Explain like I'm five · Security

What is Encryption?

Encryption, told as a story about a letter nobody can read without the key.

Buy two applesthe runner, the bystander and the thief all read it

Anyone on the road can read a letter.The runner, the bystander, the thief.

please don't readBuy two applesbuy two apples, it saysan envelope only asks; it can't stop anyone

An envelope only lasts until someone opens it.An envelope asks people not to read. It can't stop them.

Buy two applesㅋ#7@…x!Q?stolen, still unreadableBuy two applesonly someone with the key turns it back

Encryption turns the letter into marks nobody can read without the key.Stolen, it's still unreadable. Only someone with the key turns it back.

Scramblinginto unreadable marks
The keythe secret that scrambles and unscrambles
Turning it backonly with the key
Stealing gets nothingthe thief holds scribbles
two matching keyshow do I hand over the key?fast, but the hand-off is the worrypadlocks for everyone, one key for memine onlyanyone can snap one shutsnapped shutslower, but nothing to hand over

There are two ways to do keys.Matching keys are fast, but how do you hand one over? Padlocks for everyone and one key for me — slower, but nothing to hand over. Usually both are used together. The ring is this second kind.

ㅋ#7@…x!Qwhere did the key go…I can't open it eitherJimin → pharmacythe envelope still showswho wrote to whom isn't hiddenand thieves use the same padlocks

Lose the key, and I can't open it either.A seal hides only the contents — who wrote to whom still shows (what the hallway watcher sees). And thieves use the same padlocks. Key management is everything.

In one breath

Encryption = turning a letter into marks unreadable without the key. Stealing it gets nothing; only the key-holder turns it back.

Matching keys: AES. Padlock-and-key: RSA, ECC. HTTPS uses both. A hash isn't encryption — it's a fingerprint, and can't be turned back.

When grown-ups say it

Plaintext · ciphertext
The readable letter · the scrambled one. Before and after encryption.
Key
The key. The secret that scrambles and unscrambles. The longer, the harder to guess.
Decryption
Turning it back. Using the key to make the scramble a letter again.
Symmetric (AES)
Two matching keys. Fast and strong. Handing over the key is the problem.
Public · private key
Padlocks for all · one key for me. Nothing to hand over. → the ring that knows the gate
HTTPS · TLS
Sealed letters on the road. A padlock hands over a key, and the key seals the letters. → the sealed tunnel
Hash
A fingerprint. Can't be turned back. Not encryption. → the footprint
Key management (KMS · HSM)
The key vault. Lose the key and it's over, so guarding keys is the real job. → the master key in the vault
Next story
A Fingerprint for Every Thing →
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