4 ms·

If you learn the exact length of a password, you gain say 3-4 bits of entropy? 4 bits if there are sixteen possible password lengths, all equally likely. 4 bit

If you learn the exact length of a password, you gain say 3-4 bits of entropy? 4 bits if there are sixteen possible password lengths, all equally likely. 4 bits is not nothing, and it makes most cracking attacks 16 times easier, but it's hopefully never going to be the difference between good security and bad.

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tyingq 7y ago
"never going to be the difference between good security and bad." True, though there's plenty of bad out there. Combined with dumb password rules, knowing the exact length could make things pretty easy. Like the US Global Entry password rules: https://raw.githubusercontent.com/dumb-password-rules/dumb-password-rules/master/screenshots/global-entry.png https://raw.githubusercontent.com/dumb-password-rules/dumb-p... 8-12 chars with so many rules that already reduce the search space.
- pintxo 7y ago
  They might as well just supply the list of allowed passwords /s
- dmurray 7y ago
  I don't think those rules reduce the entropy significantly, maybe 1 bit compared to a rule of "any character on a US keyboard, max length 12". You can try generating a random password with that rule and see if it passes the test - more than half of the time it will. The problem with overly restrictive password rules is not generally a lack of entropy. It's that people can't remember their passwords, which might be a bug or a feature.
  - tyingq 7y ago
    I'd like the see the math on that 1 bit reduction. There's 3 "at least one char must be" and the first char must be 0-9a-Z. If the password is 8 characters, half of them, at least, have a much reduced search space.
    - dmurray 7y ago
      There are 100 million more 12-character passwords than 8-character ones, so if you randomly choose from all 8-12 character passwords, it's pretty close to just saying "choose a 12-character password". Let's say the alphabet is 93 characters (lower case, upper case, numbers, and the 31 special characters listed). So consider a random 8- to 12-character string in that alphabet. 1% of the strings have fewer than 12 characters. Lots of them will be legal, but let's eliminate them all to be conservative. 31/93 of the strings start with an illegal character, so that eliminates 33% of the strings. About 10 * 1/97 * 1/97 will have three consecutive occurrences of the same character. That eliminates another 0.1%. (83/93) to the power of 11 will have no numbers. That eliminates 28% of the remainder, or 18% of the total. By the same logic, 1% will have no special symbol. A negligible amount have no letter. One password in a million might contain the user's ID (supposing the IDs are 7-digit strings - either way, it will be negligible). Likewise there aren't many random passwords which will be the user's previous password or contain his name. That eliminated 53% of random passwords, leaving 47% for a loss of very close to one bit of entropy. This analysis wasn't completely rigorous as some of the requirements overlap, but if you try it through simulation I'm sure the result will be between 0.7 and 1.7 bits lost.
- misnome 7y ago
  That looks saner than ours; The chosen password must: - Be at least 10 (ten) characters long (and must not be longer than 127 characters) - Contain at least 8 (eight) different characters, each of which is used only ONCE - Contain at least 2 letters (case sensitive) - Contain at least 2 numbers - Contain at least 2 of the symbols !”#$%&’()*+,-/:;<=>?@[\]^_`{}~¦ The chosen password must not: - Contain a £ (Pound) character - Have 3 of any repeating character next to each other - Spell a word of 4 letters or more listed in a common dictionary (Most 4 letter combinations) - Be an abbreviation associated with the company - Be based on your user name Unsurprisingly, almost everyone just memorises the initial random password given out, since it's provided on a nice piece of paper when you start. Thankfully there isn't also an expiry policy. The max length is large but that's basically meaningless when you actually need to remember it and it takes a while to come up with even a random password that fulfills these requirements.
  - BrandoElFollito 7y ago
    I work in information security for 25 years. This is the most surrealistic password policy I have seen. Fantastic awareness training material you have here (in the section "see, others have it worse than we do") A sad part is that it is virtually impossible to use a password manager to generate a new password.
- medmunds 7y ago
  While there are still plenty of sites out there with absurd password requirements, I believe this particular example is outdated. When I used it last year, US Global Entry had changed over to the login.gov authentication service. The new rules are “It must be at least 12 characters long and not be a commonly used password. That’s it!” https://login.gov/assets/img/help/4-choose-a-password.png https://login.gov/assets/img/help/4-choose-a-password.png (Login.gov implements the relatively recent NIST 800-63 standard, which prohibits a lot of the earlier password absurdity.)
  - tyingq 7y ago
    Oh, that's good news.
vbezhenar 7y ago
If I don't know length, I need to start with length 1 and so on. If I know that password have 96 bits length, I'll skip lengths from 1 to 95. It's 2^1 + 2^2 + ... + 2^95 = 2^96 - 2 passwords. I still need to try 2^96 passwords at most. So I saved 1 bit.
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tyingq 7y ago

