are: Note the value is always considered a string.
These particular operations were chosen because they are the most common ones password crackers use. The motive for the “:” was different, though. Variables are used in tests, and sometimes tests involve theevaluation of patterns. In these cases, some values are to be treated as strings (such as the password) and oth-ers as patterns (such as a set of variables containing patterns to be used repeatedly). So, when a variablevalue is to be treated as a string, it should have the “:”.
One special variable has a value that changes. The sequence %< takes the value of the next line of the standard input, or the empty string if none. This is used to interact with programs which pass information topwcheck using the standard input. For example, here is a sequence of controls that sets variables correspond-ing to the information passwd by passwd+: true if both test1 and test2 are true true if either test1 or test2 is true true if number1 and number2 are numerically equal; any of the rela-tions >, <, >=, <= != may be used here true if string1 and string2 compare equal; may use != here also true if string1 matches the pattern string2; may use !~ here also In the above, string1 and string2 are both strings:string ::= a sequence of alphanumeric, underscore, and escaped characters the string is any line in the file filename the string is in the dbm(3) file with base name dbm_base_file the string is any line in the sorted file sorted_file_name the string is any line in the output of command Figure 3. Syntax for the boolean expressions in the test lines.
Finally, to erase a variable value, use unset; the following erases the value of the variable what: 3.3. Tests
Tests are used to determine whether or not the password is suitable for use. Each test has four compo- nents: a boolean test expression (which evaluates to true or false), a true response, a false response, and ahelp response. For example, this sequence might be used to test password length: length true “%p” is unacceptable; you need at least 6 characters length false the password passes the length test length help Passwords must be at least 6 characters long.
Suppose the password is “bis*hop”. In this case the boolean expression on the line beginning with “length test” is false, so the message on the line “length false” is printed. Note that the sense ofthe tests is that if the test succeeds (evaluates to true), the password is easy to guess. Also notice that vari-ables may be included within the messages; they will be evaluated before the message is printed.
The word “length” at the beginning of the line serves simply to link the messages with the test. When a test is evaluated, its result is stored with the label at the beginning of the line (here, “length”). Whenevera line beginning with that label is encountered, it is processed. If the control (“true” or “false”) matchesthe value of the test, the message is printed; otherwise it is ignored. If the test has not yet been seen, the mes-sages are not printed. If labels are omitted, a singe (blank) label is assumed.
The line beginning with “length help” is a help line, and that is printed when pwcheck is invoked in help mode (by giving the command line option -help). In this mode, no test processing is done; variables are
evaluated, and any test message lines with the control “help” are printed. This facility is to allow the system
administrator to provide guidance on what passwords are acceptable.
In what follows, we refer to the boolean expression which is evaluated as the “test.” The complete syn- tax of the tests is shown in figure 3; basically, all numerical tests and arithmetic operations are allowed, asare string compares and pattern matches. In addition, pwcheck can look for a password in a file or in the out-put of a program.
The tests are composed of numerical expressions, string compares, and pattern matches. For example, if l is the user’s login name and p the proposed password, the test “%p” =~ “\(%l\)*” | “%p” =~ “\(%-l\)*” is true if the proposed password is 0 or more repetitions of the login name or the login name reversed (thisrequires the GNU emacs pattern matcher).
The use of files and programs is very similar. In the test each line of the system dictionary file “/usr/dict/words” is compared to the proposed password; if any match,the test succeeds.One problem with this test from the implementation point of view is that the search is lin-ear and hence on a large file can be very slow. If the lines of the file are in sorted ASCII order, the above testmay be rewritten as and pwcheck will use the binary search technique to locate the right side (the value of the variable p) in thefile “/usr/dict/words.sorted”; note that if the file is not in sorted ASCII order, the result returned may bewrong. Finally, if the file is in the fast database format of the dbm(3) or ndbm(3) library routines, the com-mand will use the dbm functions to search the file for the value of the right side (here, the value of the variable p).
Note that the base name of the dbm files is given; in the above example, the files “/usr/dict/words.dbm.dir”and “/usr/dict/words.dbm.pag” must exist or the test fails.
The ability to use the output of a subprogram is a very powerful feature of pwcheck. For example, con- sider a test to catch all English words. If the proposed password is stored in the variable p, it would seem that would work, as “/usr/dict/words” is an on-line copy of an English dictionary. But note that some words inthat file are capitalized, and others are not; in particular, “water” is in the dictionary, but if the proposed pass-word is “Water”, the test fails. So perhaps { tr A-Z a-z < /usr/dict/words } == “%*p” (which simply copies the contents of “/usr/dict/words” to the standard output, changing every capital letterto lower case). Again, this misses plurals, present and past participles, and other derivative forms of words.
Only the spelling checker, spell(1), will catch these. So a command of the form will work. (Recall spell prints on its standard output a list of incorrectly spelled words.) Alas, while this does work, there is a serious security problem: the new proposed password will be passed to spell using the command echo(1), and for a brief time will therefore be visible to programs whichcan examine command line argument lists. To overcome this danger, pwcheck provides a special construct tosend input to a program in a test. The above test should be written as: Here, the first string following the <- is written to the command’s standard input. This solves the abovesecurity problem, and will reject any English words.1 3.4. Miscellaneous Commands
interpolates the contents of “filename” at this point in the file. Files may be nested as deeply as the systemallows (usually 16, 28, or 60). This is useful for including per-user tests; for example, if the user name isstored in the variable l, then will include a file specifically for the current user.
Note that some versions of spell record misspelled words so a systems administrator can examine
them and decide whether to add them to the dictionary or to a site-specific dictionary. If this is
done, be sure to turn it off in the test! (This often can be done with a command option such as -N.)
