Public Key Infrastructure 1 of 3 – Basics

In 2021, I wrote an intro to Public Key Infrastructure (PKI). Now that I have to host my own certificate authority, I decide to dive a little deeper into PKI in this series of posts. In software testing scenario, we need to issue (and recycle) a lot of certificates, and manage their lifecycle events such as (renewal, revocation). As a result, the corporate should establish its own private key infrastructure (PKI). This effort includes hosing their own Certificate Authority.

Concepts

IT professions often use the acronyms PKI and CA interchangeably in the context of implementation. However, strictly speaking, PKI is the entire framework (strategy, policy, etc) around managing certificate at corporate level, and CA is the specific entity that issues certificates. When it comes to architecting PKI, a key design decision is the hierarchy of CAs. Microsoft has a good page explaining the common options.

In a single-tier CA hierarchy, the root CA is also issuing CA. The root CA as the anchor of trust of the PKI also issues certificates to the end entities. Obviously, this does not scale. It is only for sandbox testing, and not suitable for any environment that needs to scale. It also carries significant risk because the private key at the anchor of trust has to remain online.

With a two-tier CA hierarchy, the root CA only issues certificates to its subordinate CAs. Therefore the root CA can go offline. As a result, the subordinate CAs issue certificates to end entities. Since the root CA can remain offline (except for issuing certificates for new CAs), the chance compromise is reduced significantly. This option also scales better with multiple issuing CAs. If one of them gets compromised, then only the end entities of the compromised CA are impacted. The root CA still needs to be online from time to time. Apart from granting new subordinate CA, the team may also need root CA online to sign CRLs, which is a management overhead.

The two-tier hierarchy introduces another problem. The root CA also needs to restrict the certificates that its subordinate CAs can issue. For example, one issuing CA can only issue certificates in the *.dev.digihunch.com domain, and another CA can only issue *.ops.digihunch.com. No subordinate CA shall issue “rogue” certificate beyond their authorized scope. In order to enforce issue boundaries on issuing CAs, RFC 5280 defines multiple ways to express constraints, such as basic constraints (including path length), name constraints, policy constraints, and EKU.

In a three-tier CA hierarchy, the top-level is still a root CA that stays offline. One level below, is a layer of CAs that also stay offline and we refer to them as intermediate CAs. Going down one more level down, there are the issuing CAs for end-entity certificates. Oftentimes, we use the intermediate CAs as policy CAs where we introduce restrictions to the subordinates. In this model, the root CA can remain offline nearly all the time because we can issue CRLs at the intermediate level. This is more flexible, but also more management overhead.

Implementation Options

The two-tier hierarchy is good in most scenarios but the three-tier hierarchy is also common for large organizations. The next consideration is implementation strategy. Take two-tier model for example. We can think about these options:

1. Implement a self-managed PKI, with an internal root CA. The internal subordinate CAs are the issuing CAs. They are chained to the internal root CA.
2. Implement a self-managed PKI, with its certificate purchased from a commercial CA. The internal subordinate CAs are the issuing CAs. They are chained to the external root CA, as the TPP (trusted third party). The external CA may or may not be a public root CA.
3. Purchase certificates from a commercial CA that are chained to a public root CA

Option 3 essentially delegates the PKI to a commercial provider. This is usually not a favourable option due to the hefty charge and minimal control. In option 2, when the third party is a public root CA, you can have all your certificates with public trust, although this is a pricey option too. Even if the third-party isn’t a public root CA, there is still a benefit of delegating the management of Root CA to commercial provider.

Option 1 is for use cases where we need a lot of certificates quick, for example, in agile development iterations. The certificates do not need trust beyond the organization, and the risk of root CA being compromised is manageable. In this option all CAs are private CAs. Option 2 on the other hand, is flexible in terms of trust boundary. You can just rely on the commercial provider as a TTP. If the TPP supports public root CA, you can issue certificates for public facing workload. In other words, you can choose either a private CA or a public CA. When we work with a private CA, we have to import the certificate to the trust stores of the organization.

