Digital Signatures and Certificates

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🔐 Digital Signatures and Digital Certificates

📘 1. What is a Digital Signature?

A Digital Signature is a cryptographic technique used to verify:

  • ✅ Authenticity (Who sent the message)

  • ✅ Integrity (Message not altered)

  • ✅ Non-repudiation (Sender cannot deny it)

It is the electronic equivalent of a handwritten signature but much more secure.

Digital signatures use asymmetric cryptography (public key cryptography).

🎯 Objectives of Digital Signature

  1. Verify sender identity

  2. Ensure message integrity

  3. Provide legal validity

  4. Prevent forgery

  5. Provide non-repudiation

🔑 How Digital Signature Works (Step-by-Step)

Step 1: Message Creation

Sender writes the message.

Step 2: Hashing

A hash function (like SHA) creates a message digest (fixed-length value).

Step 3: Encryption

Sender encrypts the hash using their private key.
This encrypted hash becomes the Digital Signature.

Step 4: Sending

Sender sends:

  • Original message

  • Digital Signature

Step 5: Verification

Receiver:

  • Decrypts signature using sender’s public key

  • Generates new hash from received message

  • Compares both hashes

If hashes match → Message is authentic and unchanged.

📌 Components of Digital Signature

  • Message

  • Hash function (SHA-256 etc.)

  • Private Key

  • Public Key

  • Digital Signature

🔥 Advantages of Digital Signatures

  • High security

  • Legally valid

  • Prevents tampering

  • Faster than manual signatures

  • Used in e-commerce and e-governance

⚠ Disadvantages

  • Requires proper key management

  • Private key loss = no signature

  • Needs digital certificate for trust

📘 2. What is a Digital Certificate?

A Digital Certificate is an electronic document that proves the ownership of a public key.

It is issued by a trusted third party called a Certificate Authority (CA).

🎯 Purpose of Digital Certificate

  • Verify identity of person/organization

  • Bind public key to identity

  • Prevent fake public keys

  • Enable secure communication

📜 Information in a Digital Certificate

  • Name of certificate holder

  • Public key

  • Certificate serial number

  • Issuer name (CA)

  • Expiry date

  • Digital signature of CA

🏢 Certificate Authority (CA)

A Certificate Authority is a trusted organization that issues digital certificates.

Examples:

  • DigiCert

  • GlobalSign

  • Let's Encrypt

  • Sectigo

🔐 Types of Digital Certificates

1️⃣ SSL/TLS Certificate

Used for securing websites (HTTPS).

2️⃣ Code Signing Certificate

Used to sign software applications.

3️⃣ Email Certificate

Used to secure email communication.

4️⃣ Document Signing Certificate

Used to sign PDFs and official documents.

🔄 Difference Between Digital Signature and Digital Certificate

Digital SignatureDigital Certificate
Used to sign a messageUsed to verify identity
Created using private keyIssued by CA
Ensures integrityEnsures authenticity
Applied on documentsContains public key

🔥 Real-Life Uses

  • Online banking

  • Income tax filing

  • E-tendering

  • Company registrations

  • Government portals

  • Secure email

📚 Legal Status in India

In India, Digital Signatures are legally recognized under:

The Information Technology Act, 2000.

They are valid for:

  • E-contracts

  • E-filing

  • E-governance transactions

🔐 Digital Signature vs Electronic Signature

Digital SignatureElectronic Signature
Uses cryptographyMay not use encryption
Highly secureBasic level security
Legally strongerLess secure
Based on PKISimple digital mark

🎓 Exam Definition (Short)

A Digital Signature is a cryptographic technique used to verify authenticity, integrity, and non-repudiation of digital messages using public key encryption. A Digital Certificate is an electronic document issued by a Certificate Authority that verifies the ownership of a public key.

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