View source for Password strength

{{Short description|Resistance of a password to being guessed}}
{{for|organizational rules on passwords|Password policy}}
[[File:Bitwarden Desktop 2024.12.1 password generator screenshot.webp|thumb|upright=1.2|Options menu of the [[random password generation]] tool in [[Software:Bitwarden|Bitwarden]]. Enabling more character subsets raises the strength of generated passwords a small amount, whereas increasing their length raises the strength a large amount.]]
'''Password strength''' is a measure of the effectiveness of a [[Password|password]] against guessing or [[Brute-force attack|brute-force attack]]s. In its usual form, it estimates how many trials an attacker who does not have direct access to the password would need, on average, to guess it correctly. The strength of a password is a function of length, complexity, and unpredictability.<ref name=CERT>{{cite web | url = http://www.us-cert.gov/cas/tips/ST04-002.html | title = Cyber Security Tip ST04-002 | work = Choosing and Protecting Passwords | date = 21 May 2009 | publisher = US CERT | access-date = June 20, 2009 | url-status = live | archive-url = https://web.archive.org/web/20090707141138/http://www.us-cert.gov/cas/tips/ST04-002.html | archive-date = July 7, 2009 }}</ref>

Using strong passwords lowers the overall [[Risk|risk]] of a security breach, but strong passwords do not replace the need for other effective [[Security controls|security controls]].<ref>{{Cite web|title=Why User Names and Passwords Are Not Enough {{!}} SecurityWeek.Com|url=https://www.securityweek.com/why-user-names-and-passwords-are-not-enough|access-date=2020-10-31|website=www.securityweek.com|date=31 January 2019|archive-date=2020-12-04|archive-url=https://web.archive.org/web/20201204204759/https://www.securityweek.com/why-user-names-and-passwords-are-not-enough|url-status=live}}</ref> The effectiveness of a password of a given strength is strongly determined by the design and implementation of the [[Authentication#Authentication factors|authentication factors]] (knowledge, ownership, inherence). The first factor is the main focus of this article.

The rate at which an attacker can submit guessed passwords to the system is a key factor in determining system security. Some systems impose a time-out of several seconds after a small number (e.g. three) of failed password entry attempts. In the absence of other vulnerabilities, such systems can be effectively secured with relatively simple passwords. However, systems store information about user passwords, and if that information is not secured and is stolen (say by breaching system security), user passwords can then be compromised irrespective of password strength.

In 2019, the United Kingdom's [[Organization:National Cyber Security Centre (United Kingdom)|NCSC]] analyzed public databases of breached accounts to see which words, phrases, and strings people used. The most popular password on the list was 123456, appearing in more than 23 million passwords. The second-most popular string, 123456789, was not much harder to crack, while the top five included "qwerty", "password", and 1111111.<ref>{{Cite web|url=https://www.bbc.com/news/technology-47974583|title=Millions using 123456 as password, security study finds|date=21 April 2019|website=BBC News|access-date=24 April 2019|archive-date=26 April 2025|archive-url=https://web.archive.org/web/20250426235754/https://www.bbc.com/news/technology-47974583|url-status=live}}</ref>

==Password creation==

Passwords are created either automatically (using randomizing equipment) or by a human; the latter case is more common. While the strength of randomly chosen passwords against a [[Brute-force attack|brute-force attack]] can be calculated with precision, determining the strength of human-generated passwords is difficult.

Typically, humans are asked to choose a password, sometimes guided by suggestions or restricted by a set of rules, when creating a new account for a computer system or internet website. Only rough estimates of strength are possible since humans tend to follow patterns in such tasks, and those patterns can usually assist an attacker.<ref name=NIST /> In addition, lists of commonly chosen passwords are widely available for use by password-guessing programs. Such lists include the numerous online dictionaries for various human languages, breached databases of [[Plaintext|plaintext]] and [[Cryptographic hash function|hashed]] passwords from various online business and social accounts, along with other common passwords. All items in such lists are considered weak, as are passwords that are simple modifications of them.

Although random password generation programs are available and are intended to be easy to use, they usually generate random, hard-to-remember passwords, often resulting in people preferring to choose their own. However, this is inherently insecure because the person's lifestyle, entertainment preferences, and other key individualistic qualities usually come into play to influence the choice of password, while the prevalence of online [[Social:Social media|social media]] has made obtaining information about people much easier.

==Password guess validation==
Systems that use passwords for [[Authentication|authentication]] must have some way to check any password entered to gain access. If the valid passwords are simply stored in a system file or database, an attacker who gains sufficient access to the system will obtain all user passwords, giving the attacker access to all accounts on the attacked system and possibly other systems where users employ the same or similar passwords. One way to reduce this risk is to store only a cryptographic hash of each password instead of the password itself. Standard cryptographic hashes, such as the Secure Hash Algorithm (SHA) series, are very hard to reverse, so an attacker who gets hold of the hash value cannot directly recover the password. However, knowledge of the hash value lets the attacker quickly test guesses offline. [[Password cracking]] programs are widely available that will test a large number of trial passwords against a purloined cryptographic hash.

Improvements in computing technology keep increasing the rate at which guessed passwords can be tested. For example, in 2010, the [[Organization:Georgia Tech Research Institute|Georgia Tech Research Institute]] developed a method of using GPGPU to crack passwords much faster.<ref name="gtri"/> Elcomsoft invented the usage of common graphic cards for quicker password recovery in August 2007 and soon filed a corresponding patent in the US.<ref name="belenko">{{ cite patent | country=US | number=7929707 | status=patent | title=Use of graphics processors as parallel math co-processors for password recovery | assign1=Elcomsoft Co. Ltd. | inventor= Andrey V. Belenko | gdate=2011-04-19 }}</ref> By 2011, commercial products were available that claimed the ability to test up to 112,000 passwords per second on a standard desktop computer, using a high-end graphics processor for that time.<ref name=elcomsoft>[https://www.elcomsoft.com/eprb.html#gpu Elcomsoft.com] {{webarchive|url=https://web.archive.org/web/20061017173506/http://www.elcomsoft.com/eprb.html |date=2006-10-17 }}, [[Company:ElcomSoft|ElcomSoft]] Password Recovery Speed table, [[NTLM]] passwords, [[Engineering:Nvidia Tesla|Nvidia Tesla]] S1070 GPU, accessed 2011-02-01</ref> Such a device will crack a six-letter single-case password in one day. The work can be distributed over many computers for an additional speedup proportional to the number of available computers with comparable GPUs. Special [[Social:Key stretching|key stretching]] hashes are available that take a relatively long time to compute, reducing the rate at which guessing can take place. Although it is considered best practice to use key stretching, many common systems do not.

