Cryptographic Hash Function
<p>I have an exam tomorrow on cryptography and came across an old exam question on hash functions and finding out the probability of collision of two hash values being the same, but I don't know how to calculate it. The question is:</p>

<p>If the hash value is a 20 bit output and allowable inputs must not exceed 2^64 bits, what is the probability of two randomly chosen values yielding a collision?</p>

<p>Was hoping someone could provide a solution.
Thanks.</p>