Crypto currency is based on public-private key encryption, which is derived from Diffie-Hellman's result. This system has worked because factoring large integers is an NP complete problem...essentially unsolvable via computer. However Peter Shor developed an algorithm on Quantum computers that can factor large integers in polynomial time ( essentially instantaneous ). 

The Crypto world claims that they have new techniques that are immune to Quantum Algorithms. These statements are unprovable, and most likely false. I love crypto, mainly because the banking segment has had a license to take 5% fees for international currency conversions. However, I see current crypto currencies as a short to medium term solution. A new breed of crypto is required to guarantee secure communication ( using entagled particle physics ).