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Certificate Standards

links: AC2 TOC - X.509 - Index

X.509 certificates are not compatible to OpenPGP or CVC.

X.509

The X.509 Certificates Authentication Framework designed by CCITT/ITU for the purpose of securing the X.500 Directory included a certificate specification where the format and data type remain important until today.

Used in:

  • The official IETF PKI standard: PKIX (Public Key Infrastructure for X.509)
  • The (older) industrial standard: PKCS (Public Key Cryptography Standards)

X.509 is an ITU standard — but also:

  • TLS servers (and sometimes clients) are identified by public key (e.g. mTLS)
  • Public keys are certified by certificate authorities
  • X.509 certificates are the format used for certificates (defines not how to distribute over network)
  • Any certificate authority can certify keys for any domain (in contrast to DNSSEC, only parent zone can certify a child zone)

Also used by S/MIME

PKCS

  • see PKCS
  • Published by RSA
  • Goal was to promote public-key techniques
  • Not open standards \(\rightarrow\) Industrial standards
    • Most were moved into PKIX

X.500

X.500 entries are based on a Distinguished Name (DN), which is part of a global unique namespace. (Hint: LDAP)

Hierarchy:

  1. Roots
  2. Countries
  3. Organisations
  4. Organisational units
  5. People

Root \(\rightarrow\) USA \(\rightarrow\) Google \(\rightarrow\) Staff \(\rightarrow\) John Doe

CN = Thawte Personal Freemail Issuing CA,
O = Thawte Consulting (Pty) Ltd, C = ZA

ASN.1 and DER

Abstract Syntax Notation One (ASN.1):

  • Defined by the ITU-T
  • Specifies type and structure of data:
    • Type of value (integer, boolean, character string, etc.)
    • Structure (containment, order, options)
  • Does not specify encoding (representation)

Distinguished Encoding Rules (DER):

Provides the advantage to have a unique way of encoding each ASN.1 value.

Privacy-Enhanced Mail (PEM) Encoding

Base64 of DER bytestream with "begin certificate" and "end certificate" markers.

-----BEGIN CERTIFICATE-----

-----END CERTIFICATE-----

Object Identifiers (OIDs)

  • An Object Identifier (OID) is an identifier used to name an object in a hierarchically-assigned namespace \(\rightarrow\) an OID number should never change!
  • In the security domain, OIDs serve to name almost every object type in X.509 certificates, such as components of Distinguished Names, Algorithms, etc.

Example: oid-info.com - oid 1.2.840.113549.1.1.5

Certificate Authorities (CA)

  • see PKI CA
  • Entities that claim to be trustworthy to verify identities and issuing public key certificates
  • CAs can be organized into a directed graph
  • X.509: Tree depth can be limited for a subtree (see X.509 basicConstraints Extension)
  • X.509: Certificates of CAs signing intermediate-level CAs have the special "CA" bit set (see X.509 keyUsage Extension)

Self-signed certificates

  • Signer is self
  • Allowed by TLS
  • Used to sign CA tree roots

links: AC2 TOC - X.509 - Index