Another situation where quick guessing is possible is when the password is used to form a cryptographic key. In such cases, an attacker can quickly check to see if a guessed password successfully decodes encrypted data. For example, one commercial product claims to test 103,000 [[Wi-Fi Protected Access|WPA]] PSK passwords per second.<ref>[http://www.elcomsoft.com/ewsa.html Elcomsoft Wireless Security Auditor, HD5970 GPU] {{webarchive|url=https://web.archive.org/web/20110219131825/http://www.elcomsoft.com/ewsa.html |date=2011-02-19 }} accessed 2011-02-11</ref>

If a password system only stores the hash of the password, an attacker can pre-compute hash values for common password variants and all passwords shorter than a certain length, allowing very rapid recovery of the password once its hash is obtained. Very long lists of pre-computed password hashes can be efficiently stored using rainbow tables. This method of attack can be foiled by storing a random value, called a cryptographic salt, along with the hash. The salt is combined with the password when computing the hash, so an attacker precomputing a rainbow table would have to store for each password its hash with every possible salt value. This becomes infeasible if the salt has a big enough range, say a 32-bit number. Many authentication systems in common use do not employ salts and rainbow tables are available on the Internet for several such systems.

===Entropy as a measure of password strength===
Password strength is specified by the amount of [[Information entropy|information entropy]], which is measured in [[Shannon (unit)|shannon]] (Sh) and is a concept from [[Information theory|information theory]]. It can be regarded as the minimum number of [[Bit|bit]]s necessary to hold the information in a password of a given type. A related measure is the  [[Binary logarithm|base-2 logarithm]] of the number of guesses needed to find the password with certainty, which is commonly referred to as the "bits of entropy".<ref>{{cite conference |url=http://www.isiweb.ee.ethz.ch/archive/massey_pub/pdf/BI633.pdf |title=Guessing and entropy |author=James Massey |year=1994 |publisher=IEEE |pages=204 |book-title=Proceedings of 1994 IEEE International Symposium on Information Theory |access-date=2019-07-12 |archive-date=2014-01-01 |archive-url=https://web.archive.org/web/20140101065020/http://www.isiweb.ee.ethz.ch/archive/massey_pub/pdf/BI633.pdf |url-status=live }}</ref> A password with 42 bits of entropy  would be as strong as a string of 42 bits chosen randomly, for example by a [[Fair coin|fair coin]] toss. Put another way, a password with  42 bits of entropy would require 2<sup>42</sup> (4,398,046,511,104) attempts to exhaust all possibilities during a brute force search. Thus, increasing the entropy of the password by one bit doubles the number of guesses required, making an attacker's task twice as difficult. On average, an attacker will have to try half the possible number of passwords before finding the correct one.<ref name=NIST />

=== Random passwords ===
{{Main|Random password generator}}

Random passwords consist of a string of symbols of specified length taken from some set of symbols using a random selection process in which each symbol is equally likely to be selected. The symbols can be individual characters from a character set (e.g., the [[ASCII]] character set), syllables designed to form pronounceable passwords or even words from a word list (thus forming a [[Passphrase|passphrase]]).

The strength of random passwords depends on the actual entropy of the underlying number generator; however, these are often not truly random, but pseudorandom. Many publicly available password generators use random number generators found in programming libraries that offer limited entropy. However, most modern operating systems offer cryptographically strong random number generators that are suitable for password generation. It is also possible to use ordinary dice to generate random passwords {{crossreference|(see {{section link|Random password generator|Stronger methods}})}}. Random password programs often can ensure that the resulting password complies with a local [[Password policy|password policy]]; for instance, by always producing a mix of letters, numbers, and special characters.

For passwords generated by a process that randomly selects a string of symbols of length, ''L'', from a set of ''N'' possible symbols, the number of possible passwords can be found by raising the number of symbols to the power ''L'', i.e. ''N''<sup>''L''</sup>. Increasing either ''L'' or ''N'' will strengthen the generated password. The strength of a random password as measured by the [[Information entropy|information entropy]] is just the [[Binary logarithm|base-2 logarithm]] or log<sub>2</sub> of the number of possible passwords, assuming each symbol in the password is produced independently. Thus a random password's information entropy, ''H'', is given by the formula:

{{align|center|
<math>H = \log_2 N^L = L\log_2 N = L {\log N \over \log 2}</math>
}}

where ''N'' is the number of possible symbols and ''L'' is the number of symbols in the password. ''H'' is measured in [[Bit|bit]]s.<ref name=NIST/><ref>Schneier, B: ''Applied Cryptography'', 2e, page 233 ff. John Wiley and Sons.</ref> In the last expression, ''log'' can be to any base.

:

A [[Binary number|binary]] [[Byte|byte]] is usually expressed using two hexadecimal characters.

To find the length, ''L,'' needed to achieve a desired strength ''H,'' with a password drawn randomly from a set of ''N'' symbols, one computes:

<math>L = {\left \lceil \frac {H}{\log_2 N} \right \rceil}</math>

where <math> \left \lceil \ \right \rceil </math> denotes the mathematical [[Floor and ceiling functions|ceiling function]], ''i.e.'' rounding up to the next largest [[Natural number|whole number]].

=== Human-generated passwords ===
People are notoriously poor at achieving sufficient entropy to produce satisfactory passwords. According to one study involving half a million users, the average password entropy was estimated at 40.54 bits.<ref>{{cite book|last1=Florencio|first1=Dinei|last2=Herley|first2=Cormac|title=Proceedings of the 16th international conference on World Wide Web |chapter=A large-scale study of web password habits |date=May 8, 2007|page=657|doi=10.1145/1242572.1242661|isbn=9781595936547|s2cid=10648989|chapter-url=http://research.microsoft.com/pubs/74164/www2007.pdf|ref=ACM 978-1-59593-654-7/07/0005.|url-status=live|archive-url=https://web.archive.org/web/20150327031521/http://research.microsoft.com/pubs/74164/www2007.pdf|archive-date=March 27, 2015}}</ref>

Thus, in one analysis of over 3 million eight-character passwords, the letter "e" was used over 1.5 million times, while the letter "f" was used only 250,000 times. A uniform distribution would have had each character being used about 900,000 times. The most common number used is "1", whereas the most common letters are a, e, o, and r.<ref name=perfect />

Users rarely make full use of larger character sets in forming passwords. For example, hacking results obtained from a MySpace phishing scheme in 2006 revealed 34,000 passwords, of which only 8.3% used mixed case, numbers, and symbols.<ref name=myspace-passwords>{{cite news | url = http://archive.wired.com/politics/security/commentary/securitymatters/2006/12/72300?currentPage=all | title = MySpace Passwords aren't so Dumb | author = Bruce Schneier | publisher = Wired Magazine | access-date = April 11, 2008 | date = December 14, 2006 | url-status = live | archive-url = https://web.archive.org/web/20140521031354/http://archive.wired.com/politics/security/commentary/securitymatters/2006/12/72300?currentPage=all | archive-date = May 21, 2014 }}</ref>

The full strength associated with using the entire ASCII character set (numerals, mixed case letters, and special characters) is only achieved if each possible password is equally likely.  This seems to suggest that all passwords must contain characters from each of several character classes, perhaps upper and lower-case letters, numbers, and non-alphanumeric characters.  Such a requirement is a pattern in password choice and can be expected to reduce an attacker's "work factor" (in Claude Shannon's terms).  This is a reduction in password "strength".  A better requirement would be to require a password ''not'' to contain any word in an online dictionary, or list of names, or any license plate pattern from any state (in the US) or country (as in the EU). If patterned choices are required, humans are likely to use them in predictable ways, such as capitalizing a letter, adding one or two numbers, and a special character. This predictability means that the increase in password strength is minor when compared to random passwords.

