Explain like I'm five · Security

What is the CIA Triad?

The CIA triad — confidentiality, integrity, availability — told as a story about the three promises a castle keeps.

as long as no thief gets in, right?someone peeksbread today30 loaves→ 80someone changes itreal guests can't get ineven with thieves out, three things can still go wrong

A castle guards against more than thieves.Even with thieves out, someone can peek at a letter, quietly change the ledger, or block the gate. Three different kinds of trouble.

no peeking: keptall in the vault… now where's the key?the recipe's locked in… no bread todayalways there: brokenguard just one, and the castle stops — you need all three

Keep just one promise, and the castle stops.The guard locked everything in the vault and lost the key. Nobody can peek — not even the cook. So there is no bread today.

the three promises a castle keepsno peekingothers can't read itsealed letters · colored stampsno tamperingnobody changes it in secretfingerprints · the king's sealalways therethere when you need itspare chests · an open gatekeep all three, or it isn't a castle

The CIA triad is the three promises a castle keeps.No peeking (sealed letters, colored stamps), no tampering (fingerprints, the king's seal), always there (spare chests). All three, together.

No peekingstolen, still unreadable
No tamperingchange it, the print won't match
Always therethrough fires and fake crowds
All three at oncepush one too hard, another falls
promisewho keeps itno peekingsealed letters · colored stamps · key ringsstolen, still unreadable; not everyone gets a keyno tamperingfingerprints · the king's seal · the logbookchange it, and the print no longer matchesalways therespare chests · line ushers · the next-day planthrough fires and fake crowdsevery tool in the castle keeps one of these three

Every tool in the castle keeps one of the three.Key rings keep no peeking, the logbook keeps no tampering, line ushers and the next-day plan keep always there. Meet a new tool, and just ask: which of the three?

no peekingno tamperingalways therethe bank castlethe market boardour castle?lock it tighter, or hand out more keys?tighter: nobody can use it · looser: it leaksthe three pull on each other — every castle sets its own balance

The three pull on each other. Every castle sets its own balance.Tighter, and nobody can use it; looser, and it leaks. Where to lean is decided on the scale, and whatever you keep, you keep in layers.

In one breath

The CIA triad = the three promises a castle keeps — no peeking, no tampering, always there. They pull on each other, so you weigh them together.

Confidentiality, Integrity, Availability — the three goals of information security. Encryption and access control serve confidentiality; hashing, signatures, and logs serve integrity; backups, redundancy, and DDoS defense serve availability. They trade off, so the balance follows the risk.

When grown-ups say it

Confidentiality
No peeking. Only those allowed get to see. → the letter no one reads without the key
Integrity
No tampering. If it changes in secret, you can tell. → a fingerprint on everything
Availability
Always there. It has to be there when needed. → the spare chest far away, fake guests at the gate
Non-repudiation
No saying you never sent it. The king's seal is on it. → the badge with the king's seal
Authentication
Proving you are you. To keep any promise, you first need to know who is asking. → the doorkeeper who checks three times
Trade-off
The three-way scale. Tighter means unusable, looser means leaky. A bank leans toward no tampering, a notice board toward always there.
Risk
Which way to lean. Keep first the promise that hurts most to break. → the scale that picks which door to guard first
Defense in depth
In layers. No promise rests on a single tool. → walls within walls
Next story
Walls Within Walls →
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