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Comment by criddell | Hacker News Reader

Can you point to a good debunking?

Here you go: https://security.stackexchange.com/questions/62832/is-the-of...

After reading through most of that, it seems that the big problem with the XKCD scheme is that it's often impractical. Too many sites restrict password length and so you are forced to use the Schneier modification to get more entropy per character.

If you make a password from an alphabet of 64 characters (upper and lower case, digits, and some special characters), there are 6 bits per character of entropy so a ten character random password could have 60 bits of entropy.

If you are choosing words from a list 4000 words long, there are 11 bits of entropy per word and so you would need to string together five random words to get a password of equal strength. The big problem then is having enough space to type that many characters.

Yep, unfortunately for passphrases the maximum acceptable password length on web sites tends to be the #1 factor limiting their use. I created sample passphrases and tried them on a handful of different sites to measure the problem. See starting slide 31 in this presentation (PDF): http://www.passwordresearch.com/files/How%20Secure%20Are%20M...

Do you know why sites would set a low limit? I can understand limiting to, say, a couple hundred characters, but 16 is ridiculous. These days, all anybody is going to store is a salted hash so the passphrase length should be mostly irrelevant.

Edit: that's a pretty nice slide deck. Thanks for the pointer.

> These days, all anybody is going to store is a salted hash so the passphrase length should be mostly irrelevant.

Since I still run into services where the "forgot password" mechanism emails your current password in the clear, this is unfortunately not as true as one would hope.

Not on the road, but IIRC there were several in the blog comments.

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