'''Password Safety Awareness Projects'''

Google developed the online interactive game Interland to teach children online safety, covering topics like cyber bullying, basic hacking, and information security. On the chapter called ''Tower Of Tresure, for example,'' it is advised to use unusual names paired with characters like (₺&@#%) with a game.<ref>{{Cite web |title=Play Interland - Be Internet Awesome |url=https://beinternetawesome.withgoogle.com/en_us/interland/ |access-date=2024-09-10 |website=Play Interland - Be Internet Awesome |language=en-us}}</ref>

==== NIST Special Publication 800-63-2 ====
NIST Special Publication 800-63 of June 2004 (revision two) suggested a scheme to approximate the entropy of human-generated passwords:<ref name=NIST>{{cite web | url = http://csrc.nist.gov/publications/nistpubs/800-63/SP800-63v6_3_3.pdf | title = SP 800-63 – Electronic Authentication Guideline | publisher = NIST | access-date = April 20, 2014 | url-status = dead | archive-url = https://web.archive.org/web/20040712152833/http://csrc.nist.gov/publications/nistpubs/800-63/SP800-63v6_3_3.pdf | archive-date = July 12, 2004 }}</ref>

Using this scheme, an eight-character human-selected password without uppercase characters and non-alphabetic characters OR with either but of the two character sets is estimated to have eighteen bits of entropy. The NIST publication concedes that at the time of development, little information was available on the real-world selection of passwords.   Later research into human-selected password entropy using newly available real-world data has demonstrated that the NIST scheme does not provide a valid metric for entropy estimation of human-selected passwords.<ref name=WeirEtAl>{{cite web | url = http://reusablesec.blogspot.com/2010/10/new-paper-on-password-security-metrics.html | title = Testing Metrics for Password Creation Policies by Attacking Large Sets of Revealed Passwords | format = PDF | author1 = Matt Weir | author2 = Susdhir Aggarwal | author3 = Michael Collins | author4 = Henry Stern | date = 7 October 2010 | access-date = March 21, 2012 | url-status = live | archive-url = https://web.archive.org/web/20120706124704/http://reusablesec.blogspot.com/2010/10/new-paper-on-password-security-metrics.html | archive-date = July 6, 2012 }}</ref> The June 2017 revision of SP 800-63 (Revision three) drops this approach.<ref>{{cite web | url = https://pages.nist.gov/800-63-3 | title = SP 800-63-3 – Digital Identity Guidelines | format = PDF | publisher = NIST | date = June 2017 | access-date = August 6, 2017 | url-status = live | archive-url = https://web.archive.org/web/20170806142240/https://pages.nist.gov/800-63-3/ | archive-date = August 6, 2017 }}</ref>

===Usability and implementation considerations===
Because national keyboard implementations vary, not all 94 ASCII printable characters can be used everywhere.<!-- The situation of assorted 7-bit ASCII character sets is long and sordid, quite indeterminate, and a dog's breakfast. The advent of Unicode / UCS has at least 'settled' what the lowest 128 characters are, though this is not exactly what most of the prior character sets had. --> This can present a problem to an international traveler who wished to log into a remote system using a keyboard on a local computer {{crossreference|(see article concerned with keyboard layouts)}}. Many handheld devices, such as [[Engineering:Tablet computer|tablet computer]]s and smart phones, require complex shift sequences or keyboard app swapping to enter special characters.

Authentication programs can vary as to the list of allowable password characters. Some do not recognize case differences (e.g., the upper-case "E" is considered equivalent to the lower-case "e"), and others prohibit some of the other symbols. In the past few decades, systems have permitted more characters in passwords, but limitations still exist. Systems also vary as to the maximum length of passwords allowed.

As a practical matter, passwords must be both reasonable and functional for the end user as well as strong enough for the intended purpose. Passwords that are too difficult to remember, an issue compounded by password complexity requirements, may be forgotten and so are more likely to be written on paper, which is a security risk if the paper is improperly secured.<ref name=Gartner>{{cite web | url = http://www.indevis.de/dokumente/gartner_passwords_breakpoint.pdf | title = Passwords are Near the Breaking Point | publisher = Gartner | author = A. Allan | access-date = April 10, 2008 | url-status = dead | archive-url = https://web.archive.org/web/20060427032938/http://www.indevis.de/dokumente/gartner_passwords_breakpoint.pdf | archive-date = April 27, 2006 }}</ref><ref name=ACMInteractions2009>{{cite journal |title=When Security Gets in the Way |journal=ACM Interactions |date=2009-11-01 |last=Norman |first=Donald |volume=16 |issue=6 |pages=60–63 |doi=10.1145/1620693.1620708 |quote=Passwords illustrate several of the difficulties with current methods of enforcing security. Passwords are the least expensive mechanism known for securing systems. But onerous password requirements reduce security and increase costs. They reduce security because many people either use trivial algorithms to generate memorable passwords or write down their passwords and store them in easily discoverable places. They increase costs because help desks must be staffed to handle the large number of users who have forgotten their passwords and can no longer log in. These requirements keep out the good guys without deterring the bad guys. }}</ref><ref name="SP800-63B"/> In contrast, Bruce Schneier argues that forcing users to remember passwords without assistance can only accommodate weak passwords, thus posing a greater security risk, and most people are good at securing their wallets or purses, which is a "great place" to store a written password.<ref name=Schneier-writedown>{{cite web | url = http://www.schneier.com/blog/archives/2005/06/write_down_your.html | title = Schneier on Security | work = Write Down Your Password | author = Bruce Schneier | date = 17 June 2005 | access-date = April 10, 2008 | url-status = live | archive-url = https://web.archive.org/web/20080413032636/http://www.schneier.com/blog/archives/2005/06/write_down_your.html | archive-date = April 13, 2008 }}</ref>

== Required bits of entropy ==
The minimum number of bits of entropy needed for a password depends on the [[Threat model|threat model]] for the given application. To limit the effectiveness of brute-force attacks, the [[National Institute of Standards and Technology]] recommends system administrators ''always'' store passwords with a [[Key derivation function|key derivation function]] that performs some [[Social:Key stretching|key stretching]], as opposed to in [[Plaintext|plaintext]] or solely with a [[Cryptographic hash function|cryptographic hash function]].<ref name="SP800-63B"/> Other implementation details, such as limiting the number of incorrect password attempts, provide additional protection.<ref name="hotsec07"/> If such methods are not used, passwords with more entropy are needed. It also pays to bear in mind that since computing power continually grows, to prevent offline attacks the required number of bits of entropy should also increase over time.

