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Now Let's Make It Physical: Enabling Physically Trusted Certificate Issuance for Keyless Security in CAs

Xiaolin Zhang, Chenghao Chen, Kailun Qin, Yuxuan Wang, Shipei Qu, Tengfei Wang, Chi Zhang, Dawu Gu

TL;DR

Armored Core addresses CA signing-key exposure by replacing digital CA keys with PUF-based, physically trusted bindings, preserving compatibility with existing PKI workflows. It introduces PUF-based X.509 certificates and a PUF transparency mechanism that logs invocations through the Certificate Transparency infrastructure. The authors implement software and hardware prototypes integrated with Let's Encrypt Pebble, Certbot, and Trillian, and provide formal proofs and extensive evaluations showing security gains with moderate improvements in performance and minimal overhead. The work demonstrates that key exposure can be mitigated by binding issuance to PUF hardware, enabling a scalable path toward physically trusted PKI while maintaining interoperability with current infrastructure.

Abstract

The signing key protection of Certificate Authorities (CAs) remains a critical challenge in PKI. Traditional approaches struggle to eliminate the risk of key exposure due to those (un)intentional human errors. This long-standing dilemma motivates us to propose Armored Core, a novel PKI security extension using the trusted binding of Physically Unclonable Function (PUF) for CAs. PUFs leverage manufacturing variations to generate unique and random responses. Combining with XOR and hash, they can make key exposure impossible for CAs through keyless certificate issuance. In Armored Core, we design a set of PUF-based X.509v3 certificate functions for CAs to generate physically trusted "signatures" without using a digital key. Moreover, we introduce a novel PUF transparency mechanism to effectively monitor the PUF operations in CAs. We integrate Armored Core into real-world PKI systems including Let's Encrypt Pebble and Certbot. We also provide a PUF-embedded hardware prototype. The evaluation results show that Armored Core can achieve keyless certificate issuance while improving the computation performance by 4.9%~73.7%. It only incurs small communication and storage overhead (<4%).

Now Let's Make It Physical: Enabling Physically Trusted Certificate Issuance for Keyless Security in CAs

TL;DR

Armored Core addresses CA signing-key exposure by replacing digital CA keys with PUF-based, physically trusted bindings, preserving compatibility with existing PKI workflows. It introduces PUF-based X.509 certificates and a PUF transparency mechanism that logs invocations through the Certificate Transparency infrastructure. The authors implement software and hardware prototypes integrated with Let's Encrypt Pebble, Certbot, and Trillian, and provide formal proofs and extensive evaluations showing security gains with moderate improvements in performance and minimal overhead. The work demonstrates that key exposure can be mitigated by binding issuance to PUF hardware, enabling a scalable path toward physically trusted PKI while maintaining interoperability with current infrastructure.

Abstract

The signing key protection of Certificate Authorities (CAs) remains a critical challenge in PKI. Traditional approaches struggle to eliminate the risk of key exposure due to those (un)intentional human errors. This long-standing dilemma motivates us to propose Armored Core, a novel PKI security extension using the trusted binding of Physically Unclonable Function (PUF) for CAs. PUFs leverage manufacturing variations to generate unique and random responses. Combining with XOR and hash, they can make key exposure impossible for CAs through keyless certificate issuance. In Armored Core, we design a set of PUF-based X.509v3 certificate functions for CAs to generate physically trusted "signatures" without using a digital key. Moreover, we introduce a novel PUF transparency mechanism to effectively monitor the PUF operations in CAs. We integrate Armored Core into real-world PKI systems including Let's Encrypt Pebble and Certbot. We also provide a PUF-embedded hardware prototype. The evaluation results show that Armored Core can achieve keyless certificate issuance while improving the computation performance by 4.9%~73.7%. It only incurs small communication and storage overhead (<4%).
Paper Structure (40 sections, 2 theorems, 1 equation, 18 figures, 6 tables, 1 algorithm)

This paper contains 40 sections, 2 theorems, 1 equation, 18 figures, 6 tables, 1 algorithm.

Table of Contents

  1. Introduction
  2. Background & Motivation
  3. PKI and Certificate Transparency
  4. Signing Key Exposure of CA
  5. Existing Mitigations and Limitations
  6. PUF for CA: Build Trust Without Keys
  7. Armored Core Overview
  8. Threat Model
  9. Design Overview
  10. Building Blocks
  11. PUF Abstraction for CA Servers
  12. PUF Abstraction
  13. Usage of PUF in CAs
  14. PUF-based X.509 Certificates
  15. Physically Trusted Signature
  16. ...and 25 more sections

Key Result

Theorem 1

When $H$ is a standard cryptographic hash function with collision resistance and XOR universality, if PUF $\mathcal{P}$ is $\epsilon_{P}-$secure PUF family, then for any PPT adversary $\mathcal{D}$ that makes at most $q$ signing queries, its advantage $\mathtt{Adv}_{\mathcal{D}}^{IND}$ of distinguis

Figures (18)

  • Figure 1: Comparison of existing approaches on CA certificate issuance in PKI.
  • Figure 2: Technical vulnerabilities that cause the exposure of CAs' signing keys since 2010s. ① Insecure implementation: Flawed logic CVE-2011-4197CVE-2017-6339CVE-2023-47640 and uncensored log CVE-2021-29792CVE-2022-23716. ② Vulnerable configurationCVE-2015-7328CVE-2018-17612CVE-2020-28053: The signing key file is erroneously configured to be publicly readable. ③ Insufficient key protection: CAs store the keys without strong protections CVE-2012-3037.
  • Figure 3: Basic arbiter PUF construction
  • Figure 4: SRAM cell
  • Figure 6: Architecture of Armored Core-enabled PKI.
  • ...and 13 more figures

Theorems & Definitions (4)

  • Definition 1
  • Definition 2: AXU Hash
  • Theorem 1
  • Theorem 2