Public Key Infrastructure (PKI)

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🔐 PUBLIC KEY INFRASTRUCTURE (PKI)

📘 Definition

Public Key Infrastructure (PKI) is a framework of hardware, software, policies, and procedures used to create, manage, distribute, store, and revoke digital certificates and public/private key pairs.

It enables secure electronic communication and authentication over insecure networks like the Internet.

🎯 Purpose of PKI

PKI is used to ensure:

  1. Confidentiality – Data is accessible only to authorized users.

  2. Integrity – Data cannot be altered without detection.

  3. Authentication – Verifies identity of users and systems.

  4. Non-Repudiation – Sender cannot deny sending a message.

🔑 Core Components of PKI

1️⃣ Certificate Authority (CA)

A Certificate Authority is a trusted organization that:

  • Issues digital certificates

  • Verifies identity before issuing certificate

  • Signs certificates digitally

Examples:

  • DigiCert

  • GlobalSign

  • Let's Encrypt

The CA acts as a trusted third party.

2️⃣ Registration Authority (RA)

  • Verifies identity of users

  • Works under the Certificate Authority

  • Approves certificate requests

3️⃣ Digital Certificate

A Digital Certificate is an electronic document that:

  • Contains public key

  • Contains owner identity

  • Is digitally signed by CA

It proves that a public key belongs to a specific individual or organization.

Information Inside a Digital Certificate

  • Owner name

  • Public key

  • Certificate serial number

  • Expiration date

  • Issuing CA

  • Digital signature of CA

4️⃣ Public and Private Keys

PKI uses asymmetric encryption:

  • Public Key → Shared openly

  • Private Key → Kept secret

Public key encrypts data.
Private key decrypts data.

5️⃣ Certificate Revocation List (CRL)

A list of certificates that are no longer valid due to:

  • Expiry

  • Compromise

  • Misuse

🔄 How PKI Works (Step-by-Step)

  1. User generates public and private key pair.

  2. User sends public key to Certificate Authority.

  3. CA verifies identity.

  4. CA issues digital certificate.

  5. Certificate is shared publicly.

  6. Others use public key to encrypt data.

  7. Only private key holder can decrypt.

🌐 Real-Life Example: HTTPS Website

When you open a secure website:

  • Browser checks digital certificate.

  • Verifies certificate with CA.

  • Establishes encrypted connection using SSL/TLS.

  • Secure communication begins.

If URL starts with:
https://
It means PKI is working in the background.

🔐 Applications of PKI

  • Secure websites (SSL/TLS)

  • Email encryption

  • Digital signatures

  • Online banking

  • E-commerce transactions

  • VPN security

  • Government e-filing systems

📌 Digital Signature in PKI

Digital Signature ensures:

  • Sender identity verification

  • Data integrity

  • Non-repudiation

It uses:

  • Hashing algorithm

  • Private key encryption

🔥 Advantages of PKI

  • High security

  • Secure online communication

  • Prevents data tampering

  • Supports digital certificates

  • Ensures authentication

⚠ Limitations of PKI

  • Expensive implementation

  • Complex management

  • Certificate expiration issues

  • Requires trusted CA

🔎 Symmetric vs Asymmetric in PKI

PKI mainly uses asymmetric encryption for:

  • Authentication

  • Secure key exchange

After key exchange, symmetric encryption is used for faster data transfer.

📊 Summary Table

ComponentFunction
CAIssues digital certificates
RAVerifies identity
Public KeyEncrypts data
Private KeyDecrypts data
CRLRevokes certificates

🎓 Exam Short Note

Public Key Infrastructure (PKI) is a framework that uses digital certificates and asymmetric cryptography to secure communication, verify identity, and ensure confidentiality, integrity, and non-repudiation in online transactions.

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