RFC 4086, "Randomness Requirements for Security", published June 2005, presents some example threat models and how to calculate the entropy desired for each one.<ref>{{cite IETF | title = Randomness Requirements for Security | rfc = 4086}}</ref> Their answers vary between 29 bits of entropy needed if only online attacks are expected, and up to 96 bits of entropy needed for important cryptographic keys used in applications like encryption where the password or key needs to be secure for a long period and stretching isn't applicable. A [[Organization:USENIX|USENIX]] conference paper from 2007 wrote that, with the proper safeguards in place, 20 bits of entropy is sufficient to protect against practical online threats.<ref name="hotsec07">{{cite conference |last1=Florêncio |first1=Dinei |last2=Herley |first2=Cormac |last3=Coskun |first3=Baris |title=Do Strong Web Passwords Accomplish Anything? |url=https://www.usenix.org/legacy/event/hotsec07/tech/full_papers/florencio/florencio.pdf |conference=2nd USENIX Workshop on Hot Topics in Security |conference-url=https://www.usenix.org/legacy/events/hotsec07/ |place=Boston, Massachusetts |year=2007 }}</ref> A 2010 [[Organization:Georgia Tech Research Institute|Georgia Tech Research Institute]] study based on ''unstretched'' keys recommended 12 characters as a minimum length requirement for a random password.<ref name="gtri">{{cite web|url=http://www.gtri.gatech.edu/casestudy/Teraflop-Troubles-Power-Graphics-Processing-Units-GPUs-Password-Security-System|title=Teraflop Troubles: The Power of Graphics Processing Units May Threaten the World's Password Security System|publisher=[[Organization:Georgia Tech Research Institute|Georgia Tech Research Institute]]|access-date=2010-11-07|url-status=live|archive-url=https://web.archive.org/web/20101230063449/http://www.gtri.gatech.edu/casestudy/Teraflop-Troubles-Power-Graphics-Processing-Units-GPUs-Password-Security-System|archive-date=2010-12-30}}</ref><ref name="msnbc">{{cite news|url=http://www.nbcnews.com/id/38771772|archive-url=https://web.archive.org/web/20130711022009/http://www.nbcnews.com/id/38771772/|url-status=dead|archive-date=July 11, 2013|title=Want to deter hackers? Make your password longer|publisher=NBC News|date=2010-08-19|access-date=2010-11-07}}</ref>

The upper end of password entropy recommendations is related to the stringent requirements of choosing keys used in encryption. In 1999, [[EFF DES cracker|an Electronic Frontier Foundation project]] broke 56-bit [[Engineering:Data Encryption Standard|DES]] encryption in less than a day using specially designed hardware.<ref name=EFF-deep-crack>{{cite web | url = http://w2.eff.org/Privacy/Crypto/Crypto_misc/DESCracker/HTML/19980716_eff_descracker_pressrel.html | title = EFF DES Cracker machine brings honesty to crypto debate | publisher = EFF | access-date = March 27, 2008 | url-status = dead | archive-url = https://web.archive.org/web/20100101001853/http://w2.eff.org/Privacy/Crypto/Crypto_misc/DESCracker/HTML/19980716_eff_descracker_pressrel.html | archive-date = January 1, 2010 }}</ref> In 2002, ''[[Distributed.net|distributed.net]]'' cracked a 64-bit key in 4 years, 9 months, and 23 days.<ref name=distributed>{{cite web | url = http://stats.distributed.net/projects.php?project_id=5 | title = 64-bit key project status | publisher = Distributed.net | access-date = March 27, 2008 | archive-url = https://web.archive.org/web/20130910051812/http://stats.distributed.net/projects.php?project_id=5 | archive-date = September 10, 2013 | url-status = dead }}</ref> As of October 12, 2011, ''distributed.net'' estimates that cracking a 72-bit key using current hardware will take about 45,579 days or 124.8 years.<ref name=distributed-72>{{cite web | url = http://stats.distributed.net/projects.php?project_id=8 | title = 72-bit key project status | publisher = Distributed.net | access-date = October 12, 2011 | archive-date = October 9, 2018 | archive-url = https://web.archive.org/web/20181009090806/http://stats.distributed.net/projects.php?project_id=8 | url-status = live }}</ref> Due to currently understood limitations from fundamental physics, there is no expectation that any digital computer (or combination) will be capable of breaking 256-bit encryption via a brute-force attack.<ref name=schneier-cyptogram>{{cite web | url = http://www.schneier.com/crypto-gram-9902.html | title = Snakeoil: Warning Sign #5: Ridiculous key lengths | author = Bruce Schneier | access-date = March 27, 2008 | url-status = live | archive-url = https://web.archive.org/web/20080418225248/http://www.schneier.com/crypto-gram-9902.html | archive-date = April 18, 2008 }}</ref> Whether or not quantum computers will be able to do so in practice is still unknown, though theoretical analysis suggests such possibilities.<ref>{{cite web |url=https://stackoverflow.com/questions/2768807/quantum-computing-and-encryption-breaking |title=Quantum Computing and Encryption Breaking |publisher=Stack Overflow |date=2011-05-27 |access-date=2013-03-17 |url-status=live |archive-url=https://web.archive.org/web/20130521043721/http://stackoverflow.com/questions/2768807/quantum-computing-and-encryption-breaking |archive-date=2013-05-21 }}</ref>

== Guidelines for strong passwords ==
{{how-to|date=January 2022}}

=== Common guidelines ===
Guidelines for choosing good passwords are typically designed to make passwords harder to discover by intelligent guessing. Common guidelines advocated by proponents of software system security have included:<ref>Microsoft Corporation, [http://www.microsoft.com/protect/yourself/password/create.mspx Strong passwords: How to create and use them] {{webarchive|url=https://web.archive.org/web/20080101132156/http://www.microsoft.com/protect/yourself/password/create.mspx |date=2008-01-01 }}</ref><ref name="schneier07">Bruce Schneier, [http://www.schneier.com/blog/archives/2007/01/choosing_secure.html Choosing Secure Passwords] {{webarchive|url=https://web.archive.org/web/20080223002450/http://www.schneier.com/blog/archives/2007/01/choosing_secure.html |date=2008-02-23 }}</ref><ref>Google, Inc., [https://www.google.com/accounts/PasswordHelp How safe is your password?] {{webarchive|url=https://web.archive.org/web/20080222225549/https://www.google.com/accounts/PasswordHelp |date=2008-02-22 }}</ref><ref name="UMD01">University of Maryland, [http://www.cs.umd.edu/faq/Passwords.shtml Choosing a Good Password] {{webarchive|url=https://web.archive.org/web/20140614022254/http://www.cs.umd.edu/faq/Passwords.shtml |date=2014-06-14 }}</ref><ref name="Bidwell000">{{cite book
| first     = Teri
| last      = Bidwell
| title     = Hack Proofing Your Identity in the Information Age
| url     = https://archive.org/details/hackproofingyour0000bidw
| url-access = registration
| publisher = Syngress Publishing
| year      = 2002
| isbn      = 978-1-931836-51-7
}}</ref>
{{pb}}
* Consider a minimum password length of 8<ref>{{cite web |title=NIST PASSWORD GUIDELINES IN 2020 |date=18 August 2020 |url=https://stealthbits.com/blog/nist-password-guidelines/#:~:text=NIST%20now%20requires%20that%20all,characters%20as%20a%20maximum%20length. |publisher=Stealthbits |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517103234/https://stealthbits.com/blog/nist-password-guidelines/#:~:text=NIST%20now%20requires%20that%20all,characters%20as%20a%20maximum%20length. |url-status=live }}</ref> characters as a general guide. Both the US and UK cyber security departments recommend long and easily memorable passwords over short complex ones.<ref>{{cite web|title=Password Policy - Updating your approach|url=https://www.ncsc.gov.uk/collection/passwords/updating-your-approach|access-date=17 May 2021|publisher=UK National Cyber Security Centre|archive-date=17 May 2021|archive-url=https://web.archive.org/web/20210517100330/https://www.ncsc.gov.uk/collection/passwords/updating-your-approach|url-status=live}}</ref><ref>{{Cite web|title=Choosing and Protecting Passwords|url=https://www.cisa.gov/news-events/news/choosing-and-protecting-passwords|date=2019-11-18|access-date=2023-10-10|publisher=US Cybersecurity & Infrastructure Security Agency (CISA)}}</ref> 
* Generate passwords randomly where feasible.
* Avoid using the same password twice (e.g. across multiple user accounts and/or software systems).
* Avoid character repetition, keyboard patterns, dictionary words, and sequential letters or numbers.
* Avoid using information that is or might become publicly associated with the user or the account, such as the user name, ancestors' names, or dates.
* Avoid using information that the user's colleagues and/or acquaintances might know to be associated with the user, such as relatives or pet names, romantic links (current or past), and biographical information (e.g. ID numbers, ancestors' names or dates).
* Do not use passwords that consist wholly of any simple combination of the aforementioned weak components.
{{np}}

