Symmetric Encryption

In symmetric encryption, the same secret key is used to encrypt the data and to decrypt it. Both communicating parties must therefore hold the same key, which means it has to be exchanged securely before any messages can be sent — and if that key is ever obtained by someone else, they can decrypt everything protected by it. Because there is only one key and the process is comparatively simple, symmetric encryption is generally efficient for encrypting large amounts of data.

Asymmetric encryption uses two mathematically related keys: a public key, which may be shared with anyone, and a private key, which must be kept secret by its owner and is never transmitted. In the confidentiality model used by this syllabus, the recipient publishes their public key; a sender encrypts the plaintext using the recipient's public key; the ciphertext is transmitted; and the recipient decrypts it using their own private key. The public key that encrypted the message cannot decrypt it — which is exactly why it is safe to publish, and why asymmetric encryption solves the key-distribution problem that symmetric encryption creates.