---
title: "Short note: Digital signature"
subject: "Computer Networks and Security"
chapter: 8
key: "semester_7/computer_networks_and_security:digital-signature-note"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/digital-signature-note
site: BCT Engineering
---

# Short note: Digital signature

Computer Networks and Security · Chapter 8: Network Security

- Asked in: 2072 Kartik, 4 marks
- Video: [What is digital signature?](https://www.youtube.com/watch?v=TmA2QWSLSPg) — Sunny Classroom
- 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/digital-signature-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-8

## Question

Write a short note on Digital signature.

## Short answer

A digital signature provides authentication, integrity, and non-repudiation by hashing the message and encrypting the digest with the sender's private key. The receiver verifies by decrypting with the sender's public key and comparing with a fresh hash of the received message. Algorithms include RSA, DSA, and ECDSA. Digital certificates bind public keys to identities. Signing does not provide confidentiality; the message can be encrypted separately.

## Answer

A **digital signature** is the electronic equivalent of a handwritten signature. It is a value created from a message and the signer's **private key**, which allows anyone to verify, with the signer's **public key**, that the message came from the signer and was not changed.

#### Services provided
Authentication (identity of signer), message integrity, and non-repudiation (signer cannot deny).

#### Working (hash then sign)
```text
Sender:  M --hash--> H(M) --encrypt with sender's private key--> signature S
         send M + S
Receiver: decrypt S with sender's public key -> H1
          hash received M -> H2
          H1 == H2 ?  valid : rejected
```
1. The sender computes the message digest with a hash function (SHA-256).
2. The digest is encrypted with the sender's private key; this is the signature.
3. The message and signature are sent.
4. The receiver decrypts the signature with the sender's public key and compares it with a fresh hash of the received message. If they match, the signature is valid.

Signing the short digest is faster than signing the full message. Algorithms: RSA, DSA (DSS), ECDSA. A **digital certificate** (X.509) issued by a certification authority binds a public key to its owner. Signing alone does not give confidentiality; the message can also be encrypted.

## Check yourself

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

1. Which key creates the signature?
   A. Signer's public key
   B. Signer's private key
   C. Receiver's public key
   D. Receiver's private key
   Answer: B. Anyone can verify with the signer's public key. (Key point: Signing key)

2. Why sign the hash?
   A. Signing the short digest is faster
   B. Hash gives confidentiality
   C. Required by law
   D. It is longer
   Answer: A. The digest is short. (Key point: Why hash)

3. The signature is valid when:
   A. Certificate expired
   B. H1 (decrypted signature) equals H2 (fresh hash of message)
   C. Message is encrypted
   D. H1 differs from H2
   Answer: B. Match shows authenticity and integrity. (Key point: Verification)

4. Which are provided?
   A. Authentication, integrity, non-repudiation
   B. Compression
   C. Confidentiality only
   D. Availability
   Answer: A. Signing alone does not give confidentiality. (Key point: Services)

## Related in other subjects

- [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