==== Length ====
A longer password is more secure. Most [[Cybersecurity engineering|cybersecurity guidelines]] suggest a minimum of 12–16 characters. Longer passphrases are more difficult for attackers to guess or break with automated software.

==== Complexity ====

* A good password will have a combination of:
* Uppercase and lowercase letters
* Numbers
* Special characters (e.g., ! @ # $ % ^ & *)

But complexity must not be at the expense of memorability. Random character strings are secure but tricky to remember; [[Passphrase|passphrases]] can offer a way to find a balance between complexity and use.

==== Uniqueness ====
Every account must have a distinct password. The use of passwords for many different accounts raises the risk of compromise if one service is attacked.{{pb}}
Forcing the inclusion of lowercase letters, uppercase letters, numbers, and symbols in passwords was a common policy but has been found to decrease security, by making it easier to crack. Research has shown how predictable the common use of such symbols are, and the US<ref name="SP800-63B">{{cite web |last1=Grassi |first1=Paul A. |last2=Fenton |first2=James L. |last3=Newton |first3=Elaine M. |last4=Perlner |first4=Ray A. |last5=Regenscheid |first5=Andrew R. |last6=Burr |first6=William E. |last7=Richer |first7=Justin P. |last8=Lefkovitz |first8=Naomi B. |last9=Danker |first9=Jamie M. |last10=Choong |first10=Yee-Yin |last11=Greene |first11=Kristen K. |last12=Theofanos |first12=Mary F. |title=Special Publication 800-63B - Digital Identity Guidelines |date=2017 |url=https://pages.nist.gov/800-63-3/sp800-63b.html#a3-complexity |publisher=National Institute for Standards and Technology |access-date=17 May 2021 |archive-date=21 April 2017 |archive-url=https://web.archive.org/web/20170421121654/https://pages.nist.gov/800-63-3/sp800-63b.html#a3-complexity |url-status=live |doi=10.6028/NIST.SP.800-63b }}</ref> and UK<ref>{{cite web |title=Password administration for system owners |url=https://www.ncsc.gov.uk/collection/passwords/updating-your-approach |publisher=UK National Cyber Security Centre |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517100330/https://www.ncsc.gov.uk/collection/passwords/updating-your-approach |url-status=live }}</ref> government cyber security departments advise against forcing their inclusion in password policy. Complex symbols also make remembering passwords much harder, which increases writing down, password resets, and password reuse, all of which lower rather than improve password security.<ref name=ACMInteractions2009 /> The original author of password complexity rules, Bill Burr, has apologized and admits they decrease security, as research has found; this was widely reported in the media in 2017.<ref name="tesla.tours">{{cite web |title=Password Rules - Founder of Password Complexity Says SORRY! |url=https://www.tesla.tours/campaigns/password-rules#h.8jxqtu8i7po2 |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094441/https://www.tesla.tours/campaigns/password-rules#h.8jxqtu8i7po2 |url-status=live }}</ref> Online security researchers<ref>{{cite web |title=CyLab Usable Privacy and Security Laboratory (CUPS) |url=https://cups.cs.cmu.edu/passwords.html |publisher=Carnegie Mellon University (USA) |access-date=17 May 2021}}</ref> and consultants are also supportive of the change<ref>{{cite web |last1=Bruce |first1=Schneier |title=Changes in Password Best Practices |date=10 October 2017 |url=https://www.schneier.com/blog/archives/2017/10/changes_in_pass.html |publisher=Schneier on Security |access-date=17 May 2021 |archive-date=13 May 2021 |archive-url=https://web.archive.org/web/20210513055046/https://www.schneier.com/blog/archives/2017/10/changes_in_pass.html |url-status=live }}</ref> in best practice advice on passwords. Despite this, complexity requirements in password policies remain widespread: a 2023 study of 20,000 websites found that about 20% still require passwords to contain characters from at least two character classes.<ref>{{cite conference |last1=Alroomi |first1=Suood |last2=Li |first2=Frank |title=Measuring Website Password Creation Policies At Scale |conference=Proceedings of the 2023 ACM SIGSAC Conference on Computer and Communications Security |publisher=Association for Computing Machinery |year=2023 |pages=3108–3122 |doi=10.1145/3576915.3623156 |isbn=9798400700507 |arxiv=2309.03384 }}</ref>

Some guidelines advise against writing passwords down, while others, noting the large numbers of password-protected systems users must access, encourage writing down passwords as long as the written password lists are kept in a safe place, not attached to a monitor or in an unlocked desk drawer.<ref name=Schneier-writedown/> Use of a [[Password manager|password manager]] is recommended by the NCSC.<ref name="National Cyber Security Centre">{{cite web|url=https://www.ncsc.gov.uk/blog-post/what-does-ncsc-think-password-managers|title=What does the NCSC think of password managers?|website=www.ncsc.gov.uk|url-status=live|archive-url=https://web.archive.org/web/20190305053922/https://www.ncsc.gov.uk/blog-post/what-does-ncsc-think-password-managers|archive-date=2019-03-05}}</ref>

