Cambridge IGCSE Computer Science · Syllabus 0478 · Data Transmission
Private Key
What is Private Key?
A private key is the key of an asymmetric key pair that must be kept secret by its owner and is never transmitted; it is used to decrypt data that was encrypted with the matching public key.
This definition is part of the Data Transmission chapter in Cambridge IGCSE Computer Science.
Private Key in context
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.
Examiner tips on Private Key
- The three sentences that must never appear in your answer. “The public key decrypts the message” — it does not, that is the private key's job. “The recipient sends their private key to the sender” — the private key is never transmitted to anybody, ever. “The two keys are both secret” — the public key is deliberately published. If you can keep those three straight, you have the whole of 2.3.2's asymmetric half.