Editing 936: Password Strength
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Another way of selecting a password is to have 2048 "symbols" (common words) and select only 4 of those symbols. log2(2048^4) = 44 bits, much better than 28. Using such symbols was again visited in one of the tips in [[1820: Security Advice]]. | Another way of selecting a password is to have 2048 "symbols" (common words) and select only 4 of those symbols. log2(2048^4) = 44 bits, much better than 28. Using such symbols was again visited in one of the tips in [[1820: Security Advice]]. | ||
| − | It is absolutely true that people make passwords hard to remember because they think they are "safer", and it is certainly true that length, all other things being equal, tends to make for very strong passwords and this can be confirmed by using [ | + | It is absolutely true that people make passwords hard to remember because they think they are "safer", and it is certainly true that length, all other things being equal, tends to make for very strong passwords and this can be confirmed by using [http://rumkin.com/tools/password/passchk.php rumkin.com's password strength checker]. Even if the individual characters are all limited to [a-z], the exponent implied in "we added another lowercase character, so multiply by 26 again" tends to dominate the results. |
In addition to being easier to remember, long strings of lowercase characters are also easier to type on smartphones and {{w|Virtual keyboard|soft keyboards}}. | In addition to being easier to remember, long strings of lowercase characters are also easier to type on smartphones and {{w|Virtual keyboard|soft keyboards}}. | ||
| − | xkcd's password generation scheme requires the user to have a list of 2048 common words (log<sub>2</sub>(2048) = 11). For any attack we must assume that the attacker knows our password generation algorithm, but not the exact password. In this case the attacker knows the 2048 words, and knows that we selected 4 words, but not which words. The number of combinations of 4 words from this list of words is (2<sup>11</sup>)<sup>4</sup> = 2<sup>44</sup>, i.e. 44 bits. For comparison, the [ | + | xkcd's password generation scheme requires the user to have a list of 2048 common words (log<sub>2</sub>(2048) = 11). For any attack we must assume that the attacker knows our password generation algorithm, but not the exact password. In this case the attacker knows the 2048 words, and knows that we selected 4 words, but not which words. The number of combinations of 4 words from this list of words is (2<sup>11</sup>)<sup>4</sup> = 2<sup>44</sup>, i.e. 44 bits. For comparison, the [http://world.std.com/~reinhold/dicewarefaq.html#calculatingentropy entropy offered by Diceware's 7776 word list is 13 bits per word]. If the attacker doesn't know the algorithm used, and only knows that lowercase letters are selected, the "common words" password would take even longer to crack than depicted. 25 ''random'' lowercase characters would have [http://www.wolframalpha.com/input/?i=log2%2826^25%29 117 bits of entropy], vs 44 bits for the common words list. |
;Example | ;Example | ||
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If you're confused, don't worry; you're in good company; even security "experts" don't understand the comic: | If you're confused, don't worry; you're in good company; even security "experts" don't understand the comic: | ||
| − | * Bruce Schneier thinks that dictionary attacks make this method "obsolete", despite the comic ''assuming'' perfect knowledge of the user's dictionary from the get-go. He advocates his own low-entropy "first letters of common plain English phrases" method instead: [https://www.schneier.com/blog/archives/2014/03/choosing_secure_1.html#:~:text=xkcd Schneier original article] and rebuttals: [https://web.archive.org/web/20160305001236/ | + | * Bruce Schneier thinks that dictionary attacks make this method "obsolete", despite the comic ''assuming'' perfect knowledge of the user's dictionary from the get-go. He advocates his own low-entropy "first letters of common plain English phrases" method instead: [https://www.schneier.com/blog/archives/2014/03/choosing_secure_1.html#:~:text=xkcd Schneier original article] and rebuttals: [https://web.archive.org/web/20160305001236/http://robinmessage.com/2014/03/why-bruce-schneier-is-wrong-about-passwords/ 1] [http://security.stackexchange.com/a/62881/10616 2] [http://www.reddit.com/r/technology/comments/1yxgqo/bruce_schneier_on_choosing_a_secure_password/cfp2z9k 3] [http://www.reddit.com/r/YouShouldKnow/comments/232uch/ysk_how_to_properly_choose_a_secure_password_the/cgte7lp 4] [http://www.reddit.com/r/YouShouldKnow/comments/232uch/ysk_how_to_properly_choose_a_secure_password_the/cgszp62 5] [http://www.reddit.com/r/YouShouldKnow/comments/232uch/ysk_how_to_properly_choose_a_secure_password_the/cgt6ohq 6] |