# this is a sample password configuration file for pwcheckset version beta-testpattern gnu# this file loads the variables (see section 3.2 of this paper)include /etc/passwd.siv#================= some stuff to make patterns easier to readset let \[A-Za-z]set nlet \[^A-Za-z]#================= fix up first, last name, etc.
setpat “%(Gecos)” “^\([^,]*\),\([^,]*\),\(.*\)$” name office extensionsetpat “%(name)” “^\(%(let)*\)%(nlet)*%(nlet)\(%(let)%(let)*\)%(nlet)%(nlet)*\(%(let)%(let)*\)$” \ setpat “%(name)” “^\([A-Za-z]+\)[^A-Za-z]+\([A-Za-z]+\)$” first lastset initials %1.1(first)%1.1(middle)%1.1(last)#================= test password lengthlength test %#p<=6length true “%p” is unacceptable; you need at least 6 characterslength false the password passes the length testlength help Passwords must be at least 6 characters long#================= test if password is an English wordEnglish test {spell} <- “%p\n” == ““English true “%p” is unacceptable; it is an English wordEnglish false the password passes the no-English-words testEnglish help Passwords must not be an English word#================= test login namelogin test “%p” =~ “\(%l\)*” | “%p” =~ “\(%-l\)*”login true “%p” is unacceptable; it cannot be your login name repeatedlogin false the password passes the repeated login name testlogin help Passwords must not be repetitions of your login name (or reversed login name)#================= sorted dictionary (ASCII order)dictionary test $/usr/dict/words.sorted$ == “%p”dictionary true “%p” is in the system dictionarydictionary false the password is not in the system dictionarydictionary help Passwords must not reside in a system dictionary#================= DBM filedbmdictionary test @/usr/dict/[email protected] == “%p”dbmdictionary true “%p” is in the system’s dbm dictionarydbmdictionary false the password is not in the system’s dbm dictionarydbmdictionary help Passwords must not reside in the system’s dbm dictionary Figure 4. A sample pwcheck configuration file.
Finally, although UNIX passwords may be of any length, only the first 8 characters are significant.
Hence strings which differ in the ninth character are really the same so far as the UNIX password system isconcerned. When the control is used, all string comparisons are done only for the first 8 character. (Any transformation to variable valuesare done first; the truncation occurs only during the comparison and only for purposes of the comparison.) 3.5. Example Configuration File
An example configuration file is given in Figure 4. Note that variables may contain patterns; however, the first backslash is needed. Were it not there, the “[“ and “]” would mean the characters between were afile name, and the first line of the file would become the value of the variable let.
The assumption made about the GECOS field here is that it is of the form name, office, extension (just as the RIACS format shown in section 2.5). Note the use of the variables let and nlet in the patterns.
4. Future Directions and Work
The password changing program, passwd+, is stable; no major changes to the design have been made for some time, and it is in the process of being cleaned up and documented so that it can be distributed forbeta test. The thrust of future development will most likely be in developing library routines to handle differ-ent password storage schemes and password distribution mechanisms.
The password checker is another matter. It is undergoing changes in both design and syntax. The pro- gram pwcheck was designed to be independent of passwd+, and as such lacks some features and transforma-tions which would be of great use, such as the ability to count letters, non-letters, and so forth. These may bedone using other programs such as sed(1) or awk(1); however, this requires a subprogram be run, and is veryawkward. Hence it is clear these abilities need to be added, but it is not clear how they should be integratedwith the rest of pwcheck.
Part of the problem is that little, if any, research has been done on language design for this purpose; this suggests that perhaps too little research on the usefulness of proactive password checking itself is docu-mented. Logic and intuition says that proactive password checking is far more effective than passwordcracking, and should be as good as (if not better than) other forms of password assignment. No experimentalevidence has been gathered to support this opinion, and human nature being the crux of the problem, itwould be very wise to test these beliefs.
5. Conclusion
The goals of passwd+ and pwcheck are to provide a friendly, powerful tool for system administrators to improve the quality of the passwords users select. Sufficient facilities are provided to allow per-site and per-user tests.
The software should be available soon; its location will be announced through the cert-tools mailing list when it is available. The version which will be released will be beta test software, so people who take it willbe asked to report bugs (and fixes), as well as any new password file routines they write. Documentation onthe software and its internal structure will be available to help any hardy souls willing to work with it! 6. References
Matt Bishop, “A Proactive Password Checker,” in Information Security, David T. Lindsay and WynL. Price (eds.), North-Holland, New York, NY pp. 169-180 (1991) M. Gasser, “A Random Word Generator for Pronounceable Passwords,” Technical Report ESD-TR-75-97, The MITRE Corporation, Bedford, MA (Nov. 1975) L. Grant, “DES Key Crunching for Safer Cipher Keys,” SIG Security Audit and Control Review 5(3)
pp. 9-16 (Summer 1987).
J. Haskett, “Pass-Algorithms: A User Validation Scheme Based on Knowledge of Secret Algo-
rithms,” Communications of the ACM 27(8) pp. 777-784 (Aug. 1984).
Daniel V. Klein, ““Foiling the Cracker”: A Survey of, and Improvements to, Password Security,” Pro-ceedings of the UNIX Security Workshop II pp. 5-14 (Aug. 1990) Robert Morris and Ken Thompson, “Password Security: A Case History,” Communications of the
22(11) pp. 594-597 (Nov. 1979)
UNIX User’s Reference Manual, 4.3 Berkeley Software Distribution Virtual VAX-11 Version, Com-puter Systems Research Group, Computer Science Division, Department of Electrical Engineeringand Computer Science, University of California, Berkeley CA (Apr. 1986); reprinted by the USENIX

Source: http://www.solver.io/external/articles/secure-programming/bishop-1992-anatomy-of-a-proactive-password-changer.pdf


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