It's important to note that quantum computers, have not yet reached the level of maturity where they can break modern encryption algorithms efficiently. While quantum computers have the potential to break certain types of cryptographic algorithms, the development of large-scale, error-corrected quantum computers capable of breaking commonly used encryption standards is still a significant technical challenge.
However, if you encountered a claim that a cipher was broken in an hour on a single-core PC, it is highly likely that the cipher was not resistant to classical attacks in the first place. Classical computers are still very capable of breaking many encryption algorithms if they have vulnerabilities or weaknesses.
There are several possibilities for such a quick "break" of a cipher on a classical computer:
Brute force attack: If the cipher's key size is small or the algorithm is weak, it may be vulnerable to a brute force attack. In this case, an attacker systematically tries all possible keys until the correct one is found. With a small key space, the search can be completed quickly, even on a single-core PC.
Cryptanalysis: A weakness or vulnerability may have been discovered in the cipher that allows for an efficient attack. Cryptanalysis involves exploiting the structure or mathematical properties of the cipher to break it more quickly than a brute force attack. If such a weakness exists, it can significantly reduce the time required to break the cipher.
Implementation flaws: The issue may not lie with the cipher itself but with its implementation. Poorly implemented encryption algorithms can introduce vulnerabilities that make them easier to attack. If a flaw exists in the implementation, an attacker might exploit it to break the encryption more quickly.
It's important to keep in mind that a claim of breaking a cipher quickly on a classical computer does not imply that the cipher is inherently vulnerable to quantum attacks. Quantum computers introduce new computational capabilities, particularly in the field of factorization and discrete logarithm problems, which could potentially render some currently used encryption algorithms insecure. However, these claims should be scrutinized carefully, and the broader cryptographic community closely monitors advancements in quantum computing to develop new encryption standards that are resistant to quantum attacks.