The possible character set for a password can be constrained by different websites or by the range of keyboards on which the password must be entered.<ref>e.g. for a keyboard with only 17 nonalphanumeric characters, see one for a BlackBerry phone in [http://www.hardwaresecrets.com/fullimage.php?image=18705 an enlarged image]  {{webarchive|url=https://web.archive.org/web/20110406121058/http://www.hardwaresecrets.com/fullimage.php?image=18705 |date=2011-04-06 }} in support of [http://www.hardwaresecrets.com/article/795/2 Sandy Berger, ''BlackBerry Tour 9630 (Verizon) Cell Phone Review'', in Hardware Secrets (August 31, 2009)] {{webarchive|url=https://web.archive.org/web/20110406121111/http://www.hardwaresecrets.com/article/795/2 |date=April 6, 2011 }}, both as accessed January 19, 2010. That some websites don’t allow nonalphanumerics is indicated by [http://forums.theregister.co.uk/post/527230 Kanhef, ''Idiots, For Different Reasons'' (June 30, 2009) (topic post)] {{webarchive|url=https://web.archive.org/web/20110406121058/http://forums.theregister.co.uk/post/527230 |date=April 6, 2011 }}, as accessed January 20, 2010.</ref>

=== Examples of weak passwords ===
{{See also|Password cracking|List of the most common passwords}}

As with any security measure, passwords vary in strength; some are weaker than others. For example, the difference in strength between a dictionary word and a word with obfuscation ({{em|e.g.}} letters in the password are substituted by, say, {{nowrap|numbers{{px2}}{{mdash}}{{px2}}}}‌a common approach) may cost a password-cracking device a few more seconds; this adds little strength. The examples below illustrate various ways weak passwords might be constructed, all of which are based on simple patterns which result in extremely low entropy, allowing them to be tested automatically at high speeds:<ref name=perfect>{{cite book
  | last = Burnett
  | first = Mark
  | editor = Kleiman, Dave
  | title = Perfect Passwords
  | publisher = Syngress Publishing
  | year = 2006
  | location = Rockland, Massachusetts
  | isbn = 978-1-59749-041-2
  | page = 181
* [[Default password]]s (as supplied by the system vendor and meant to be changed at installation time): ''password'', ''default'', ''admin'', ''guest'', etc. Lists of default passwords are widely available on the internet.
* Reused passwords: Passwords should be unique to a particular account. Altering reused passwords, such as changing a few letters or numbers, does not provide sufficient security.
* Dictionary words: ''chameleon'', ''RedSox'', ''sandbags'', ''bunnyhop!'', ''IntenseCrabtree'', etc., including words in non-English dictionaries.
* Words with numbers appended: ''password1'', ''deer2000'', ''john1234'', etc., can be easily tested automatically with little lost time.
* [[Munged password]]s (words with simple obfuscation): ''p@ssw0rd'', ''l33th4x0r'', ''g0ldf1sh'', etc., can be tested automatically with little additional effort. For example, a domain administrator password compromised in the DigiNotar attack was reportedly ''Pr0d@dm1n.''<ref>{{cite web |url=http://thehackernews.com/2011/09/comodohacker-responsible-for-diginotar.html |title=ComodoHacker responsible for DigiNotar Attack – Hacking News |publisher=Thehackernews.com |date=2011-09-06 |access-date=2013-03-17 |url-status=live |archive-url=https://web.archive.org/web/20130517204022/http://thehackernews.com/2011/09/comodohacker-responsible-for-diginotar.html |archive-date=2013-05-17 }}</ref>
* Doubled words: ''crabcrab'', ''stopstop'', ''treetree'', ''passpass'', etc.
* Common sequences from a keyboard row: ''qwerty'', ''123456'', ''asdfgh'', etc. including diagonal or backward sequences (qazplm, ytrewq, etc).
* Numeric sequences based on well known numbers such as 911 <sup>(9-1-1, [[Religion:September 11 attacks|9/11]])</sup>, 314159... <sup>([[Pi|pi]])</sup>, 27182... <sup>([[E (mathematical constant)|e]])</sup>, 112 <sup>(1-1-2)</sup>, etc.
* Identifiers: ''jsmith123'', ''1/1/1970'', ''555–1234'', one's username, etc.
* Weak passwords in non-English languages, such as contraseña (Spanish) and ji32k7au4a83 (bopomofo keyboard encoding from Chinese)<ref>{{cite news|url=https://www.iheart.com/content/2019-03-08-heres-why-ji32k7au4a83-is-a-surprisingly-common-password/|title=Here's Why 'ji32k7au4a83' Is A Surprisingly Common Password|access-date=25 March 2019|date=8 March 2019|author=Dave Basner|archive-date=25 March 2019|archive-url=https://web.archive.org/web/20190325154357/https://www.iheart.com/content/2019-03-08-heres-why-ji32k7au4a83-is-a-surprisingly-common-password/|url-status=live}}</ref>
* Anything personally related to an individual: license plate number, Social Security number, current or past telephone numbers, student ID, current address, previous addresses, birthday, sports team, relative or pet names (or nicknames, birthdays, initials, etc.) can easily be tested automatically after a simple investigation of a person's details.
* Dates: dates follow a pattern and make your password weak.
* Names of well-known locations: New York, Texas, China, London, etc.
* Names of brands, celebrities, sports teams, musical groups, TV shows, movies, etc.
* Short passwords: Even if a password doesn't have any of the weaknesses listed above, if it is too short, it can be easily cracked.
{{np}}{{pb}}
There are many other ways a password can be weak,<ref>Bidwell, p. 87</ref> corresponding to the strengths of various attack schemes; the core principle is that a password should have high entropy (usually taken to be equivalent to randomness) and ''not'' be readily derivable by any "clever" pattern, nor should passwords be mixed with information identifying the user. Online services often provide a restore password function that a hacker can figure out and by doing so bypass a password.

=== Rethinking password change guidelines ===

In the landscape of 2012, as delineated by William Cheswick in an article for ACM magazine, password security predominantly emphasized an alpha-numeric password of eight characters or more. Such a password, it was deduced, could resist ten million attempts per second for a duration of 252 days. However, with the assistance of contemporary GPUs at the time, this period was truncated to just about 9 hours, given a cracking rate of 7 billion attempts per second. A 13-character password was estimated to withstand GPU-computed attempts for over 900,000 years.<ref>{{Cite web|url=https://queue.acm.org/detail.cfm?id=2422416|title=HTML version - Rethinking Passwords|last=William|first=Cheswick|date=2012-12-31|website=[[Organization:Association for Computing Machinery|Association for Computing Machinery]] (ACM)|url-status=live|archive-url=https://archive.today/20191103172648/https://queue.acm.org/detail.cfm?id=2422416|archive-date=2019-11-03|access-date=2019-11-03}}</ref><ref>{{Cite journal|title=ACM Digital Library - Rethinking Passwords|last=William|first=Cheswick|date=2012-12-31|journal=ACM Queue|volume=10|issue=12|pages=50–56|doi=10.1145/2405116.2422416|doi-access=free}}</ref>

In the context of 2023 hardware technology, the 2012 standard of an eight-character alpha-numeric password has become vulnerable, succumbing in a few hours. The time needed to crack a 13-character password is reduced to a few years. The current emphasis, thus, has shifted. Password strength is now gauged not just by its complexity but its length, with recommendations leaning towards passwords comprising at least 13-16 characters. This era has also seen the rise of Multi-Factor Authentication (MFA) as a crucial fortification measure. The advent and widespread adoption of password managers have further aided users in cultivating and maintaining an array of strong, unique passwords.<ref>{{Cite web |title=The State of Password Security 2023 Report {{!}} Bitwarden Resources |url=https://bitwarden.com/resources/the-state-of-password-security/ |access-date=2023-09-24 |website=Bitwarden |language=en-US |archive-date=2023-10-11 |archive-url=https://web.archive.org/web/20231011012838/https://bitwarden.com/resources/the-state-of-password-security/ |url-status=live }}</ref>

