There is currently no efficient algorithm to calculate a discrete logarithm on a classical computer, this is known as the discrete logarithm problem. Much of cryptography is based on this problem, however there has been no proof that no efficient algorithm exists. Only much work has been done, and no easy way has been found. See: https://en.wikipedia.org/wiki/Discrete_logarithm_problem
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How efficient is an "efficient algorithm" (for the purposes of this market)?
If you just want something on the edge of being practical, then index calculus does an amazing job for discrete logarithms over finite fields. OTOH if you want a provably polytime algorithm then I think you'd probably want P vs NP to be resolved.
@duck_master This is about provably polynomial time, also for the edge cases, not just a heuristic that works well in most cases. Such that it's no longer practical to use . ElGamal encryption, Diffie–Hellman key exchange in say peoples browsers.