Q19Information Security Systems

The X.509 standard, defined by the ITU-T, dictates the structure and contents of a digital certificate, which is used to bind an entity's identity to their public key. An X.509 certificate is a signed data structure containing the following key fields:

• Version: Indicates the X.509 version in use (commonly v3, which supports extensions).
• Serial Number: A unique identifier assigned by the issuing CA, used for revocation tracking.
• Signature Algorithm ID: Specifies the algorithm the CA used to sign the certificate (e.g., SHA-256 with RSA).
• Issuer Name: The distinguished name of the Certificate Authority that issued the certificate.
• Validity Period: The 'not before' and 'not after' dates that bound the certificate's usable lifetime.
• Subject Name: The distinguished name of the entity (person, server, organization) that owns the public key.
• Subject Public Key Info: The public key itself and the algorithm it is used with.
• Extensions (v3): Additional fields such as Key Usage, Subject Alternative Name, and CRL Distribution Points.

The entire certificate body is hashed and signed with the issuing CA's private key, so any tampering invalidates the signature when verified against the CA's public key.

PKI is the overarching framework of hardware, software, policies, and procedures needed to create, manage, distribute, and revoke digital certificates. Its key components are:

• Certificate Authority (CA): The highly trusted root entity that issues, signs, and manages digital certificates, vouching for the bound identity of each certificate holder. CAs are often organized hierarchically, with a root CA delegating trust to intermediate CAs.
• Registration Authority (RA): A subordinate entity that offloads identity verification work from the CA, checking applicant credentials and approving or rejecting certificate signing requests before forwarding them to the CA for signing.
• Certificate Repository: A centralized, publicly accessible directory (often LDAP-based) where issued certificates and public keys are stored and can be looked up by relying parties.
• Certificate Revocation List (CRL) / OCSP: A CRL is a periodically published, signed list of certificates the CA has revoked before expiration (e.g., due to key compromise). The Online Certificate Status Protocol (OCSP) provides a faster, real-time alternative for checking a single certificate's revocation status.
• End Entities: The users, servers, or devices that hold certificates and use them to prove identity during protocols such as TLS.

In operation, these components interact as follows: an end entity generates a key pair and submits a Certificate Signing Request (CSR) to the RA; the RA verifies the requester's identity and forwards the approved request to the CA; the CA signs and issues the X.509 certificate; relying parties who receive this certificate during a TLS handshake validate it by checking the CA's signature, the validity period, and its revocation status via CRL or OCSP before trusting the enclosed public key.

Trust in this system is transitive and hierarchical: browsers and operating systems ship with a built-in list of trusted root CA certificates, and any certificate that chains up through a sequence of valid signatures to one of these roots is trusted automatically, without the relying party needing prior knowledge of the specific server. This chain-of-trust model is what allows a browser to trust millions of previously unseen websites the moment it verifies an unbroken signature path back to a known root, making PKI the foundation of secure web browsing, code signing, and email encryption.