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Modern password crackers are pulling all of wikipedia and youtube for seed words. If your words are in either of those, don't expect the password to stand to a dedicated attacker

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There are 1160290625000000000000000 combinations of 5 words with a dictionary of 65000 words. That's not brute-forceable. If you take existing phrases it's another story, but random words works well.

not sure what your calculation is, but permutations is what you should have calculated.

His calculation was (65000 Choose 5) * 5!. His premise required a combination then a permutation.

Being a little loose with my estimates and a bit of Fermi Math, thats only about 300 years of computing time on a small home built GPU cluster.

Basically tells me that 4 random words are definitely crackable and 5 are theoretically possible (and definitely doable with 5-10 years of Moore's law)

lg(65k^4) is very nearly 64. If you worry about 4 random words being brute forced, you should worry about 64 bit symmetric keys being brute forced. I don't know where the current recommendations come down on that.

If you're picking with structure (including "phrases that spring to mind"), agreed. If you genuinely include enough entropy, then it doesn't much matter what mnemonics you layer on top.

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