Wednesday, September 29 2004

Dicing with passwords

Last year I posted a reference to Arnold Reinhold’s Diceware page, and included a copy of my favorite passphrase generator, which attempts to generate pronouncable nonsense words.

I’ve always been a big fan of pronounceable nonsense, even in the days when passwords were limited to eight characters, but I think it’s particularly useful for long passphrases. My problem was that it can actually be pretty difficult to get a good nonsense phrase out of the original table. So I made my own.

Now, the instinctive reaction to someone creating their own security tool instead of using one created by an expert is (or ought to be) an anguished cry of “Noooooo, you fooooool!”. This is a special case, though, because the beauty of the Diceware scheme is that the contents of the table don’t actually matter, as long as each cell is unique. You could fill the first column with colors and the rest of the cells with the names of different superheroes, and the resulting passphrases would contain just as much entropy.

So here’s my new favorite method of generating passphrases. Roll three six-sided dice (one to choose a consonant, two more to choose the rest of the syllable), repeat at least ten times, and assemble into a phrase.


Update: Sorry if I didn’t make it clear. Split the results up with spaces to create two- or three-syllable “words”.

Also, a word on the relative strength of passphrases. Each syllable contains ~7.75 bits of entropy (log2(6*6*6)), so ten syllables produces a 77.5-bit passphrase, which is likely good enough for data that isn’t kept under lock and key 24x7 (e.g. login password on a laptop). See Reinhold’s FAQ on passphrase length for details. Note that the dictionary-based Diceware system requires longer passphrases to get the same strength (5d6 per word versus 6d6 for nonsense syllables).