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A password policy is a set of rules designed to enhance computer security by encouraging users to employ strong passwords and use them properly. A password policy is often part of an organization's official regulations and may be taught as part of security awareness training. The password policy may either be advisory or mandated by technical means. Some governments have national authentication frameworks that define requirements for user authentication to government services, including requirements for passwords.
Aspects of password policy
Typical components of a password policy include:
Password length and formation
Many policies require a minimum password length (eight characters is typical but may not be appropriate). A more appropriate length is 15 characters. Some systems impose a maximum length for compatibility with legacy systems.
Some policies suggest or impose requirements on what type of password a user can choose, such as:
- the use of both upper- and lower-case letters (case sensitivity)
- inclusion of one or more numerical digits
- inclusion of special characters, e.g. @, #, $ etc.
- prohibition of words found in a dictionary or the user's personal information
- prohibition of passwords that match the format of calendar dates, license plate numbers, telephone numbers, or other common numbers
- prohibition of use of company name or an abbreviation
Other systems create the password for the users or let the user select one of a limited number of displayed choices.
Some policies require users to change passwords periodically, e.g. every 90 or 180 days. The benefit of password expiration, however, is debatable. Systems that implement such policies sometimes prevent users from picking a password too close to a previous selection.
This policy can often backfire. Since it is hard to come up with 'good' passwords that are also easy to remember, if people are required to come up with many passwords because they have to change them often, they end up using much weaker passwords; the policy also encourages users to write passwords down. Also, if the policy prevents a user from repeating a recent password, this means that there is a database in existence of everyone's recent passwords (or their hashes) instead of having the old ones erased from memory. Finally, users may change their password repeatedly within a few minutes, and then change back to the one they really want to use, circumventing the password change policy altogether.
The human factors aspects of passwords must also be considered. Unlike computers, human users cannot delete one memory and replace it with another. Consequently changing a memorized password is very difficult, and most users resort to choosing a password that is easy to guess. Users are often advised to use mnemonic devices to remember complex passwords. However if the password must be repeatedly changed, mnemonics are useless because the user would not remember which mnemonic to use.
Administration factors can also be an issue. Users sometimes have older devices that require a password that was used before the password duration expired. In order to manage these older devices, users may have to resort to writing down all old passwords for the eventual case where they need to log in to an older device.
Requiring a very strong password and not requiring it be changed is often better. However, this approach does have a major drawback: if an unauthorized person acquires a password and uses it without being detected, that person may have access for an indefinite period.
It is necessary to weigh these factors: the likelihood of someone guessing a password because it is weak, vs. the likelihood of someone managing to steal, or otherwise acquire without guessing, a stronger password.
Common password practice
Password policies often include advice on proper password management such as:
- never share a computer account
- never use the same password for more than one account
- never tell a password to anyone, including people who claim to be from customer service or security
- never write down a password
- never communicate a password by telephone, e-mail or instant messaging
- being careful to log off before leaving a computer unattended
- changing passwords whenever there is suspicion they may have been compromised
- operating system password and application passwords are different
- password should be alpha-numeric
Password policies may include progressive sanctions beginning with warnings and ending with possible loss of computer privileges or job termination. Where confidentiality is mandated by law, e.g. with classified information, a violation of password policy could be a criminal offense. Some[who?] consider a convincing explanation of the importance of security to be more effective than threats of sanctions.
The level of password strength required depends, in part, on how easy it is for an attacker to submit multiple guesses. Some systems limit the number of times a user can enter an incorrect password before some delay is imposed or the account is frozen. At the other extreme, some systems make available a specially hashed version of the password so anyone can check its validity. When this is done, an attacker can try passwords very rapidly and much stronger passwords are necessary for reasonable security. (See password cracking and password length equation.) Stricter requirements are also appropriate for accounts with higher privileges, such as root or system administrator accounts.
Password policies are usually a tradeoff between theoretical security and the practicalities of human behavior. For example:
- Requiring excessively complex passwords and forcing them to be changed frequently can cause users to write passwords down in places that are easy for an intruder to find, such as a Rolodex or post-it note near the computer.
- Users often have dozens of passwords to manage. It may be more realistic to recommend a single password be used for all low security applications, such as reading on-line newspapers and accessing entertainment web sites.
- Similarly, demanding that users never write down their passwords may be unrealistic and lead users to choose weak ones. An alternative is to suggest keeping written passwords in a secure place, such as a safe or an encrypted master file. The validity of this approach depends on what the most likely threat is deemed to be. While writing down a password may be problematic if potential attackers have access to the secure store, if the threat is primarily remote attackers who do not have access to the store, it can be a very secure method.
- Inclusion of special characters can be a problem if a user has to log onto a computer in a different country. Some special characters may be difficult or impossible to find on keyboards designed for another language.
- Some identity management systems allow Self Service Password Reset, where users can bypass password security by supplying an answer to one or more security questions such as "where were you born?," "what's your favorite movie?," etc. Often the answers to these questions can easily be obtained by social engineering, phishing or simple research.
A 2010 examination of the password policies  of 75 different web-sites concludes that security only partly explains more stringent policies: monopoly providers of a service (e.g. government sites) have more stringent policies than sites where consumers have choice (e.g. retail sites and banks). The study concludes that sites with more stringent polices "do not have greater security concerns, they are simply better insulated from the consequences from poor usability."
Enforcing a policy
The more complex a password policy the harder it may be to enforce, due to user difficulty in remembering or choosing a suitable password.
Most companies will require users to familiarise themselves with any password policy, much in the same way a company would require employees to be aware of Health & Safety regulations, or building fire exits, however it is often difficult to ensure that the relevant policies are actually being followed.
- Changing Passwords for Key User Accounts
- "Is It Just My Imagination?" article by Suzanne Ross "Inkblots not only help users create a strong password, but people also seem to enjoy using them."
- Search for password requirements on websites
- Improving Usability of Password Management with Standardized Password Policies. Retrieved on 2012-10-12.
- Where do security polices come from? Proc. Symp. Usable Privacy and Security, 2010