== Password policy ==
{{Main|Password policy}}

A password policy is a guide to choosing satisfactory passwords. It is intended to:
{{pb}}
* assist users in choosing strong passwords
* ensure the passwords are suited to the target population
* Provide recommendations for users concerning the handling of their passwords
* impose a recommendation to change any password which has been lost or suspected of compromise
* use a [[Blacklist (computing)#Usernames and passwords|password blacklist]] to block the use of weak or easily guessed passwords.
{{np}}{{pb}}
Previous password policies used to prescribe the characters which passwords must contain, such as numbers, symbols, or upper/lower case. While this is still in use, it has been debunked as less secure by university research,<ref>{{cite web |title=Practical Recommendations for Stronger, More Usable Passwords Combining Minimum-strength, Minimum-length, and Blocklist Requirements |url=https://www.andrew.cmu.edu/user/nicolasc/publications/Tan-CCS20.pdf |publisher=Carnegie Mellon University |access-date=17 May 2021}}</ref> by the original instigator<ref>{{cite web |title=Bill Burr, Founder of Password complexity rules says SORRY! |url=https://www.tesla.tours/campaigns/password-rules#h.8jxqtu8i7po2 |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094441/https://www.tesla.tours/campaigns/password-rules#h.8jxqtu8i7po2 |url-status=live }}</ref> of this policy, and by the cyber security departments (and other related government security bodies<ref>{{cite web |title=Passwords in online services |url=https://ico.org.uk/for-organisations/guide-to-data-protection/guide-to-the-general-data-protection-regulation-gdpr/security/passwords-in-online-services/#whatrequirementsshould |publisher=UK Information Commissioner's Office (ICO) |access-date=17 May 2021 |archive-date=4 May 2021 |archive-url=https://web.archive.org/web/20210504043052/https://ico.org.uk/for-organisations/guide-to-data-protection/guide-to-the-general-data-protection-regulation-gdpr/security/passwords-in-online-services/#whatrequirementsshould |url-status=dead }}</ref>) of USA<ref name="SP800-63B"/> and UK.<ref>{{cite web |title=Password guidance |url=https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/458857/Password_guidance_-_simplifying_your_approach.pdf |publisher=Cyber Security, UK Government Communications Headquarters |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094441/https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/458857/Password_guidance_-_simplifying_your_approach.pdf |url-status=live }}</ref> Password complexity rules of enforced symbols were previously used by major platforms such as Google<ref>{{cite web |title=Create a Strong Password |url=https://support.google.com/accounts/answer/32040?hl=en#:~:text=Meet%20password%20requirements,accented%20characters%20aren't%20supported. |publisher=Google Inc. |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094441/https://support.google.com/accounts/answer/32040?hl=en#:~:text=Meet%20password%20requirements,accented%20characters%20aren't%20supported. |url-status=live }}</ref> and Facebook,<ref>{{cite web |title=Login and Password Help |url=https://www.facebook.com/help/1573156092981768/ |publisher=FaceBook Inc |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094439/https://www.facebook.com/help/1573156092981768/ |url-status=live }}</ref> but these have removed the requirement following the discovery that they actually reduced security. This is because the human element is a far greater risk than cracking, and enforced complexity leads most users to highly predictable patterns (number at the end, swap 3 for E, etc.) which helps crack passwords. So password simplicity and length (passphrases) are the new best practice and complexity is discouraged. Forced complexity rules also increase support costs, and user friction and discourage user signups.

Password expiration was in some older password policies but has been debunked<ref name="tesla.tours"/> as best practice and is not supported by USA or UK governments, or Microsoft which removed<ref>{{cite web |title=Security baseline (FINAL) for Windows 10 v1903 and Windows Server v1903 |date=23 May 2019 |url=https://docs.microsoft.com/en-au/archive/blogs/secguide/security-baseline-final-for-windows-10-v1903-and-windows-server-v1903 |publisher=Microsoft |access-date=17 May 2021 |archive-date=17 May 2021 |archive-url=https://web.archive.org/web/20210517094440/https://docs.microsoft.com/en-au/archive/blogs/secguide/security-baseline-final-for-windows-10-v1903-and-windows-server-v1903 |url-status=live }}</ref> the password expiry feature. Password expiration was previously trying to serve two purposes:<ref name=LOPSA>{{cite web | url = http://lopsa.org/node/295 | title = In Defense of Password Expiration | publisher = League of Professional Systems Administrators | access-date = April 14, 2008 | url-status = dead | archive-url = https://web.archive.org/web/20081012063918/http://lopsa.org/node/295 | archive-date = October 12, 2008 }}</ref>{{np}}{{pb}}
* If the time to crack a password is estimated to be 100 days, password expiration times fewer than 100 days may help ensure insufficient time for an attacker.
* If a password has been compromised, requiring it to be changed regularly may limit the access time for the attacker.
{{np}}{{pb}}
However, password expiration has its drawbacks:<ref name=WEB>
{{cite web
 |url         = https://www.cesg.gov.uk/articles/problems-forcing-regular-password-expiry
 |title       = The problems with forcing regular password expiry
 |date        = 15 April 2016
 |work        = IA Matters
 |publisher   = CESG: the Information Security Arm of GCHQ
 |access-date  = 5 Aug 2016
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20160817223701/https://www.cesg.gov.uk/articles/problems-forcing-regular-password-expiry
 |archive-date = 17 August 2016
}}
</ref><ref name=CERIAS>{{cite web | url = http://www.cerias.purdue.edu/weblogs/spaf/general/post-30/ | title = Security Myths and Passwords | publisher = The Center for Education and Research in Information Assurance and Security | author = Eugene Spafford | access-date = April 14, 2008 | url-status = live | archive-url = https://web.archive.org/web/20080411123000/http://www.cerias.purdue.edu/weblogs/spaf/general/post-30/ | archive-date = April 11, 2008 }}</ref>{{pb}}
* Asking users to change passwords frequently encourages simple, weak passwords.
* If one has a truly strong password, there is little point in changing it. Changing passwords that are already strong introduces a risk that the new password may be less strong.
* A compromised password is likely to be used immediately by an attacker to install a [[Backdoor (computing)|backdoor]], often via [[Privilege escalation|privilege escalation]]. Once this is accomplished, password changes won't prevent future attackers from accessing them.

