4.1Security Fundamentals, Cryptography, and Authentication (PKI, Digital Signatures)

In AP-level questions on cryptography, the differentiator is precisely stating which direction a key is used in. Depending on who encrypts with which key and who decrypts with which key, public-key cryptography can deliver either confidentiality or authenticity. This section first organizes the CIA evaluation axis, then dives into how the differing mathematical properties of symmetric-key cryptography, public-key cryptography, and hash functions translate into practical usage.

4.1.1CIA and complementary properties

• Confidentiality means only authorized parties can access information; integrity means information stays accurate, free of unauthorized tampering; availability means it can be used whenever needed. Complementary properties include authenticity (assurance that a party genuinely is who it claims), accountability (tracing who did what), non-repudiation (preventing later denial of an action), and reliability (consistent intended behavior).
• Which CIA element is affected depends on the situation, even for similar incidents (e.g., a leaked customer list = confidentiality; a defaced website = integrity; a DoS-caused outage = availability). A digital signature is an application of encryption yet delivers authenticity, integrity, and non-repudiation rather than confidentiality — a key reason to avoid equating "encryption" with "confidentiality."