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@ryanc interesting.
back of the envelope calculation for that attack, assuming 6 letter words, chosen randomly, 3 letters, chosen randomly, 5 guesses, attacker has full knowledge of those 3 letters => almost 70% chance of guessing.
Much harder if the attacker knows the letters but not the exact locations.
Also a lot harder if the bank is deliberate in its choosing of letters rather than completely random - many words have some combination of 3 letters that are a complete 100% giveaway and other combinations that are very much not.