# Certificates and Trust Models 🔑🔍

## **X.509 Standard** 📜

* **Overview**:
  * Defines the format of digital certificates.
  * First issued in 1988, with Version 3 being the current standard.
  * Includes specifications for a Certificate Revocation List (CRL) to manage invalid certificates.
* **Fields in an X.509 Certificate**:
  * **Version**: Indicates the X.509 standard version used.
  * **Serial Number**: A unique identifier for the certificate assigned by the Certificate Authority (CA).
  * **Signature Algorithm**: Specifies the public key and hashing algorithms used to sign the certificate.
  * **Issuer Name**: Details the authority that issued the certificate.
  * **Validity**: Contains two subfields:
    * **Not Before**: Start date of certificate validity.
    * **Not After**: End date of certificate validity.
  * **Subject**: Information about the entity to which the certificate was issued.
  * **Subject Public Key Info**: Details the public key algorithm and the public key itself.
  * **Certificate Signature Algorithm**: Must match the algorithm used in the Subject Public Key Info.
  * **Certificate Signature Value**: The digital signature data of the certificate.
* **Certificate Fingerprints**:
  * Hash digests of the entire certificate, used for validation and inspection.

## **Web of Trust** 🌐🔗

* **Concept**:
  * An alternative to the centralized PKI model.
  * Individuals sign each other's public keys rather than relying on a CA.
  * Involves verifying identities through agreed-upon methods (e.g., ID checks) before signing keys.
* **Process**:
  * **Key Signing Parties**: Events where participants verify and sign each other's keys.
  * **Reciprocal Trust**: Each party signs the public key of the other, establishing mutual trust.
  * **Web Expansion**: As new members are introduced and trusted, they extend the web of trust, linking separate trust networks.

## **Summary** 📈

* X.509 certificates provide a structured format for managing digital identities and secure communications.
* The web of trust offers a decentralized approach to establishing trust and verifying identities without relying on centralized authorities.

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