=== Creating and handling passwords ===
The hardest passwords to crack, for a given length and character set, are random character strings; if long enough they resist brute force attacks (because there are many characters) and guessing attacks (due to high entropy). However, such passwords are typically the hardest to remember. The imposition of a requirement for such passwords in a password policy may encourage users to write them down, store them in [[Engineering:Mobile device|mobile device]]s, or share them with others as a safeguard against memory failure. While some people consider each of these user resorts to increase security risks, others suggest the absurdity of expecting users to remember distinct complex passwords for each of the dozens of accounts they access. For example, in 2005, security expert Bruce Schneier recommended writing down one's password:

{{Blockquote|quote=Simply, people can no longer remember passwords good enough to reliably defend against dictionary attacks, and are much more secure if they choose a password too complicated to remember and then write it down. We're all good at securing small pieces of paper. I recommend that people write their passwords down on a small piece of paper, and keep it with their other valuable small pieces of paper: in their wallet.<ref name=Schneier-writedown/>}}

The following measures may increase acceptance of strong password requirements if carefully used:

* a training program. Also, updated training for those who fail to follow the password policy (lost passwords, inadequate passwords, etc.).
* rewarding strong password users by reducing the rate, or eliminating, the need for password changes (password expiration). The strength of user-chosen passwords can be estimated by automatic programs which inspect and evaluate proposed passwords when setting or changing a password.
* displaying to each user the last login date and time in the hope that the user may notice unauthorized access, suggesting a compromised password.
* allowing users to reset their passwords via an automatic system, which reduces help desk call volume. However, some systems are themselves insecure; for instance, easily guessed or researched answers to password reset questions bypass the advantages of a strong password system.
* using randomly generated passwords that do not allow users to choose their passwords, or at least offering randomly generated passwords as an option.

=== Memory techniques ===

Password policies sometimes suggest [[Biology:Memory improvement|memory technique]]s to assist remembering passwords:
* mnemonic passwords: Some users develop [[Biology:Mnemonic|mnemonic]] phrases and use them to generate more or less random passwords which are nevertheless relatively easy for the user to remember. For instance, the first letter of each word in a memorable phrase. Research estimates the password strength of such passwords to be about 3.7 bits per character, compared to the 6.6 bits for random passwords from ASCII printable characters.<ref>{{cite conference |url=https://www.internetsociety.org/sites/default/files/ndss2017_03A-4_Kiesel_paper.pdf |title=A Large-scale Analysis of the Mnemonic Password Advice |author=Johannes Kiesel |author2=Benno Stein |author3=Stefan Lucks |year=2017 |publisher=Internet Society |book-title=Proceedings of the 24th Annual Network and Distributed System Security Symposium (NDSS 17) |url-status=dead |archive-url=https://web.archive.org/web/20170330174637/https://www.internetsociety.org/sites/default/files/ndss2017_03A-4_Kiesel_paper.pdf |archive-date=2017-03-30 |access-date=2017-03-30 }}</ref> Silly ones are possibly more memorable.<ref>[http://uc.iupui.edu/uploadedFiles/Learning_Center_Site/Mnemonic%20Devices.pdf ''Mnemonic Devices'' (Indianapolis, Ind.: Bepko Learning Ctr., University College)], as accessed January 19, 2010 {{webarchive |url=https://web.archive.org/web/20100610000727/http://uc.iupui.edu/uploadedFiles/Learning_Center_Site/Mnemonic%20Devices.pdf |date=June 10, 2010 }}</ref> Another way to make random-appearing passwords more memorable is to use random words {{crossreference|(see: [[Diceware]])}} or syllables instead of randomly chosen letters.
* after-the-fact mnemonics: After the password has been established, invent a mnemonic that fits.<ref>[http://changingminds.org/techniques/memory/remembering_passwords.htm Remembering Passwords (ChangingMinds.org)] {{webarchive|url=http://archive.wikiwix.com/cache/20100121181700/http://changingminds.org/techniques/memory/remembering_passwords.htm |date=2010-01-21 }}, as accessed January 19, 2010</ref> It does not have to be reasonable or sensible, only memorable. This allows passwords to be random.
* visual representations of passwords: a password is memorized based on a sequence of keys pressed, not the values of the keys themselves, e.g. a sequence !qAsdE#2 represents a [[Rhomboid|rhomboid]] on a US keyboard. The method to produce such passwords is called PsychoPass.<ref name=":10">{{cite journal | last1 = Cipresso | first1 = P | last2 = Gaggioli | first2 = A | last3 = Serino | first3 = S | last4 = Cipresso | first4 = S | last5 = Riva | first5 = G | year = 2012 | title = How to Create Memorizable and Strong Passwords | journal = J Med Internet Res | volume = 14 | issue = 1| page = e10 | doi = 10.2196/jmir.1906 | pmid = 22233980 | pmc=3846346 | doi-access = free }}</ref> Passwords produced by this method are much weaker than their length suggests, since successive keys are not independent and common keyboard sequences are included in password dictionaries. But some improvements can be made.<ref>{{cite journal | pmc = 3742392 | pmid=23942458 | doi=10.2196/jmir.2366 | volume=15 | issue=8 | title=Security analysis and improvements to the PsychoPass method. | year=2013 | journal=J Med Internet Res | page=e161 | last1 = Brumen | first1 = B | last2 = Heričko | first2 = M | last3 = Rozman | first3 = I | last4 = Hölbl | first4 = M | doi-access=free }}</ref><ref>{{cite web|url=https://blogs.dropbox.com/tech/2012/04/zxcvbn-realistic-password-strength-estimation/|title=zxcvbn: realistic password strength estimation|website=Dropbox Tech Blog|url-status=live|archive-url=https://web.archive.org/web/20150405131234/https://blogs.dropbox.com/tech/2012/04/zxcvbn-realistic-password-strength-estimation/|archive-date=2015-04-05}}</ref>
* password patterns: Any pattern in a password makes guessing (automated or not) easier and reduces an attacker's work factor.

==Password managers==
{{Main|Password manager}}
A reasonable compromise for using large numbers of passwords is to record them in a password manager program, which include stand-alone applications, web browser extensions, or a manager built into the operating system. A password manager allows the user to use hundreds of different passwords, and only have to remember a single password, the one which opens the encrypted password database.<ref>{{Cite web |title=The Emperor's New Password Manager: Security Analysis of Web-based Password Managers {{!}} EECS at UC Berkeley |url=https://www2.eecs.berkeley.edu/Pubs/TechRpts/2014/EECS-2014-138.html |access-date=2023-10-01 |website=www2.eecs.berkeley.edu}}</ref> Needless to say, this single password should be strong and well-protected (not recorded anywhere). Most password managers can automatically create strong passwords using a cryptographically secure [[Random password generator|random password generator]], as well as calculating the entropy of the generated password. A good password manager will provide resistance against attacks such as key logging, clipboard logging and various other memory spying techniques.

==See also==

* [[Keystroke logging]]
* [[Passphrase]]
* [[Phishing]]
* [[Vulnerability (computing)]]

== References ==
{{reflist}}

== External links ==
* [https://tools.ietf.org/html/rfc4086 RFC 4086: Randomness Requirements for Security]
* [https://web.archive.org/web/20160416035311/http://www.architectingsecurity.com/2010/09/11/password-patterns/ Password Patterns:The next generation dictionary attacks]

{{DEFAULTSORT:Password Strength}}
[[Category:Cryptography]]
[[Category:Password authentication]]

{{Sourceattribution|Password strength}}