17.1 Digital Certificates

Digital certificates are a crucial component of secure communication on the internet. They provide a way to verify the identity of a website or user, ensuring that communications are not intercepted or tampered with. This section will explore the concept of digital certificates, their structure, how they are issued, and their role in establishing trust.

What is a Digital Certificate?

A digital certificate is an electronic document that binds a public key to an entity (e.g., a website, a person, or an organization). It's essentially a digital ID card.

Key Components of a Digital Certificate

A digital certificate typically contains the following information:

• Subject's Public Key: The public key of the entity the certificate belongs to.
• Subject's Distinguished Name (DN): Information identifying the subject, such as name, organization, and email address.
• Issuer's Digital Signature: A digital signature from a Certificate Authority (CA) verifying the certificate's authenticity.
• Serial Number: A unique identifier for the certificate.
• Valid From and Valid To Dates: The period during which the certificate is valid.
• Certificate Version: Indicates the version of the certificate standard used.
• Extensions: Additional information, such as key usage and certificate policies.

How are Digital Certificates Issued?

Digital certificates are issued by trusted third-party organizations called Certificate Authorities (CAs). The process generally involves the following steps:

1. Certificate Signing Request (CSR): The entity requesting the certificate generates a CSR, which contains its public key and identifying information.
2. Verification by the CA: The CA verifies the identity of the entity requesting the certificate. This may involve checking documents or other verification methods.
3. Certificate Issuance: If the verification is successful, the CA issues a digital certificate containing the entity's public key and its own digital signature.

The Role of Certificate Authorities (CAs)

CAs are trusted entities that maintain a list of trusted public keys. Web browsers and other applications have a pre-installed list of root CAs. When a user visits a website with a digital certificate, the browser checks if the certificate was issued by a trusted CA.

How Digital Certificates Work (SSL/TLS)

Digital certificates are fundamental to the Secure Sockets Layer (SSL) and its successor, Transport Layer Security (TLS) protocols. These protocols are used to encrypt communication between a web browser and a web server.

Here's a simplified overview of how SSL/TLS uses digital certificates:

1. Handshake: When a browser connects to a secure website, it initiates a handshake with the server.
2. Certificate Presentation: The server presents its digital certificate to the browser.
3. Certificate Verification: The browser verifies the certificate's authenticity by checking if it was issued by a trusted CA and if it is valid.
4. Key Exchange: If the certificate is valid, the browser uses the public key in the certificate to establish a secure session key with the server.
5. Encrypted Communication: All subsequent communication between the browser and the server is encrypted using the session key.

Element	Description
Public Key	The key used for encryption and decryption.
Private Key	The key used for decryption and signing. Must be kept secret.
Digital Signature	A cryptographic mechanism used to verify the authenticity and integrity of a message.
Certificate Authority (CA)	A trusted third-party organization that issues digital certificates.

Benefits of Using Digital Certificates

Using digital certificates provides several important benefits:

• Authentication: Verifies the identity of a website or user.
• Confidentiality: Encrypts communication to prevent eavesdropping.
• Integrity: Ensures that data has not been tampered with during transmission.
• Non-repudiation: Provides evidence that a message was sent by a specific entity.

Certificate Revocation

Sometimes, a digital certificate may become invalid before its expiry date. This could happen if the private key has been compromised. To address this, Certificate Authorities maintain Certificate Revocation Lists (CRLs) or use the Online Certificate Status Protocol (OCSP). These mechanisms allow applications to check if a certificate has been revoked.