Public-key cryptography vs Symmetric encryption

Why they differ

Symmetric encryption uses one shared key for both locking and unlocking; public-key cryptography uses a linked pair, so no secret has to be shared first.

Public-key cryptography

Cryptography

Encryption that uses a pair of keys - one shared openly, one kept private - so strangers can protect data for you without a shared secret.

Formal

A family of methods in which each party has a mathematically linked pair of keys; data locked with the public key can only be unlocked with the private key, and anything signed with the private key can be checked by anyone holding the public key.

In plain English

Like a mailbox with a slot - anyone can drop a letter in, but only the owner has the key to take letters out.

In practice

A social worker in a municipality sends a case file by encrypted email to a hospital. Her mail program locks it with the hospital's public key, so only the hospital's private key can open it.

Why it matters

It solves the problem of sharing a secret safely with strangers, and makes digital signatures possible, which is what lets people trust who is on the other end.

Symmetric encryption

Cryptography

Encryption where the same secret key both locks and unlocks the data, so sender and receiver must share it in advance.

Formal

A class of methods, such as the Advanced Encryption Standard, in which one cryptographic key both turns readable data into scrambled data and turns it back. It is fast enough for large amounts of data but does not solve how the key is shared safely.

In plain English

Like a padlocked box where you and a friend each hold a copy of the same key - easy to use, but you had to hand the copy over in person first.

In practice

A home-care worker's laptop is stolen from a car in a municipality. Its disk is protected with symmetric encryption, unlocked only at login, so the thief cannot read the citizens' data on it.

Why it matters

It does the heavy lifting of protecting almost all stored and sent data, so the whole protection is only as strong as how well the shared key is kept secret.

Shared connections

Cryptographic key Encryption TLS