Security

• Plain text is encrypted with a key into cipher text; decryption reverses it.
• Symmetric encryption uses one shared secret key; it is fast but the key must be distributed safely.
• Asymmetric encryption uses a key pair: a public key anyone may have and a private key kept secret. Data encrypted with one key can only be decrypted with the other.
• Private message to a person: encrypt with the recipient's public key. Verified message from an organisation: sign with the organisation's private key.
• Quantum cryptography uses photons to distribute keys so that any eavesdropping is detected.
• SSL/TLS secures client–server communication: certificates for authentication, asymmetric encryption to agree a session key, symmetric encryption for the data.
• A digital certificate, issued by a Certificate Authority, binds a public key to its owner; it is used to verify digital signatures.

Symmetric and asymmetric cryptography

Plain text is the original readable data. An encryption algorithm and a key turn it into cipher text, which is meaningless to anyone without the right key. Encryption does not stop data being intercepted; it stops intercepted data being understood.

Sending a private message from the public to an individual or organisation

The sender obtains the recipient's public key (it can be published) and encrypts the message with it. Only the recipient's private key can decrypt it, and only the recipient has that key, so nobody else can read the message, even someone who has the public key.

Sending a verified message to the public

The organisation encrypts (signs) with its own private key. Anyone can decrypt it with the organisation's public key. The message is not secret, but if the public key decrypts it, it can only have come from the holder of the private key, so its origin is verified. In practice only a hash of the message is signed: this is a digital signature (see below).

Purpose, benefits, drawbacks

Quantum cryptography

Securing client–server communication