I'm trying to find simple mathematical proof that lengthy passwords are more secure than complex (with multiple symbols) ones. I don't want to get into the concept of entropy - it is not useful to me atm. I've found this explanation at Security Stack, which it seemed pretty reasonable to me. My question is: is this explanation valid and, if so, is there a limit to it's validity?. My doubt comes from testing it out considering a password with $n$ digits, but restrict to a group of 26 characters (the alphabet) vs a password with fixed 10 digits, with $n$ possible characters:
$$f(n) = 26^n \qquad\text{vs}\qquad f(n) = n^{10}.$$
For these two functions it just doesn't work out as the answer in Security Stack explained. So, how does this work?
I guess the title may be a bit misleading, because this question markes as duplicate does not answer my question! I get that exponential growth is faster than polynomial growth, but why doesn't it apply to these two functions I'm comparing? I mean, what is wrong in my line of thought?
Answer
It's certainly true that $a^x,\,a>0$ eventually grows faster than $x^b,\,b>0$. (Perhaps the easiest proof is the fact that $\int_0^\infty xe^{-cx}dx=\frac{1}{c^2},\,c>0$ is finite. Thus $\lim_{x\to\infty}xe^{-cx}=0$, so $x^b$ can't keep up with $e^{bcx}$; now take $c=b^{-1}\ln a$.) So $26^x>x^{10}$ for all sufficiently large $x$.
No comments:
Post a Comment