define a password for THEMSELVES. Two benefits

One very serious drawback: ten percent of your users' passwords will be brute-forceable given fifty tries.

If you are an ISP, this may be okay. Warn the users that if anyone guesses their password they're fully responsible, don't give them access to anything but their own stuff, patch against whatever local root exploits are discovered, and that'll probably do.

If your users have to have access to stuff that you have to protect, then you have problems. If possible, the mechanism users use to change their passwords should check against a list of common passwords and a dictionary and reject any password that's very close to anything on the list or in the dictionary.

Of course, if you can give them a secure password that they can remember... There was an article on slashdot recently about using inkblots as password clues, but what about using pronounceable sequences or words?

I did a little arithmetic on this issue, and I have concluded that a sufficient number of dictionary words (at least three IIRC, but it of course depends on the size of your dictionary) strung together with hyphens can be as secure as a much harder-to-remember random password. It takes longer to type the words, but they are easier to remember. There's your tradeoff. Say your dictionary has twenty thousand words and you use $n of them, that's (20000)**$n possibilities. The traditional approach (using sequences of random characters from [A-Za-z0-9]) gets more like (62)**$n and so requires a somewhat higher n. Thus, you don't have to string eight words together to surpass the security of an eight-character random password. Three words may be adequate for some purposes (especially if you use a large dictionary) -- and it will be easier to remember. Even a four-word password (so your dictionary only needs to have 64*64 words in it) may be easier to remember than jEp3WkbG, if you consider a word to be easier to recall than two random characters.

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$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}}
split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/
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