The Discrete Log Problem
Mathematics may be all about solving problems, but there are certain unsolved problems, problems for which no efficient algorithm exists. Strangely, this is actually a good thing, because the field of cryptography, which forms a basis for computer security, makes use of such problems to create secure algorithms that cannot be broken easily.
One such problem is the discrete logarithm problem -
Given:
p, a very large prime
a (mod p)
ax (mod p)
Find: x
This problem is believed to be a ‘hard’ problem, but not proved to be a ‘hard’ problem. Apparently, mathematicians are having a hard time with the proof!

April 18th, 2006 at 01:20
Of course there are many instances when the problem is easy (surely Wikipedia has a reasonable entry for this)!
For instance, if p-1 has only relatively small prime factors there’s an algorithm for it.