What Is a TLS Certificate?

What Is a TLS Certificate?

A TLS certificate is a digital certificate that authenticates server identity and enables TLS encryption for secure communication. TLS certificates are issued by Certificate Authorities (CAs) and contain: server domain name, public key, certificate validity period, issuer information, and digital signature.

TLS certificates enable: server authentication (verifying server identity), TLS encryption (enabling encrypted connections), and trust establishment (building trust in secure communications).

TLS certificates are essential for HTTPS, secure email (SMTP with STARTTLS), and other TLS-encrypted services. Learn more about TLS and TLS handshake.

Certificate Purpose

1. Server Authentication

TLS certificates authenticate server identity, verifying that clients are connecting to legitimate servers and not imposters.

2. TLS Encryption

TLS certificates enable TLS encryption by providing public keys used to establish encrypted connections.

3. Trust Establishment

TLS certificates establish trust in secure communications by providing cryptographic proof of server identity.

4. Data Protection

TLS certificates enable data protection by facilitating encrypted communication, protecting sensitive data.

5. Security Indicators

Valid TLS certificates enable security indicators (lock icon) in browsers, building user trust.

Certificate Components

TLS certificates contain several components:

Domain Name

Certificate includes domain name(s) it's valid for (e.g., example.com, *.example.com for wildcard).

Public Key

Certificate contains public key used for TLS encryption and key exchange.

Validity Period

Certificate has validity period (not before, not after dates) - certificates expire and must be renewed. Learn more about why TLS certificates expire.

Issuer Information

Certificate includes Certificate Authority (CA) information that issued the certificate.

Digital Signature

Certificate is digitally signed by CA, providing cryptographic proof of certificate authenticity.

Certificate Authority

What Is a CA?

Certificate Authority (CA) is a trusted organization that issues TLS certificates, verifying certificate requests and signing certificates.

CA Trust

CAs are trusted by browsers and operating systems, enabling certificates issued by CAs to be automatically trusted.

Certificate Chain

TLS certificates are part of certificate chain (certificate → intermediate CA → root CA). Learn more about certificate chains.

CA Types

• Public CAs (Let's Encrypt, DigiCert, etc.) - Trusted by browsers
• Private CAs - Internal certificates for organizations
• Self-signed certificates - Not trusted by default

Certificate Validation

Clients validate certificates by checking CA signature, certificate chain, and certificate validity.

Certificate Validation

Validation Process

Clients validate TLS certificates by: checking certificate signature (verifying CA signature), validating certificate chain (checking chain to trusted root CA), verifying domain match (ensuring certificate matches domain), checking expiration (ensuring certificate hasn't expired), and verifying revocation status (checking if certificate is revoked).

Validation Success

If validation succeeds, TLS connection is established and secure communication begins.

Validation Failure

If validation fails, TLS connection is rejected. Learn more about TLS handshake failures.

Common Validation Issues

• Expired certificates
• Invalid certificate chain
• Domain name mismatch
• Revoked certificates
• Untrusted CA

Certificate Monitoring

Monitor certificate expiration dates and renew certificates before expiration to prevent validation failures.