ITU-T recommendation X.509 is part of the X.500 series, defining a directory service comprising servers that maintain a database of user information, including mappings from user names to network addresses. This service acts as a repository for public-key certificates, each containing a user’s public key signed with the private key of a trusted certification authority (CA). X.509 outlines the authentication services framework provided by the X.500 directory and defines alternative protocols based on public-key certificates. Widely used across various applications like S/MIME, IP Security, and SSL/TLS, X.509 has become a crucial standard since its initial issuance in 1988, with subsequent revisions addressing security concerns.
**X.509 Certificate Structure**
At the core of X.509 are public-key certificates created by trusted CAs and stored in the directory, which makes them accessible to users without needing the directory itself to generate keys or certificates. A typical X.509 certificate includes several key elements: version, serial number, signature algorithm identifier, issuer name, validity period, subject name, subject’s public key information, issuer and subject unique identifiers, and extensions. The certificates are signed by the CA’s private key, allowing users to verify validity using the CA’s public key. This process ensures that certificates cannot be altered undetectably by unauthorized parties.
**Obtaining and Verifying Certificates**
Users can acquire certificates directly from the directory or from each other. A chain of certificates can be established to obtain the necessary public keys securely, especially when multiple CAs are involved. This chain, expressed in notation as X1 X2 X2 B for instance, allows users to navigate through a series of CAs, reinforcing the trustworthiness of the certificates. However, if a user's public key is part of a broader network with multiple CAs, navigating this network requires maintaining directories with clear path structures for efficient certificate verification.
**Certificate Revocation and X.509 Version 3**
Certificates have a defined period of validity but can be revoked before expiration for various reasons, such as compromised private keys. For this purpose, CAs maintain lists of revoked certificates, which are posted in the directory and need to be checked by users. X.509 version 3 introduced enhancements over the earlier version 2, addressing issues such as inadequate subject field specification and the need for additional security policy information. Version 3 allows for optional extensions, which provide greater flexibility in specifying key and policy information, subject and issuer attributes, and certification path constraints.
**Key and Policy Information, Subject Attributes, and Path Constraints**
Version 3 extensions categorize into key and policy information (like authority key identifiers and policy mappings), subject and issuer attributes (such as alternative names), and certification path constraints (including basic constraints indicating if a subject can act as a CA). These enhancements cater to modern requirements, ensuring heightened flexibility and security in managing digital certificates, facilitating operations such as encrypted communication and policy management across diverse applications and user environments.
0 Comments