Is a 2-Tier PKI Architecture Better Than a 3-Tier?

Introduction

One of the first and most critical decisions when building a Public Key Infrastructure (PKI) solution is determining the right architecture. This includes considerations such as:

• Separating your Validation Authority (VA), Registration Authority (RA), and Certificate Authority (CA) for security purposes
• Deciding where to host Certificate Revocation Lists (CRLs)
• Determining whether Online Certificate Status Protocol (OCSP) is necessary
• Choosing the depth of your PKI hierarchy; 2, 3, or even 4 tiers?

In this article, we’ll focus on the final topic: how deep your PKI hierarchy should be, and whether a 2-tier or 3-tier approach is more suitable. While 2 and 3 tiers are the most common configurations, they are by no means limitations. It’s entirely feasible to implement a single-tier hierarchy, or even extend to 4 or 5 tiers.

Note: If you’re considering a 1-tier or a 4+ tier PKI hierarchy, it’s strongly recommended to consult with PKI professionals. These more complex or less secure configurations require expert guidance to ensure best practices and robust security.

Why Choose a 2-Tier PKI Hierarchy?

A 2-tier PKI hierarchy is often favoured for its simplicity and ease of management. Typically, this structure involves a single Root CA issuing certificates to one or more Subordinate (Issuing) CAs:

Root CA → Issuing CA → Leaf Certificates

Key Benefits:

• Reduced Complexity: With fewer components to manage, the overall architecture is easier to maintain.
• Lower Operational Overhead: Fewer virtual machines, containers, or physical servers to monitor reduces both staffing needs and ongoing costs.
• Faster Deployment: Simpler infrastructure can accelerate project timelines, potentially turning a 12-month deployment into a 9-month one for example.
• Improved Certificate Lifecycle Management (CLM): With fewer Issuing CAs, CRLs, and OCSP responders to manage, visibility and control over certificate expiration becomes more manageable, reducing the risk of outages.
• Cost-Effective: Simpler architecture means reduced training requirements and fewer PKI-skilled professionals needed for ongoing operations.
• Security Improvement Over 1-Tier: While not as granular as a 3-tier approach, a 2-tier model still offers a significant security improvement over a flat, single-tier PKI.

This architecture is ideal for small to mid-sized organisations where the scale of operations doesn’t justify the complexity of additional tiers.

Why Choose a 3-Tier PKI Hierarchy?

While a 2-tier PKI provides simplicity, a 3-tier hierarchy offers enhanced flexibility and granular control. The structure typically looks like this:

Root CA → Intermediate CA → Issuing CA → Leaf Certificates

Key Benefits:

• Enhanced Security Capabilities: Although the security leap from 1-tier to 2-tier is significant, a 3-tier system offers improved capabilities. For instance, separate Issuing CAs can be created for specific departments or use cases.
• Delegated Administration: Role-Based Access Control (RBAC) can be applied more effectively by assigning administration rights per tier or department.
• Granular Policy Management: Unique certificate issuance policies can be enforced for each Issuing CA, allowing better compliance and auditing practices.
• Compartmentalization: In the event of a compromise, only a limited subset of devices or users are affected. For example, revoking a single Issuing CA does not necessitate reissuing certificates across your entire infrastructure.
• Tailored Usage: Issuing CAs can be configured for specific tasks such as:
  • Device auto-enrollment (e.g., for workstations)
  • Server certificate issuance
  • Application-specific needs

Example Scenario:

Imagine a business with separate PKI requirements for the Operations, Development, and IT Infrastructure departments. A 3-tier PKI could be structured as follows:

+------------------+
|   Root CA (1x)   |
| Self-signed CA   |
+--------+---------+
         |
     -----------------------------------
     |            |                    |
Intermediate   Intermediate       Intermediate  
 CA (Ops)       CA (Dev)          CA (IT Infra)
     |                                 |
     |            ---------------------------------
     |            |                               |
Issuing CA    Issuing CA                      Issuing CA
(Ops)         (IT Servers)                (IT Workstations)
                                              

This structure enables dedicated issuing authorities for servers and workstations, each with distinct policies and administrative controls.

Such an architecture is ideal for:

• Government agencies needing to segregate certificate issuance across functions
• Multinational corporations facing different compliance requirements by region or department

Note: A 3-tier PKI requires more administrative effort and resources. If you’re considering this approach, professional PKI consultation is advisable.

Summary: 2-Tier vs. 3-Tier PKI Hierarchies

Both 2-tier and 3-tier PKI models have their place, depending on your organization’s size, security needs, and operational complexity.

Feature	2-Tier PKI	3-Tier PKI
Complexity	Low	Medium to High
Deployment Time	Faster	Slower
Security	Good	Excellent (more granular)
Certificate Management	Easier	More segmented, flexible
Policy Granularity	Limited	High
Administration Requirements	Lower	Higher

There is no universal “best” option. The right choice depends on the specific needs and constraints of your organization. Consider your available resources, compliance requirements, security posture, and growth plans when determining your PKI architecture.

References

• Microsoft: Designing and Implementing a PKI