* Steve Gibson basically gets it, but calculates entropy incorrectly in order to promote his own method and upper-bound password-checking tool: [https://www.grc.com/sn/sn-313.htm#:~:text=math%20is%20wrong Steve Gibson Security Now transcript] and [https://subrabbit.wordpress.com/2011/08/26/how-much-entropy-in-that-password/ rebuttal] | * Steve Gibson basically gets it, but calculates entropy incorrectly in order to promote his own method and upper-bound password-checking tool: [https://www.grc.com/sn/sn-313.htm#:~:text=math%20is%20wrong Steve Gibson Security Now transcript] and [https://subrabbit.wordpress.com/2011/08/26/how-much-entropy-in-that-password/ rebuttal] | ||
* Computer security consultant Mark Burnett ''almost'' understands the comic, but then advocates adding numerals and other crud to make passphrases less memorable, which completely defeats the point (that it is human-friendly) in the first place: [https://web.archive.org/web/20150319220514/https://xato.net/passwords/analyzing-the-xkcd-comic/ Analyzing the XKCD Passphrase Comic] | * Computer security consultant Mark Burnett ''almost'' understands the comic, but then advocates adding numerals and other crud to make passphrases less memorable, which completely defeats the point (that it is human-friendly) in the first place: [https://web.archive.org/web/20150319220514/https://xato.net/passwords/analyzing-the-xkcd-comic/ Analyzing the XKCD Passphrase Comic] | ||
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==External links== | ==External links== | ||
*An [https://en.wikipedia.org/wiki/Request_for_Comments RFC], RFC7997 ''The Use of Non-ASCII Characters in RFCs'', uses "Correct Horse Battery Staple" in ''Table 3: A sample of legal passwords'' on page 10. [https://www.rfc-editor.org/rfc/pdfrfc/rfc7997.txt.pdf#page=10] | *An [https://en.wikipedia.org/wiki/Request_for_Comments RFC], RFC7997 ''The Use of Non-ASCII Characters in RFCs'', uses "Correct Horse Battery Staple" in ''Table 3: A sample of legal passwords'' on page 10. [https://www.rfc-editor.org/rfc/pdfrfc/rfc7997.txt.pdf#page=10] | ||
| − | *Some info was used from the highest voted answer given to the question of "how accurate is this XKCD comic" at StackExchange [ | + | *Some info was used from the highest voted answer given to the question of "how accurate is this XKCD comic" at StackExchange [http://security.stackexchange.com/questions/6095/xkcd-936-short-complex-password-or-long-dictionary-passphrase]. |
| − | *Similarly, a question of "how right this comic is" was made at AskMetaFilter [ | + | *Similarly, a question of "how right this comic is" was made at AskMetaFilter [http://ask.metafilter.com/193052/Oh-Randall-you-do-confound-me-so] and [[Randall]] responded [http://ask.metafilter.com/193052/Oh-Randall-you-do-confound-me-so#2779020 there]. |
*Also the Wikipedia article on '{{w|Passphrase}}' is useful. | *Also the Wikipedia article on '{{w|Passphrase}}' is useful. | ||
| − | *In case you missed it in the explanation, GRC's Steve Gibson has a fantastic page [https://www.grc.com/haystack.htm] about this (and may have prompted this comic, as his podcast [ | + | *In case you missed it in the explanation, GRC's Steve Gibson has a fantastic page [https://www.grc.com/haystack.htm] about this (and may have prompted this comic, as his podcast [http://www.grc.com/sn/sn-303.htm] about this was posted the month before this comic). |
| − | * This comic inspired [ | + | * This comic inspired [http://blog.acolyer.org/2015/10/29/how-to-memorize-a-random-60-bit-string/ How to memorize a random 60-bit string] scientific paper (link is to the article about paper, with paper itself linked) |
* [https://github.com/dropbox/zxcvbn zxcvbn password strength estimator] thanks this comic for the inspiration in acknowledgements. | * [https://github.com/dropbox/zxcvbn zxcvbn password strength estimator] thanks this comic for the inspiration in acknowledgements. | ||
| − | * CMU paper: [ | + | * CMU paper: [http://cups.cs.cmu.edu/soups/2012/proceedings/a7_Shay.pdf Correct horse battery staple: Exploring the usability of system-assigned passphrases] |
| − | * [ | + | * [http://research.microsoft.com/pubs/265143/Microsoft_Password_Guidance.pdf Microsoft Password Guidance] (page 8) |
| − | * [ | + | * [http://gizmodo.com/the-guy-who-invented-those-annoying-password-rules-now-1797643987 The Guy Who Invented Those Annoying Password Rules Now Regrets Wasting Your Time], August 8, 2017 (this comic is reproduced in the article). |
{{comic discussion}} | {{comic discussion}} | ||
