Are there any encryption suites which can be cracked by classical computers but not quantum computers?

Are there any encryption suites that can be cracked by usual computers or super computers, but not quantum computers?

If that's possible, what assumptions will it depend on? (Factorizing big numbers, $a^b\pmod d$ $a^c\pmod d$ $a^{bc}\pmod d$ etc...)

• – Sanchayan Dutta Mar 15 '18 at 15:32
• A quantum computer can theoretically do anything that a classical computer can do, in which case your question only makes sense as a question about the technological state of the art. All it would take is a cryptosystem which can easily be solved by a classical computer using basic arithmetic (such as simple addition modulo N) on sufficiently large numbers that those numbers cannot be stored on today's relatively minuscule prototype devices. – Niel de Beaudrap Mar 15 '18 at 16:53

However, even then, it likely won't be answered definitively any more than we can answer definitively what cryptography we can't break with classical computers: nobody has found a realistically efficient classical algorithm for factoring a product $n$ of uniform random 1024-bit primes whose totient $\phi(n)$ is coprime with 3, nor has anyone found a realistically efficient classical algorithm for computing cube roots modulo $n$, nor has anyone even ascertained whether factoring is harder than computing cube roots (though certainly it's not easier).