Cert Decoder - Online Certificate Decoder for X.509 Certificates

Cert Decoder is a free online tool for decoding X.509 SSL/TLS certificates in PEM format. Instantly extract key certificate details, including the issuer, validity period, Subject Alternative Names (SAN), serial number, and fingerprint. All processing happens securely in your browser—no data is sent to a server.

Easily verify the details of your SSL/TLS certificate, including expiration dates and domain associations. A PEM certificate is a base64-encoded block that starts with "-----BEGIN CERTIFICATE-----" and ends with "-----END CERTIFICATE-----". Use Cert Decoder to check the authenticity of your certificate instantly.

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X.509 Certificates Explained

X.509 is an international standard defined by the International Telecommunication Union (ITU) that specifies the format and structure of public key certificates. These certificates play a fundamental role in establishing secure communications over the internet, particularly within protocols such as TLS (Transport Layer Security) and SSL (Secure Sockets Layer), which form the basis for HTTPS connections.

An X.509 certificate binds an entity's identity—such as a domain name, organization, or individual—to a public key through a digital signature. This binding ensures that users can trust the authenticity of the public key presented during secure communications. Certificates can either be issued by trusted third-party entities known as Certificate Authorities (CAs) or be self-signed for internal or testing purposes.

Main Components of an X.509 Certificate:

Version Number: Indicates the certificate format version (typically v3).
Serial Number: A unique identifier assigned by the issuing CA.
Signature Algorithm: Specifies the cryptographic algorithm used to sign the certificate.
Issuer Name: Identifies the Certificate Authority that issued the certificate.
Validity Period: Defines the timeframe during which the certificate is considered valid.
Subject Name: The identity of the entity associated with the public key.
Subject Public Key Info: Contains details about the public key itself, including algorithm and key length.
Extensions: Optional fields providing additional information, such as usage constraints, alternative names, and revocation details.

Types of X.509 Certificates:

Certificate Authority (CA) Certificates: Used to issue and validate other certificates, creating a chain of trust.
End-Entity (Leaf) Certificates: Issued to end-users, servers, or applications to authenticate their identity and cannot issue further certificates.

Common Uses of X.509 Certificates:

Secure Web Browsing (HTTPS): Ensuring secure communication between web browsers and servers.
Email Encryption and Signing (S/MIME): Securing email communications through encryption and digital signatures.
Code Signing: Verifying software authenticity and integrity before installation or execution.
User Authentication: Providing secure user authentication in various systems and networks.

X.509 certificates are typically validated through a hierarchical trust model known as a certificate chain, where each certificate is verified by tracing back to a trusted root CA certificate. Proper management, renewal, and revocation of these certificates are essential practices for maintaining secure digital environments.

This content is adapted from information available on Wikipedia's X.509 article, licensed under Creative Commons Attribution-ShareAlike 4.0 International License.