Understanding the dynamics of digital certificates can be overwhelming, especially for new users. With so many technical terms around, navigating the cyber security space requires advanced web encryption and authentication knowledge.
The distinction between a client certificate vs. server certificate often confuses non-technical users, but knowing what they are and how they work is essential in online environments dealing with user authentication and sensitive data protection.
This article discusses the difference between a client and a server certificate and their part in web security. Once you learn how they function, you’ll know what goes behind the scenes during the communication between your device (client) and a remote server.
Table of Contents
- What Is a Client Certificate?
- What Is a Server Certificate?
- What Is the Difference Between a Client and a Server Certificate?
What Is a Client Certificate?
A client certificate is a digital certificate issued to and used by a client, typically an end user’s device or application, to authenticate its identity when connecting to a secure server, enabling secure and mutually authenticated communication in various online transactions and interactions.
The term “client” refers to the entity initiating a connection to a secure server. This client could be a computer, a smartphone, or an application that seeks to communicate securely with a server.
How Does a Client Certificate Work?
To better understand the role of client and server certificates, you’ll first need to learn about Public Key Infrastructure (PKI). A client authentication certificate validates the client to the server. This digital certificate is issued by a Certificate Authority (CA) and contains the client’s public key. It’s paired with a private key, stored securely on the client’s end.
During the SSL handshake, the server and client present their certificates. The server verifies the client’s certificate by checking the signature against the CA’s public key. If successful, it establishes mutual authentication.
With mutual authentication confirmed, the server and client certificate exchange a symmetric session key. This key encrypts and decrypts data transmitted during the session, ensuring confidentiality.
The client certificate verifies the client’s identity through trusted certificate authorities and establishes a secure and authenticated channel for data exchange between the client and the server.
What Is a Server Certificate?
A server certificate is a digital certificate that verifies the authenticity of a server in a computer network. Issued by a trusted CA to a server, it establishes a secure communication channel between a client (such as a web browser) and the server.
The certificate contains information about the server, including its public key. The server certificate is part of secure communication protocols such as HTTPS. When we mention server certificates, we refer to the SSL certificates websites use to ensure data confidentiality.
How Does a Server Certificate Work?
When a user (client) attempts to connect to a secure website, the server presents its SSL certificate to the user’s browser.
The browser checks the server certificate to ensure it’s valid, not expired, and issued by a trusted CA. If valid, the browser proceeds to establish a secure connection.
The server’s certificate contains a public key. The browser generates a random symmetric key and encrypts it using the server’s public key. Then, it sends it to the server.
The server, possessing the matching private key, decrypts the symmetric key.
The encrypted communication between the user and the server continues using the symmetric key, ensuring confidentiality and integrity of the data exchanged during the session.
What Is the Difference Between a Client and a Server Certificate?
You’re about to explore the difference between client and server certificates.
We’ll discuss SSL Server Authentication versus Client Authentication, understand the importance of OID, and go through some practical examples.
SSL Server Authentication Vs Client Authentication
To understand the difference between server and client certificates, it’s essential first to comprehend what SSL server authentication and client authentication entail.
During server validation, the server authentication certificate verifies the server identity to the client using a public key, ensuring a secure connection. On the other hand, client authentication involves a client certificate. Similar to an email client certificate, this offers proof of client identity to the server. The server verifies this client certificate to maintain encrypted communication.
Client certificates, while not directly involved in server authentication, prove the client’s identity and are commonly used in APIs, VPNs, and enterprise systems.
In SSL client authentication, the server certificate verifies the server’s identity to the client. Client certificates, if implemented, actively participate in the handshake, proving the client’s identity to the server. This adds a crucial layer of security, restricting access to authorized clients, particularly valuable in sensitive systems.
Object Identifier (OID)
OID, or Object Identifiers, are unique numbers used in an SSL/TLS certificate to identify different entities within a cryptographic system.
The OID identifies the client’s specific software or hardware in a client certificate. It’s like a digital fingerprint, confirming the client’s identity. In contrast, in a server certificate, the OID pinpoints the server’s identity and encryption capabilities.
Essentially, the OID in a client certificate ensures you’re using trusted and legitimate software or hardware, whereas the OID in a server certificate guarantees you’re communicating with the right server and that your data is encrypted for protection.
Below are some common OIDs used in the X.509 certificates for client and server authentication:
Common OIDs:
- Common Name (CN): 2.5.4.3 (Both)
- Subject Alternative Name (SAN): 2.5.29.17 (Both)
- Key Usage: 2.5.29.15 (Both)
- Extended Key Usage (EKU): 2.5.29.37 (Both)
- Authority Key Identifier: 2.5.29.35 (Both)
- Subject Key Identifier: 2.5.29.14 (Both)
Additional OIDs for Server Certificates:
TLS Web Server Authentication: 1.3.6.1.5.5.7.3.1
Additional OIDs for Client Certificates:
TLS Web Client Authentication: 1.3.6.1.5.5.7.3.2
Server Certificates / Client Certificate Applications
Consider a secure website transaction: the server certificate authenticates the site to you, reassuring you it’s safe to share sensitive info.
Unlike server certificates, a client certificate might be used in a business setting where you must prove your identity to access secure data.
Server Certificates in Practice
- Secure Website Communication: HTTPS protocol with server certificates encrypts data between the user’s browser and server.
- Email Encryption: TLS/SSL certificates on email servers encrypt messages for secure communication.
- API Security: Server SSL certificates ensure secure communication between servers and APIs.
- Virtual Private Networks (VPNs): SSL certificates are crucial for VPN servers as they provide encryption for data transmitted over virtual private networks.
Client Certificates in Practice
- Authentication for Users: Two-factor authentication with client certificates enhances security for authorized entry and protects against brute force attacks.
- Access to Secure Systems: Client certificates verify user identity for access to secure databases.
- Digital Signatures in Emails: Client certificates, like S/MIME, enable digital signatures in email security.
- Code Signing: Code signing certificates authenticate software integrity and source.
Bottom Line
In summary, you’ve learned that client and server certificates play distinct roles in digital security. While a client certificate authenticates the user to a server, a server certificate ensures the server’s legitimacy to the client.
The key distinction lies in who they’re authenticating to whom. Knowing the client certificate vs server certificate difference can make your web interactions safer and more secure.
Remember, a sound understanding of a client vs server certificate will help you navigate the complex world of cybersecurity.
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