Root of Trust

---

The root of trust model defines how users and different CAs can trust one another.

• Each CA issues itself a certificate
  • called a root certificate
    • is self-signed
    • uses an RSA key size of 2,048 or 4,096 bits or the ECC equivalent
    • serves as the trust anchor
    • can be used to sign other certificates issued by the CA
  • Installing the CA’s root certificate means that hosts will automatically trust any certificates signed by that CA

Single CA

• single root CA issues certificates directly to users and computers
• often used on private networks
• problem with this approach
  • the single CA server is very exposed
  • If compromised the whole PKI collapses

Third-party CAs

• most third-party CAs operate a hierarchical model
  • root CA issues certificates to one or more intermediate CAs
  • intermediate CAs issue certificates to subjects (leaf or end entities)
• advantage:
  • different intermediate CAs can be set up with certificate policies
    • enables users to perceive clearly what a particular certificate is designed for
  • Each leaf certificate can be traced to the root CA along the certification path
    • referred to as certificate chaining or a chain of trust

Self-signed Certificates

• PKI can be difficult or expensive to manage
• Any machine, web server, or program code can be deployed with a self-signed certificate
  • has been signed by the entity that issued it, rather than by a CA
• e.g., web administrative interfaces of consumer routers are often only protected by a self-signed certificate
• useful in development and test environments
• OS or browser will mark self-signed certificates as untrusted
  • user can choose to override this
• very difficult to validate
• should not be used to protect critical hosts and applications