---
title: "What is public key cryptography"
subject: "Computer Networks and Security"
chapter: 8
key: "semester_7/computer_networks_and_security:public-key-cryptography"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/public-key-cryptography
site: BCT Engineering
---

# What is public key cryptography

Computer Networks and Security · Chapter 8: Network Security

- Asked in: 2071 Chaitra, 2 marks
- Video: [Public Key Cryptography - Computerphile](https://www.youtube.com/watch?v=GSIDS_lvRv4) — Computerphile
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/public-key-cryptography
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-8

## Question

What is public key cryptography?

## Short answer

Public-key cryptography uses a public key known to all and a private key kept secret. Data encrypted with the public key decrypts only with the private key, and signatures made with the private key verify with the public key. It provides confidentiality, authentication, and digital signatures without preshared secrets.

## Answer

**Public-key (asymmetric) cryptography** uses a pair of mathematically related keys: a **public key** that is known to everyone and a **private key** that is kept secret by the owner. Data encrypted with the public key can be decrypted only with the matching private key, and data signed with the private key can be verified by anyone with the public key.

It removes the need to share a secret key beforehand and provides confidentiality, authentication and digital signatures. Examples: RSA, ElGamal, Diffie-Hellman, ECC.

```text
Alice --encrypt with Bob's PUBLIC key--> Bob decrypts with his PRIVATE key
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In public-key cryptography, which key is kept secret?
   A. Both
   B. Private key
   C. Neither
   D. Public key
   Answer: B. The owner keeps the private key; the public key is known to everyone. (Key point: Key pair)

2. Data encrypted with a public key can be decrypted with:
   A. Any key
   B. A hash
   C. The same public key
   D. The matching private key
   Answer: D. Only the matching private key decrypts it. (Key point: Encryption)

3. Who can verify data signed with the private key?
   A. Nobody
   B. Only the receiver
   C. Anyone with the public key
   D. Only the signer
   Answer: C. The public key is available to all. (Key point: Signature)

## Related in Computer Networks and Security

- [Symmetric vs asymmetric key encryption](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Asymmetric-vs-asymmetric) — Computer Networks and Security, chapter 8, asked 3×

## Related in other subjects

- [Short note: public key cryptography](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Apublic-key-cryptography-note) — Operating System, chapter 8
- [Public key cryptography and digital signature](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Adigital-signature-public-key) — Operating System, chapter 8
- [Public speaking and key strategies](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Apublic-speaking-strategies) — Communication English, chapter 4