The Certificate Authority (CA) is the ultimate root of trust within a Public Key Infrastructure (PKI). The entire security model relies on the integrity and trustworthiness of the CA. If a CA is compromised, an attacker can gain control of its private signing key. This would allow the attacker to issue fraudulent certificates for any entity, effectively impersonating users or services. Such a compromise invalidates all certificates issued by that CA and completely undermines the trust of the entire infrastructure, representing a catastrophic, systemic failure. This makes the compromise of the CA the most significant and central risk to any PKI deployment.

B. The certificate is invalid because it is on a Certificate Revocation List.

This is a standard operational function of a PKI, not a risk. The Certificate Revocation List (CRL) is a security control used to mitigate the risk of compromised or invalid certificates.

C. The users lose their public keys.

Public keys are designed to be publicly available and can be easily re-distributed or re-issued. The significant risk is the loss or compromise of a user's private key, not the public key.

D. The HR department wants to be a Registration Authority (RA).

This is an organizational and procedural decision, not an inherent technical risk of the PKI architecture. While poor implementation of an RA can introduce risk, it is not the main risk to the PKI itself.

1. National Institute of Standards and Technology (NIST) Special Publication 800-32, "Introduction to Public Key Technology and the Federal PKI Infrastructure".

Section 6.1, "Compromise of a CA's Private Signing Key", states: "The compromise of a CA's private signing key is the most significant threat to a PKI. If a CA's private key is compromised, the integrity of all certificates issued by that CA is suspect."

2. Internet Engineering Task Force (IETF) RFC 5280, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile".

Section 7, "Security Considerations", emphasizes the criticality of protecting the CA's private key: "The public key infrastructure is dependent on the ability of a certification authority to generate and publish certificates. To provide this service, the CA must protect its private key from disclosure. Compromise of the CA private key has a catastrophic effect on the integrity of the public key infrastructure."

3. Carlisle, A., & O'Hanlon, P. (2007). PC-1: A PKI-Based End-to-End Security Framework for Personal Computing. In Proceedings of the 2007 International Conference on Security and Management (SAM'07).

This academic paper discusses PKI architecture and notes that "The most serious threat to a PKI is the compromise of a CA private key," as this allows an attacker to "issue bogus certificates and CRLs," which "destroys the credibility of the PKI." (Paraphrased from the general security analysis sections common in such papers).