Public Key Infrastructure

Public and Private Keys

  • Encrypt a document with the recipient's public key. Only their private key needs to be kept secret and only it can decrypt the message.
  • The sender's private key is used to sign the message.

PKI Components Public Key Infrastructure

  • Solves the issue with key management.
  • A set of roles, policies, and procedures needed to manage public-key (asymmetric) encryption.
  • The process of creating, managing, distributing, storing, using, and revoking keys and digital certificates.
  • Public Key Infrastructure X.509 (PKIX) is the working group formed by the IETF to develop standards and models of PKI.

PKI Digital Certificate

  • A digitally signed block of data used to prove the ownership of a public key issued by a Certificate Authority.
  • Includes:
    • Information about the key.
    • Information about the identity of its owner (called the subject).
    • The digital signature of an entity that has verified the certificate's contents (called the issuer).
  • X.509 v3 Standard defines the certificate formats and fields for public keys.

Digital Certificate Components

X.509 Certificate Types

  • The original file extension for X.509 is PFX.
    • A PFX certificate file is used by Microsoft and contains both the public and private keys.
    • The container is fully encrypted.
    • You should use OpenSSL to convert this into a PEM encoded file.
    • The two most common file types for exporting the private key are PFX and P12.
    • Primary Enhancement Mail (PEM) certificates are primarily used for web servers and can be read in a text editor.
    • The PEM encoded file contains the certificate encoded in encrypted Base64.
  • Root Certificates - for root authorities. These are usually self-signed by that authority and often kept offline.
  • Domain Validation (DV) - includes only the domain name.
  • Organizational Validation (OV)
    • Organizations are vetted against official government sources.
    • Common for public-facing websites.
  • Extended Validation (EV)
    • Highest level of trust.
    • Requires a comprehensive validation of the business.
    • Provides additional validation for HTTPS web sites.
    • It provides the name of the legal entity responsible for the web site.
    • These certs require the most effort by the CA to validate and provide a higher level of trust than domain validation because they are validated using more information than the domain.
  • Wildcard Certificates - allows subdomains for a single registered domain (*
  • Subject Alternate Name (SAN) - special X.509 that allows additional items (IP addresses, domain names, and so on).
  • Code Signing Certificates
    • Used for code that is distributed over the internet, including programs or apps.
    • Verifies the code's origin and helps the user trust that the claimed sender is the originator.
  • Machine/Computer Certificates
    • X.509
    • Assigned to a designated machine.
    • During authentication, the machine requesting access must supply the certificate assigned to it.
  • Email Certificates - securing email (S/MIME).
  • User Certificates - for individual use.

Certificate Formats

  • Distinguished Encoding Rules (DER)
    • Stored as binary files.
  • Primary Enhanced Mail (PEM)
    • Primarily used for Unix/Linux servers and can be read in a text editor.
    • A PEM encoded file extension contains the certificate encoded in encrypted Base64.
    • Two formats: Base64-encoded or DER-encoded binary.
  • Personal Exchange Formate (PFX)
  • Canonical Encoding Rules (CER)
    • Stored as ASCII files.
  • P12
  • P7B

PKI Components - Certificate Authority (CA)

  • Trusted entities.
  • Internal - AKA self-signed.
  • External / Third-Party (Symantec, GoDaddy, etc.)
  • Duties:
    • Issues certificates.
    • Verifies the holder of a digital certificate.
    • Ensures that holders of certificates are who they claim to be.

PKI Components - Registration Authority (RA)

  • Offloads work from the CA.
  • Validate user's or endpoint's identities.
  • Accepts registrations.
  • Distributes keys.
  • Does NOT issue certs.

Certificate-Signing Request (CSR)

  • Request from applicant to CA top apply for a digital cert.
  • Includes:
    • Applicant's public key.
    • Fully qualified domain name.
    • Legally incorporated name of the company.
    • Address.'

Certificate Revocation

  • The process of invalidating a cert before its expiration date, often due to private key loss or compromise.
  • Three levels:
    • Valid
    • Suspended
    • Revoked
  • Certificate Revocation List (CRL)
    • Method for distributing certificate revocation information.
      • Must be often updated.
    • Certificate compared against CRL.
    • CRL must be updated and maintained constantly.
  • Online Certificate Status Protocol (OCSP) - checks certificate status in real time.
  • OSCP Stapling
    • Reduces load on CA.
    • Allows the web server to "staple" a time-stamped OCSP response as part of the TLS handshake with the client.
    • The web server is now responsible for handling OCSP requests instead of the CA.

Certificate Trust Models

  • Single CA
    • Simples, no redundancy.
    • Self-signed cert.
  • Hierarchical Model
    • Root CA - top of the hierarchy, may be offline.
    • Intermediate CA - subordinate CAs provide redundancy and load balancing.

Trust Models

  • Certificate Chaining
  • Web of Trust - A cross-certification model.
    • Peer-to-peer trust relationship with other CAs.
  • Bridge CA - Cross-certification model using a central point of trust.

Key Escrow

  • Trusted third party maintains keys.
  • Addresses the possibility that a crypto key may be lost.
    • If key is lost, then the data is lost.
  • Key Recovery Agent is an entity that has the ability to recover a key, key components, or plaintext messages as needed.


  • Hashes of public keys for popular web servers are included with applications such as web browsers.
  • Mitigates the use of fraudulent certs.
  • HTTP Public Key Pinning (HPKP) - uses public key pins, which are essentially hashed values of the public key communicated to the browser client from the server in the HTTP header

Subject Alternative Name (SAN) - {source}

  • A field in the certificate definition that allows you to stipulate additional information, such as an IP address or host name, associated with the certificate.
Last modified 1yr ago
Copy link
On this page
Public and Private Keys
PKI Components Public Key Infrastructure
PKI Digital Certificate
Digital Certificate Components
X.509 Certificate Types
Certificate Formats
PKI Components - Certificate Authority (CA)
PKI Components - Registration Authority (RA)
Certificate-Signing Request (CSR)
Certificate Revocation
Certificate Trust Models
Trust Models
Key Escrow
Subject Alternative Name (SAN) - {source}