Quote:
Originally Posted by Dhomochevsky
Entropy in information science is basicly the amount of variation a message can possibly contain, that means it's uncertainty.
Say you have a 2 bit number. Then it could hold 2^2 different meanings: 00,01,10,11
So the entropy here is 2 bits (the entropy equaivalent of a 2 bit number).
Characters are not bits. For ecample ASCII uses 8 bits to code a single character. You could code all small letters into 5 bits though.
With 26 possible letters, a random sequence of letters of length 6 would have 26^6 possibilities. That is hugely more than 6 bits of entropy. And it is also much more than 11 bits.
He uses only 44 bits, because he assumes that an attacker knows that
a) he's using only common english words, not random sequences
b) he's using exactly 4 of them
and it is STILL reasonably safe.
This is because the obvious rebuttal to this comic would be "but dictionary attacks will crack it in no time".
He's getting ahead of that by assuming such an attack and still coming out with a good security.
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So the 11 entropy (2^11) actually just a number he came up with to assume that's how many common words a human can think of?
Google around a bit, 2^11 is a bit more than 2000. And most kid starts school knowing 500-5000 words, so i guess it has a point