One extreme end goes like this: Imagine a world where the most widely-used cryptographic methods turn out to be broken: quantum computers allow encrypted Internet data transactions to become readable by anyone who happened to be listening. No more HTTPS, no more PGP. It sounds a little bit sci-fi, but that’s exactly the scenario that cryptographers interested in
If you take the development of serious quantum computing power as a given, all of the encryption methods based on factoring primes or doing modular exponentials, most notably RSA, elliptic curve cryptography, and Diffie-Hellman are all in trouble as Shor’s algorithm, when applied on a quantum computer, will render the previously difficult math problems trivially easy. This will make current public-key crypto and key exchange pretty much useless.
But it does not mean that all crypto algorithms are in suuch big damage: Strong symmetric ciphers, algorithms that use the same key for encryption and decryption (AES, Blowfish, etc.) will also be easier to crack with quantum computers, but only by roughly a factor of two.
Quantum computing is still in its infancy, but maybe you should think if you should prepare to it (transitioning away from susceptible technologies) – it is estimated that quantum computers are practical in ten-to-thirty year range. And that’s what some cryptographers are doing: developing algorithms that are not easy for quantum computers to solve. For example McEliece cryptosystems look like a good alternative to the current public-key infrastructure – but it needs to be researched more to know that it is really safe.