Yes, another one of those "I have found a great password-generation strategy" waiting to be shot down!
It seems that although diceware aims to be a secure and user-friendly password generation method, there is a consensus that its output is not typically user-friendly. Over the years I have read many posts asking how to generate "nicer" passwords from it. For example 1 2 3 and in particular 4.
To get "nicer" passwords, whatever that means, the suggestions are typically:
- create your own word list (a lot of effort and need a longer password for same entropy since manual word lists are typically shorter)
- use a selection method but be aware you lose entropy (e.g. pick $1$ out of $16$ passwords at the cost of $4$ bits of entropy)
But it seems to me that there is an easy way to get a more memorable yet shorter password, especially if you're bilingual. What you need is a set of $N$ disjoint diceware word lists, each with $6^5$ words.
When drawing a word, lookup the dice combination in the $N$ lists and pick the word you like most.
This is exactly as secure as drawing from a single list. Indeed, assuming there is no overlap between the lists, this is equivalent to processing the $6^5$ sets of $N$ words to create your own "preferred" word list and use a standard diceware on that list.
1 - Am I correct that generating a password in this fashion does not decrease entropy?
In practice, it is hard to find non-overlapping lists (unless you speak several languages, and even then there might be duplicates). The impact of overlaps is trivial to quantify when $N = 2$ but
2 - What is the impact of overlaps between lists when $N > 2$?
Even better, pick your $2$ preferred out of $N$ and flip a coin to choose. You just increased the entropy ($+1/$word) and still get a nicer password than single-list diceware.
Pushing this idea further: there are around $500$K words in the English language, from which one can create $\dfrac{500\cdot 10^3}{6^5} \approx 64$ non-overlapping word lists. For each dice combination, pick your favorite $K$ out of $64$ and pick a final word at random.
That's $\log_2(K)$ extra bits of entropy per word, so with $K=4$ you have $14.9$ $bits/word$ instead of the standard $12.9/word$.
Nicer and shorter password for the same entropy / more entropy for the same number of words.
You control the trade-off between entropy and nicer words by adjusting $K$. With $K=8$ you gain $3$ $bits/word$ whereas with $K=1$ you get your preferred words but standard diceware entropy.
3 - Why is nobody doing this? This is easy enough to automate. Unless something above is wrong?