A cipher is the rule set or algorithm used to transform readable data into encrypted data and back again. The cipher defines the mathematical steps of encryption and decryption, while a separate secret key controls the specific result. Modern ciphers may be block ciphers, which process fixed-size chunks of data, or stream ciphers, which encrypt data as a continuous flow.
Ciphers matter in cybersecurity because they protect confidentiality in transit and at rest. They are used in secure messaging, disk encryption, VPNs, and web traffic. Attackers may try to break weak ciphers, exploit poor implementations, or steal keys instead of defeating the algorithm itself. Defenders choose well-studied ciphers, use them with strong key management, and retire outdated designs before they become practical targets.