A quantum computer is a computing system that uses quantum mechanics, such as superposition and entanglement, to process certain problems in ways that differ from classical machines. For cyber security, the key concern is not everyday malware but the possibility that sufficiently advanced quantum computers could break some widely used public-key cryptography, especially algorithms that rely on factoring or discrete logarithms.
This matters because encrypted data, certificates, digital signatures, and key exchange systems protect long-lived records, banking traffic, and authentication systems. Attackers may also use a “harvest now, decrypt later” approach: steal encrypted data today and wait for better tools in the future. Defenders respond by inventorying where cryptography is used, planning migrations, and adopting quantum-resistant encryption before vulnerable systems become a liability.