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Especially in cryptocurrencies you often hear people say that private key/seeds should not be typed "randomly" by humans, but rather use a truly random number generator. I understand, that humans behave in a certain way, so there must be patterns that would shrink the search space of a brute-force attack significantly.

That's the theory. But in practice, how would someone model such an attack? I know there are researched cognitive biases in perception and production of randomness but are there models, how you would model a search space of a human "randomly" hitting a keyboard?

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    you could run a mechanical turk asking for 50 key presses. – dandavis Jan 12 '18 at 7:07
  • Type something randomly on your computer for a very long time, now count the occurences of asdf qwer zxc, ... It's a bug in humans, they often confuse typing fast with typing random, or think speed somehow increases randomness. :-) – Nomad Apr 12 '18 at 16:00
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A simple answer is that a human will tend to hit keys in the middle and/or on the 'home row' of the keyboard more and will tend to hit keys near each other together more often (due to double hits). There may also be more 'non-shifted' special characters and number row characters. Numbers and Special characters can capitals are more likely to be grouped (or just appended to) the main 'random' string.

You will therefore get a higher frequency of asdfghjkltyuivbnm, sequences like sw qd l; etc. and more in the format of oipynKJIU8767@~{&$

These can be consciously avoided, but this could from its own patten or have a too even mix (nearly the same number of each type).: dR%6Ni9)lI&6...

This could weaken things somewhat and give the attacker some assistance, but for a long string it's unlikely to be significant. It would also require the attacker knowing it was human generated to they a modified brute force attack

None of my suggestions on frequency is comprehensive and a bit of effort can produce a very random string.

Personally I often use a password generator then change a couple of items or to remove any link to the pseudo-random seed (a bit paranoid really).

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    Another pattern you missed is that humans almost never generate repeats. Doubled characters will occur about once every 26 random alphabetics, but a human randomly typing letters will probably never generate a sequence such as "mm" or "ss" (and never something like "wwww"). – Mark Mar 13 '18 at 21:02

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