# Can we estimate the loss of entropy when applying a N-bit hash function to and N-bit random input? [duplicate]

Someone pointed out recently to me that a cryptographic hash function " is not designed as a bijective mapping from N bit input to N bit output".

So if I feed an N-bit cryptographic hash function with N bits of random input, there's a loss of entropy between the input and output of the hash function.

Considering the md5 hash function, is there a way to estimate that loss of entropy? And is this loss cumulative so I could say, if I apply the hash function enough times, I end up with a 50% loss of entropy?

Actually, no. If it is a good Hash, you should roughly have $$N-k$$ bits of output entropy for some $$k$$ of much lower order than $$N$$.

The problem arises when the input is much longer than $$N$$ bits.

One way to estimate the entropy loss of such a Hash applied to $$N$$ bit inputs is to model it as a randomly chosen function on $$N$$ bits. This was first done by Odlyzko and Flajolet. There is a nice review with updated results here

Let $$\tau_m$$ be the image size of the $$m$$th iterate of the function. The entropy can be related to its behaviour.

If the function is a permutation, $$\tau_m=2^N$$ for all $$m\geq 1$$ and there is no entropy loss.

Edit: See the comment and link by @fgrieu which is an estimate of what I called $$\tau_1.$$ He is saying that $$\tau_1\approx 2^{128-0.8272\cdots }$$ for $$N=128.$$

• Is there any estimate for the $k$? – kelalaka Feb 10 '19 at 21:12
• Except for small $N$, the $k$ depends very little on $N$, and for $N=128$ is already very close to its asymptotic value $\eta=\displaystyle{1\over e}\sum_{j=1}^\infty{j\;\log_2j\over j!}\;\;=0.8272\dots\text{bit}$. See this. – fgrieu Feb 10 '19 at 22:14