Some PKI topologies splits a CA into two sub-components: the Certificate Authority Service (CAS, or simply CA) and the Registration Authority (RA), as the diagram above shows. With many locations, each location has an RA that communicates with a with a central CAS. The RAs receive requests from local clients and are responsible for authenticating these requests and pass the validated ones along to the CAS. The validation can take place in ACME protocol. The CAS implicitly trusts RAs and will sign the validated requests from RAs before sending them via API calls. The RA then relays the certificates back to the local requestors. This topology is more scalable with the CAS focusing only on signing and the RAs on validation and passing the requests along. Large organizations may have their central CAS hosted on-prem, and remote RAs in the CSP regions.

Components for X.509 Certificate

X.509 is the standard for digital certificate. The X.509 standard has the following most important fields:

• Subject: The name of the subject (e.g. a user, service, device), commonly represented as X.500 formate distinguished name (DN). For website, the value can be CN=digihunch.com
• Serial Number: A unique identifier for each certificate that a CA issues.
• Issuer: DN of the CA. For a self-signed root CA, the issuer is the subject. They are different otherwise, such as in subordinate CA certificates and end entity certificates.

These fields were introduced in version 1. In addition, X.509 version 3 certificates introduces extensions that provide additional functionality and features to the certificate. Each extension comes in two flavours: critical and non-critical. A certificate-using system MUST reject the certificate if it encounters a critical extension it does not recognize or a critical extension that contains information that it cannot process. A non-critical extension MAY be ignored if it is not recognized, but MUST be processed if it is recognized.

Here are some common extensions that a lot of implementations use:

• Subject Alternative Name (SAN): only used in end entity certificates, not in CA certificates. The format of SAN is flexible and it does not have to be X.500 DN. For website certificate, we often place alternative DNS names here.
• Key Usage: The intended scope of usage for a private key is specified through the Key Usage and Extended Key Usage (EKU) extensions in the associated certificate. Example: “Certificate Sign, CRL Sign”
• Basic Constraints: Used to distinguish between end-entity cert and CA cert. You should also specify path length. The value can be: “CA:TRUE, pathlen:1”. If the path length constraint is 0, the CA may have one more level of subordinate CA. But these subordinate CAs must have path length of 0 on their own certs, and cannot extend one more level as their own subordinates. Those subordinate CAs can only issue end-entity certificates. If the path length value is none, then there is no restriction on the levels of subordinate CAs.
• Name Constraints: for CA certs only, defined in RFC5280, to limit the scope to certain names on the certificate that the CA issues. Client must verify that a certificate is allowed to be signed by CA.
• CDP (CRL distribution points): URL(s) where the application or service can retrieve the certificate revocation list (CRL).
• AIA (Authority Information Access): URL(s) where the application or service can retrieve the revocation list for CA’s certificate
• SKI (Subject Key Identifier): the SHA-1 hash of the subject’s public key
• AKI (Authority Key Identifier): the SHA-1 hash of the issuer’s public key
• EKU (Extended Key Usage, also Enhanced Key Usage): an object identifier (OID) for each application or service a certificate can be used for. It needs to align with Key Usage
• Certificate Policy: reference to the certification practice statement (CPS) of the issuer. During exchange, any relying party can access the assurance level associated with the certificate, and decide on the level of trust to put in the certificate.
• Policy Constraints: for path validation, it can be used to prohibit policy mapping or to require that each certificate in a path contain an acceptable policy identifier.
• Policy Mappings: in CA certificates to restrict the certificates that the CA can issue.

This isn’t a complete list. Plus, X.509 V3 also supports custom extensions. Here is a sample certificate. In real life, the way each implementation uses these extensions may vary slightly, so interoperability issues between PKIs exist. When designing your own PKI, it is important to examine the usage of extensions to comply with the standard. For example, Microsoft has the following recommendations to restrict certificates:

• For subordinate CA certificates, the Basic Constraints extension should be present and marked as critical
• The cA field should be set to TRUE
• The pathLenConstraint field should be set to the minimum value required to enable the business scenario (i.e. 0 if that CA will issue certificates only to End Entities)
• The EKU extension should be present and contain the minimum set of EKU object identifiers (OIDs) to enable the business scenario. Furthermore, the anyExtendedKeyUsage OID (2.5.29.37.0) should not be specified.

Summary

This post discussed the basic concepts in public key infrastructure. In the next post, I’ll cover the automation of certificate issuance.