Encryption

Every idea in Topic 2 sits somewhere on one journey. Data has to travel; it is divided into packets so that many users can share the same network; routers choose a route for each packet independently; the method of transmission (how the bits travel and in which directions) is chosen to suit the situation; interference on the way can corrupt the data; an error-detection method reveals that corruption; automatic repeat query asks for the damaged data again; and encryption keeps the contents private from anyone who intercepts them on the way. Learn the chapter as that single chain and every individual method has an obvious place to hang.

Data travelling across a network passes through equipment and links that the sender does not control, so it can be intercepted. Encryption is the process of scrambling readable data, called plaintext, into an unreadable form called ciphertext, under the control of an encryption key. Decryption uses the appropriate key to turn the ciphertext back into readable plaintext. The purpose is confidentiality: if the transmission is intercepted, what the interceptor obtains is ciphertext, which should be very difficult to understand without the required key. Encryption does not prevent the interception itself, and it does not stop the data being deleted or corrupted on the way.

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.