Why do endpoint-based signing models create so much risk during a post-quantum cryptography transition?

Endpoint-based signing spreads private keys across many devices, which makes algorithm upgrades inconsistent and difficult to govern. It increases the chance of shadow keys, legacy algorithms, and missed replacements. In a PQC migration, that decentralization turns cryptographic change into a device-by-device project, which raises operational error, compliance gaps, and the risk of weak keys remaining active.

Why This Matters for Security Teams

Endpoint-based signing makes post-quantum migration risky because it turns cryptographic change into a distributed device management problem instead of a controlled identity and policy program. Every laptop, server, appliance, and build runner that holds a key becomes a potential exception, and exceptions are where legacy algorithms survive. That creates uneven coverage, delayed retirement of weak primitives, and audit evidence that is hard to trust.

This is especially dangerous because cryptographic agility is not just a library upgrade. It depends on inventory, key ownership, certificate lifecycle discipline, and enforcement across heterogeneous systems. NIST guidance on operational resilience and identity controls, including the NIST Cybersecurity Framework 2.0 and NIST SP 800-53 Rev 5 Security and Privacy Controls, both push organisations toward managed, repeatable control execution rather than ad hoc endpoint exceptions. NHIMG research shows why this matters operationally: Ultimate Guide to NHIs — Why NHI Security Matters Now notes that 80% of identity breaches involved compromised non-human identities such as service accounts and API keys.

In practice, many security teams discover their cryptographic inventory is incomplete only after a migration has already exposed the weakest endpoints.

How It Works in Practice

The core issue is that endpoint-based signing places private keys close to the workload that uses them. That can be acceptable for narrow, tightly managed use cases, but during a PQC transition it multiplies the number of places where algorithm support, certificate profiles, and trust anchors must be updated. A single missed endpoint can keep an obsolete signature scheme alive long after the rest of the environment has moved on.

Practitioners usually need to treat this as a lifecycle and control-plane problem. That means establishing authoritative inventory, identifying where keys are generated and stored, and validating whether the signing path is embedded in an application, an agent, a device firmware module, or a user-managed certificate store. It also means separating discovery from enforcement. Discovery finds where signing exists; enforcement ensures only approved algorithms can be used.

• Map all signing endpoints, including hidden service accounts, CI/CD runners, and embedded systems.
• Classify which keys are static, which are rotated, and which cannot be centrally governed.
• Set migration rules for algorithm allowlists, certificate lifetimes, and deprecation milestones.
• Test whether policy updates actually reach every endpoint before old algorithms are retired.

For NHI-heavy environments, the problem is often wider than it first appears. NHIMG’s Top 10 NHI Issues highlights how hidden identities and poor rotation practices persist across enterprise systems, which is exactly the kind of sprawl that slows cryptographic replacement. The ISO/IEC 27001:2022 Information Security Management model is useful here because it emphasizes controlled change and governance, not just technical substitution.

These controls tend to break down in environments with offline devices, embedded firmware, or unmanaged third-party endpoints because the organisation cannot reliably push or verify cryptographic updates.

Common Variations and Edge Cases

Tighter cryptographic control often increases operational overhead, requiring organisations to balance migration speed against service continuity and device heterogeneity. That tradeoff is real in OT networks, air-gapped systems, and legacy appliances, where endpoint-based signing may be the only practical path until the platform can be replaced.

Current guidance suggests a segmented approach rather than a universal cutover. High-assurance systems should move first, but legacy endpoints may need compensating controls such as shorter certificate lifetimes, constrained trust paths, and stronger monitoring for algorithm drift. In some environments, dual-stack support for classical and post-quantum algorithms will be unavoidable for a period, but that should be treated as a temporary bridge, not a stable end state.

The hardest edge case is when keys are tied to software that cannot be centrally reissued, such as vendor-managed agents or appliances with limited update windows. In those cases, the risk is not only that weak algorithms remain, but that no one can prove where they remain. NHIMG’s Ultimate Guide to NHIs — Key Challenges and Risks is a good reminder that incomplete visibility is itself a security failure. Endpoint-based signing is workable only when the organisation can inventory, govern, and retire every signing point on schedule.

Related resources from NHI Mgmt Group

• Why does endpoint-based signing create identity risk?
• Why do non-human identities create more audit risk than human accounts?
• Why do non-human identities create audit risk in modern environments?
• Why do non-human identities create compliance risk even when policies exist?