The technological hurdles still being heavily challenging, with enormous efforts invested in developing Quantum-Computing (QC), science and industry evolve continuously in the direction of reliable and usable hardware systems to tackle heavy and complex mathematical problems. This development also paves the way to vulnerability of state-of-the-art encryption which relies on the assumption the corresponding mathematical operations and tasks are practically unsolvable (on time scales of a million years or more). Used for communication innumerable times per day and all around the world, from popular private messenger chats over sensitive economic data transfer to (inter-)national security reports, present standards of electronic communication are about to lose confidentiality with upcoming QC systems. This makes concerted and swift action to identify weak spots and harden telecommunication against QC inevitable tasks postponing is unacceptably risky.

Risk analysis starts assessing the level of vulnerability against between the two main branches of modern cryptography, the symmetric (shared private key) and asymmetric (public key) ciphers. For symmetric cryptosystems, all communication partners share a common private/secret key. Here, a QC attack to intercept information is based on a brute force key search, using the quantum search algorithm by Grover. Fortunately, this approach reduces the effort only by an exponent of ½ , keeping it basically exponentially complex with respect to the key length. Doubling it (i.e., from 128 to 256 bits) remedies the QC vulnerability of symmetric ciphers and implementations of the corresponding AES-256 standard already protect suitably well against QC attacks. Therefore, the main focus of QC resilience lies on state-of-the-art asymmetric ciphers which use both a public and a private key, for encryption and decryption, respectively. The private can be extracted from the public key by means of a mathematical operation, either a prime factorization or calculation of the discrete logarithm, for the respective asymmetric approach. Since the public key is an extremely large number in current asymmetric ciphers, this task is exponentially heavy, and therefore unfeasible on conventional computing hardware. On a quantum computer, however, the quantum algorithms by Shor, for factorization and calculation of discrete logarithms, reduce the effort to a polynomial complexity in the length of the public key, making asymmetric cryptography vulnerable to QC the day it becomes available.