Cambridge O Level Computer Science · Syllabus 2210 · Data Transmission
Public Key
What is Public Key?
A public key is the key of an asymmetric key pair that may be shared with anyone; it is used to encrypt data intended for the key pair's owner and cannot decrypt that data.
This definition is part of the Data Transmission chapter in Cambridge O Level Computer Science.
Public 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 Public 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.
Questions students ask about Public Key
What is the difference between symmetric and asymmetric encryption?
In symmetric encryption, the same secret key encrypts and decrypts the data, so both parties must hold that key and it must be exchanged securely beforehand. In asymmetric encryption, two mathematically related keys are used: a public key, which may be shared with anyone and encrypts data for its owner, and a private key, kept secret by its owner and used to decrypt it. Because the public key that encrypts a message cannot decrypt it, asymmetric encryption avoids the problem of exchanging a secret key securely.