---
title: "Symmetric vs asymmetric key encryption"
subject: "Computer Networks and Security"
chapter: 8
key: "semester_7/computer_networks_and_security:symmetric-vs-asymmetric"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/symmetric-vs-asymmetric
site: BCT Engineering
---

# Symmetric vs asymmetric key encryption

Computer Networks and Security · Chapter 8: Network Security

- Asked in: 2073 Shrawan, 3 marks; 2069 Chaitra, 4 marks; 2071 Shrawan, 6 marks (asked 3 times)
- Video: [Encryption - Symmetric Encryption vs Asymmetric Encryption - Cryptography - Practical TLS](https://www.youtube.com/watch?v=o_g-M7UBqI8) — Practical Networking
- 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/symmetric-vs-asymmetric
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-8

## Question

Compare symmetric key encryption method with asymmetric (public) key encryption.

## Short answer

Symmetric encryption uses one shared key for both operations, is fast, and suits bulk data. Asymmetric uses a public/private key pair, is slower, and enables digital signatures, authentication, and secure key exchange without pre-shared secrets.

## Answer

**Symmetric-key encryption** uses one shared secret key for both encryption and decryption. **Asymmetric (public-key) encryption** uses a pair of keys: a public key to encrypt and a related private key to decrypt.

| Point | Symmetric | Asymmetric |
|---|---|---|
| Keys | One shared secret key | Public key + private key |
| Key distribution | Hard: the key must be shared secretly | Easy: public key can be published |
| Speed | Fast | Slow (heavy mathematics) |
| Key size | 128 to 256 bits | 2048 bits or more (RSA) |
| Keys for n users | n(n − 1)/2 (4,950 for 100 users) | 2n (200 for 100 users) |
| Services | Confidentiality | Confidentiality, authentication, digital signature, key exchange |
| Non-repudiation | Not provided | Provided by digital signatures |
| Examples | DES, 3DES, AES, RC4, Blowfish | RSA, Diffie-Hellman, ElGamal, ECC |
| Typical use | Bulk data encryption | Exchanging session keys, signatures |

```text
Symmetric : Plain --[K]--> Cipher --[K]--> Plain
Asymmetric: Plain --[Pub B]--> Cipher --[Priv B]--> Plain
```

In practice both are combined: the public-key method securely sends a symmetric session key, and the symmetric cipher encrypts the data (SSL/TLS, PGP).

## Check yourself

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

1. How many keys does symmetric encryption need for n users?
   A. 2n
   B. n(n-1)/2
   C. n
   D. n^2
   Answer: B. Each pair needs a key: n(n-1)/2, 4,950 for 100 users; asymmetric needs 2n. (Key point: Key number)

2. Which statement on speed is correct?
   A. Symmetric is slow due to heavy mathematics
   B. Symmetric is fast, asymmetric is slow
   C. Asymmetric is faster
   D. Both equal
   Answer: B. Asymmetric uses heavy mathematics so it is slow; symmetric is used for bulk data. (Key point: Speed)

3. Which type provides non-repudiation via digital signatures?
   A. Asymmetric
   B. Both
   C. Neither
   D. Symmetric
   Answer: A. Only key pairs allow a signature unique to the signer. (Key point: Non-repudiation)

4. How are both combined in SSL/TLS or PGP?
   A. They are never combined
   B. Asymmetric encrypts bulk data
   C. Symmetric key encrypts the public key
   D. Public key method sends a session key, symmetric cipher encrypts the data
   Answer: D. The slow public-key method secures the session key and the fast symmetric cipher encrypts the data. (Key point: Hybrid use)

## Related in Computer Networks and Security

- [What is public key cryptography](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Apublic-key-cryptography) — Computer Networks and Security, chapter 8

## Related in other subjects

- [Symmetric and asymmetric data](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Asymmetric-asymmetric-data) — Data Mining, chapter 1
- [Asymmetric vs symmetric DSL; ADSL modems](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aasync-sync-dsl-adsl-modems) — Telecommunications, chapter 8
- [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 speaking and key strategies](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Apublic-speaking-strategies) — Communication English, chapter 4