"never going to be the difference between good security and bad." True, though there's plenty of bad out there. Combined with dumb password rules, knowing the exact length could make things pretty easy. Like the US Global Entry password rules: https://raw.githubusercontent.com/dumb-password-rules/dumb-password-rules/master/screenshots/global-entry.png https://raw.githubusercontent.com/dumb-password-rules/dumb-p... 8-12 chars with so many rules that already reduce the search space.

pintxo 7y ago
They might as well just supply the list of allowed passwords /s
dmurray 7y ago
I don't think those rules reduce the entropy significantly, maybe 1 bit compared to a rule of "any character on a US keyboard, max length 12". You can try generating a random password with that rule and see if it passes the test - more than half of the time it will. The problem with overly restrictive password rules is not generally a lack of entropy. It's that people can't remember their passwords, which might be a bug or a feature.
- tyingq 7y ago
  I'd like the see the math on that 1 bit reduction. There's 3 "at least one char must be" and the first char must be 0-9a-Z. If the password is 8 characters, half of them, at least, have a much reduced search space.
  - dmurray 7y ago
    There are 100 million more 12-character passwords than 8-character ones, so if you randomly choose from all 8-12 character passwords, it's pretty close to just saying "choose a 12-character password". Let's say the alphabet is 93 characters (lower case, upper case, numbers, and the 31 special characters listed). So consider a random 8- to 12-character string in that alphabet. 1% of the strings have fewer than 12 characters. Lots of them will be legal, but let's eliminate them all to be conservative. 31/93 of the strings start with an illegal character, so that eliminates 33% of the strings. About 10 * 1/97 * 1/97 will have three consecutive occurrences of the same character. That eliminates another 0.1%. (83/93) to the power of 11 will have no numbers. That eliminates 28% of the remainder, or 18% of the total. By the same logic, 1% will have no special symbol. A negligible amount have no letter. One password in a million might contain the user's ID (supposing the IDs are 7-digit strings - either way, it will be negligible). Likewise there aren't many random passwords which will be the user's previous password or contain his name. That eliminated 53% of random passwords, leaving 47% for a loss of very close to one bit of entropy. This analysis wasn't completely rigorous as some of the requirements overlap, but if you try it through simulation I'm sure the result will be between 0.7 and 1.7 bits lost.
misnome 7y ago
That looks saner than ours; The chosen password must: - Be at least 10 (ten) characters long (and must not be longer than 127 characters) - Contain at least 8 (eight) different characters, each of which is used only ONCE - Contain at least 2 letters (case sensitive) - Contain at least 2 numbers - Contain at least 2 of the symbols !”#$%&’()*+,-/:;<=>?@[\]^_`{}~¦ The chosen password must not: - Contain a £ (Pound) character - Have 3 of any repeating character next to each other - Spell a word of 4 letters or more listed in a common dictionary (Most 4 letter combinations) - Be an abbreviation associated with the company - Be based on your user name Unsurprisingly, almost everyone just memorises the initial random password given out, since it's provided on a nice piece of paper when you start. Thankfully there isn't also an expiry policy. The max length is large but that's basically meaningless when you actually need to remember it and it takes a while to come up with even a random password that fulfills these requirements.
- BrandoElFollito 7y ago
  I work in information security for 25 years. This is the most surrealistic password policy I have seen. Fantastic awareness training material you have here (in the section "see, others have it worse than we do") A sad part is that it is virtually impossible to use a password manager to generate a new password.
medmunds 7y ago
While there are still plenty of sites out there with absurd password requirements, I believe this particular example is outdated. When I used it last year, US Global Entry had changed over to the login.gov authentication service. The new rules are “It must be at least 12 characters long and not be a commonly used password. That’s it!” https://login.gov/assets/img/help/4-choose-a-password.png https://login.gov/assets/img/help/4-choose-a-password.png (Login.gov implements the relatively recent NIST 800-63 standard, which prohibits a lot of the earlier password absurdity.)
- tyingq 7y ago
  Oh, that's good news.

vbezhenar 7y ago

If I don't know length, I need to start with length 1 and so on. If I know that password have 96 bits length, I'll skip lengths from 1 to 95. It's 2^1 + 2^2 + ... + 2^95 = 2^96 - 2 passwords. I still need to try 2^96 passwords at most. So I saved 1 bit.

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