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Comment by furyg3 | Hacker News Reader

If someone wants to use a 250 character truly random password with crazy characters, let them do it. If they used a long password with an excellent mix of upper-case, lower-case, and special characters, but no numbers, it's a good password, take it.

I had a password for a credit card account which could not be more than 8 characters and couldn't contain 'special' characters or punctuation. This was probably done to either make the password human readable (over-the-phone, horrible idea), or because of some legacy system on their end. At some point the organization got smart and made a minimum of 6 characters, of which two needed to be numbers (but kept the other restrictions). This effectively narrows down pool the possible passwords for an attacker to guess, the opposite of what you want to do.

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Have fun running bcrypt on 100TB of /dev/urandom. Insanely large password limit? Sure sounds like good business to me. No password length limit? Sounds like a DOS attack waiting to happen.

What would you suggest as a sane upper bound?

No idea, especially as pass phrases have become more popular the max length of a password that you might see in the wild has gone up quite a bit. Lets say someone is willing to type for 1 min to login to a site. According to Google the fastest typing speed recorded is 216 words per min. According to the words per min Wikipedia entry a "word"(They give examples of "I run" counts as one word, but "rhinoceros" counts as 2) is 5 characters long so that gives us a max password length of 1080 characters. According to wikipedia War and Peace is just shy of 600 thousand words long or approximately 3 million characters.

Therefore 1000 is a good minimum max length but 3 million is way to long.

const int PW_MAX = strlen("correct horse battery staple")

You're probably good with 1k FWIW Django added a 4k limit (4096 bytes) last September: https://www.djangoproject.com/weblog/2013/sep/15/security/

While it's true their changes made the key space smaller and, even with a good KDF, 8 chars was never long enough to begin with, the reduction is negligible. Assume 36 char (alphanumeric, case insensitive) passwords:

Originally:

    36^8 + 36^7 + 36^6 + 36^5 + 36^4 + 36^3 + 36^2 + 36
    2,901,713,047,668 possible passwords
    ~41.4 bits

After:

    (36^8 - 26^2 * (8*7)/2) + (36^7 - 26^2 * (7*6)/2) + (36^6 - 26^2 * (6*5)/2)
    2,901,650,810,624 possible passwords
    still ~41.4 bits

Only 62,237,044 possibilities eliminated. Length is the most important factor, so this was probably a good decision if most of their users were using <= 6 char passwords, or not using numbers at all. And frankly, the financial sector should be using HSMs anyway, making weak KDFs irrelevant.

Making the minimum password length 6 characters reduces the possible outcomes by a negligible ammount.

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