heimdal-strength

(Heimdal password quality check embedding CrackLib)

SYNOPSIS

heimdal-strength [principal]

DESCRIPTION

heimdal-strength is an external password quality check program for Heimdal that verifies the strength of a password. Passwords can be tested with CrackLib, checked against a CDB database of known weak passwords, checked for length, checked for non-printable or non-ASCII characters that may be difficult to enter reproducibly, required to contain particular character classes, or any combination of these tests. It is normally run via kpasswdd(8) using the Heimdal password quality check interface rather than directly.

To use this program, it must be configured in krb5.conf via settings in [appdefaults] for the application name krb5-strength. A typical setting would be:

    krb5-strength = {
        password_dictionary = /usr/local/lib/kadmind/dictionary
    }

which says to check passwords with CrackLib using the given path as the base path of the CrackLib dictionary. See CONFIGURATION below for details on the supported configuration options.

heimdal-strength then expects the Heimdal password quality check information on standard input, specifically:

    principal: <principal>
    new-password: <password>
    end

where <principal> is the principal whose password would be changed and <password> is the new password. If the password appears to be strong, it prints APPROVED on standard output and exits with a status of 0. If the password is rejected as being too weak, it will print the reason for rejecting the password on standard error and exit with a status of 0. If some fatal error occurs, it will print that error to standard error and exit with a non-zero status.

CONFIGURATION

The following krb5.conf configuration options are supported:

cracklib_maxlen

Normally, all passwords are checked with CrackLib if a CrackLib dictionary is defined. However, CrackLib's rules were designed for a world in which most passwords were four to eight characters long, and tends to spuriously reject a lot of passphrases. If this option is set to something other than its default of 0, passwords longer than that length bypass CrackLib checks. (Using a SQLite dictionary for longer passwords is strongly recommended.)

minimum_different

If set to a numeric value, passwords with fewer than this number of unique characters will be rejected. This can be used to reject, for example, passwords that are long strings of the same character or repetitions of small numbers of characters, which may be too easy to guess.

minimum_length

If set to a numeric value, passwords with fewer than that number of characters will be rejected, independent of any length restrictions in CrackLib. Note that this setting does not bypass the minimum length requirements in CrackLib itself.

password_dictionary

Specifies the base path to a CrackLib dictionary and enables password strength testing using CrackLib. The provided path should be the full path to the dictionary files, omitting the trailing *.hwm, *.pwd, and *.pwi extensions for the CrackLib dictionary.

password_dictionary_cdb

Specifies the base path to a CDB dictionary and enables CDB password dictionary lookups. The path must point to a CDB-format database whose keys are the known passwords or dictionary words. The values are ignored. You can use the krb5-strength-wordlist utility to generate the CDB database from a word list.

The CDB dictionary lookups do not do the complex password mangling that CrackLib does. Instead, the password itself will be checked against the dictionary, and then variations of the password formed by removing the first character, the last character, the first and last characters, the first two characters, and the last two characters. If any of these strings are found in the CDB database, the password will be rejected; otherwise, it will be accepted, at least by this check.

A CrackLib dictionary, a CDB dictionary, and a SQLite dictionary may all be configured at the same time or in any combination, in which case CrackLib will be run first, followed by CDB and then SQLite as appropriate.

password_dictionary_sqlite

Specifies the base path to a SQLite dictionary and enables SQLite password dictionary lookups. The path must point to a SQLite 3 database with a table named passwords. This table should have two columns, password and drowssap, which, for each dictionary word, holds the word and the reversed form of the word. You can use the krb5-strength-wordlist utility to generate the SQLite database from a word list.

The SQLite dictionary lookups do not do the complex password mangling that CrackLib does, but they will detect and reject any password that is within edit distance one of a word in the dictionary, meaning that the dictionary word can be formed from the password by adding, deleting, or modifying a single character.

A CrackLib dictionary, a CDB dictionary, and a SQLite dictionary may all be configured at the same time or in any combination, in which case CrackLib will be run first, followed by CDB and then SQLite as appropriate.

require_ascii_printable

If set to a true boolean value, rejects any password that contains non-ASCII characters or ASCII control characters. Spaces are allowed; tabs are not (at least assuming the POSIX C locale). No canonicalization or character set is defined for Kerberos passwords in general, so you may want to reject non-ASCII characters to avoid interoperability problems with computers with different default character sets or Unicode normalization forms.

require_classes

This option allows specification of more complex character class requirements. The value of this parameter should be one or more whitespace-separated rule. Each rule has the syntax:

    [<min>-<max>:]<class>[,<class>...]

where <class> is one of upper, lower, digit, or symbol. The symbol class includes all characters other than alphanumeric characters, including space. The listed classes must appear in the password. Separate multiple required classes with a comma (and no space).

The character class checks will be done in whatever locale the plugin or password check program is run in, which will normally be the POSIX C locale but may be different depending on local configuration.

A simple example:

    require_classes = upper,lower,digit

This requires all passwords contain at least one uppercase letter, at least one lowercase letter, and at least one digit.

If present, <min> and <max> specify the minimum password length and maximum password length to which this rule applies. This allows one to specify character class requirements that change with password length. So, for example:

    require_classes = 8-19:upper,lower 8-15:digit 8-11:symbol

requires all passwords from 8 to 11 characters long contain all four character classes, passwords from 12 to 15 characters long contain upper and lower case and a digit, and passwords from 16 to 19 characters long contain both upper and lower case. Passwords longer than 20 characters have no character class restrictions. (This example is probably used in conjunction with minimum_length = 8.)

require_non_letter

If set to a true boolean value, the password must contain at least one character that is not a letter (uppercase or lowercase) or a space. This may be helpful in combination with passphrases; users may choose a stock English phrase, and this will force at least some additional complexity.

SEE ALSO

krb5-strength-wordlist(1), kadm5-strength(3), kpasswdd(8), krb5.conf(5)

The "Password changing" section of the Heimdal info documentation describes the interface that this program implements and how to configure Heimdal to use it.

The current version of this program is available from its web page at <https://www.eyrie.org/~eagle/software/krb5-strength/> as part of the krb5-strength package.

AUTHOR

Russ Allbery <eagle@eyrie.org>

COPYRIGHT AND LICENSE

Copyright 2016 Russ Allbery <eagle@eyrie.org>

Copyright 2010, 2013, 2014 The Board of Trustees of the Leland Stanford Junior University

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.

Last spun 2017-02-12 from POD modified 2016-12-25