4 ms·

How did you find the entropy of these passwords? By my calculations 8 random characters from a set of 62 is 47.6 bits, not 36. (log(2,62)* 8 = 47.6) On a logari

by beala 14y ago

How did you find the entropy of these passwords? By my calculations 8 random characters from a set of 62 is 47.6 bits, not 36. (log(2,62)* 8 = 47.6) On a logarithmic scale, this is a huge difference. For the passphrase, 30 characters chosen from 73 is 185.7 bits (log(2,73)* 30). Even this isn't correct though, because the characters aren't randomly chosen. As the author says: "Not so for human-created passwords, where a sea of associations and language rules greatly reduces the search space." In other words, knowing that the first word is "I" limits the possibilities for the next word, and so on.

pixl97 14y ago
The site I used was http://rumkin.com/tools/password/passchk.php http://rumkin.com/tools/password/passchk.php How would an attacker know what the first letter is? Yes an attacker can guess if sentence passwords are used that (a|A),(the|The),and (i|I) would be very common starters but after that the bigger search space complicates things. https://www.grc.com/haystack.htm https://www.grc.com/haystack.htm Read the sections on entropy and padding. Just using sentences alone does present attackers the ability to use an encyclopedia attack, but a few randomly placed symbols (padding) turns the endeavor back in to an exhaustive search.
- beala 14y ago
  An attacker might be looking over your shoulder, and see the first characters of your password. Or he might read HN, and see that your preferred passphrase method is composed of english sentences with common substitutions. Maybe a website you're using leaks its password DB, and your "padding method" is exposed, reducing the search space back to what it was before the padding. I'm going to have to disagree with GRC here. "D0g.........." is a very bad password, especially if you use ".........." as your padding on every website. Even without this, common substitutions like '3' for 'e' or '!' or 'i' are accounted for in everything but the most naive cracking tools. Perhaps less likely, a timing attack in the auth system exposes the first few characters of your password, but not the rest. Why put up with these issues, when something like xkcd's 4 random words works well, is easy to remember and type, and calculates entropy in a uncontroversial way?
- sp332 14y ago
  Randomly chosen letters have fairly high entropy, about 4 or 5 bits per character. Written English has much, much lower entropy, generally around 1 bit per character. You can see this if you compress written text. Most compressors on normal English will get to 12-15% of the original size.
- DanBC 14y ago
  A diceware passphrase is much more secure than a regular English sentence. (http://world.std.com/~reinhold/diceware.html http://world.std.com/~reinhold/diceware.html) And substituting some symbols for letters doesn't add much security. Depending on the mode of attack there are precomputed sentences with a variety of substitution. Be careful with GRC. Some people don't have a high opinion of his analyses: (http://radsoft.net/news/roundups/grc/20060119,00.shtml http://radsoft.net/news/roundups/grc/20060119,00.shtml)

by beala 14y ago

pixl97 14y ago

The site I used was http://rumkin.com/tools/password/passchk.php http://rumkin.com/tools/password/passchk.php How would an attacker know what the first letter is? Yes an attacker can guess if sentence passwords are used that (a|A),(the|The),and (i|I) would be very common starters but after that the bigger search space complicates things. https://www.grc.com/haystack.htm https://www.grc.com/haystack.htm Read the sections on entropy and padding. Just using sentences alone does present attackers the ability to use an encyclopedia attack, but a few randomly placed symbols (padding) turns the endeavor back in to an exhaustive search.

beala 14y ago
An attacker might be looking over your shoulder, and see the first characters of your password. Or he might read HN, and see that your preferred passphrase method is composed of english sentences with common substitutions. Maybe a website you're using leaks its password DB, and your "padding method" is exposed, reducing the search space back to what it was before the padding. I'm going to have to disagree with GRC here. "D0g.........." is a very bad password, especially if you use ".........." as your padding on every website. Even without this, common substitutions like '3' for 'e' or '!' or 'i' are accounted for in everything but the most naive cracking tools. Perhaps less likely, a timing attack in the auth system exposes the first few characters of your password, but not the rest. Why put up with these issues, when something like xkcd's 4 random words works well, is easy to remember and type, and calculates entropy in a uncontroversial way?
sp332 14y ago
Randomly chosen letters have fairly high entropy, about 4 or 5 bits per character. Written English has much, much lower entropy, generally around 1 bit per character. You can see this if you compress written text. Most compressors on normal English will get to 12-15% of the original size.
DanBC 14y ago
A diceware passphrase is much more secure than a regular English sentence. (http://world.std.com/~reinhold/diceware.html http://world.std.com/~reinhold/diceware.html) And substituting some symbols for letters doesn't add much security. Depending on the mode of attack there are precomputed sentences with a variety of substitution. Be careful with GRC. Some people don't have a high opinion of his analyses: (http://radsoft.net/news/roundups/grc/20060119,00.shtml http://radsoft.net/news/roundups/grc/20060119,00.shtml)