the CIA triad

Ask what it actually means for information to be secure and you get not one answer but three, and they pull in different directions. Think about your medical records. You want them kept private — only your doctor should read them (confidentiality). You want them correct and untampered — no one should secretly change your blood type (integrity). And you want them there when needed — they must be retrievable in an emergency, not locked away or destroyed (availability). These three goals — confidentiality, integrity, availability — are the CIA triad, the standard frame for reasoning about security.

Confidentiality means information is disclosed only to those authorized to see it; its enemies are eavesdropping, leaks, and theft, and its defenses include encryption and access control. Integrity means information is modified only in authorized ways and is not corrupted or forged; its enemies are tampering and unauthorized writes, and its defenses include checksums, digital signatures, and write permissions. Availability means the system and its data are usable when legitimate users need them; its enemy is denial — crashes, deletion, or a denial-of-service attack that floods the system — and its defenses include redundancy, backups, and rate limiting. Every concrete security mechanism in an operating system can be mapped to defending one or more of these three.

The triad is useful precisely because the three goals can conflict, forcing honest trade-offs. The most confidential file is one nobody can read — but that destroys availability. The safest-from-tampering system might be read-only — but then you cannot do legitimate work. Locking an account after three wrong passwords improves confidentiality and integrity, yet an attacker can exploit it to deny you access (harming availability). Naming all three keeps designers honest: a real security decision is rarely about maximizing one property, but about balancing the three against the threats and the cost. A common extension adds authenticity and non-repudiation, but CIA remains the core.