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Teaching an Old Dog New Tricks: Verifiable FHE Using Commodity Hardware

Jules Drean, Fisher Jepsen, Edward Suh, Srini Devadas, Aamer Jaleel, Gururaj Saileshwar

TL;DR

Argos tackles verifiability in fully homomorphic encryption by relocating attestation secrets to a discrete TPM, yielding an integrity-only enclave design that mitigates microarchitectural side channels. It formalizes circuit-level verifiable FHE (vFHE) and demonstrates how to compose standard FHE with TPM-based remote attestation to achieve malicious security with minimal overhead, including extensions to authenticated PIR and PSI. The implementation on commodity hardware shows low overhead for FHE evaluation and modest costs for complex protocols, aided by batching to amortize TPM usage, and an intentionally small TCB built around a minimal security monitor. By combining cryptography with trusted hardware, Argos enables practical deployment of FHE-based protocols beyond semi-honest settings while avoiding the prohibitive proof overhead of prior approaches.

Abstract

We present Argos, a simple approach for adding verifiability to fully homomorphic encryption (FHE) schemes using trusted hardware. Traditional approaches to verifiable FHE require expensive cryptographic proofs, which incur an overhead of up to seven orders of magnitude on top of FHE, making them impractical. With Argos, we show that trusted hardware can be securely used to provide verifiability for FHE computations, with minimal overhead relative to the baseline FHE computation. An important contribution of Argos is showing that the major security pitfall associated with trusted hardware, microarchitectural side channels, can be completely mitigated by excluding any secrets from the CPU and the memory hierarchy. This is made possible by focusing on building a platform that only enforces program and data integrity and not confidentiality. All secrets related to the attestation mechanism are kept in a separate coprocessor (e.g., a TPM)-inaccessible to any software-based attacker. Relying on a discrete TPM typically incurs significant performance overhead, which is why (insecure) software-based TPMs are used in practice. As a second contribution, we show that for FHE applications, the attestation protocol can be adapted to only incur a fixed cost. Argos requires no dedicated hardware extensions and is supported on commodity processors from 2008 onward. Our prototype implementation introduces 3% overhead for FHE evaluation, and 8% for more complex protocols. In particular, we show that Argos can be used for real-world applications of FHE, such as private information retrieval (PIR) and private set intersection (PSI), where providing verifiability is imperative. By demonstrating how to combine cryptography with trusted hardware, Argos paves the way for widespread deployment of FHE-based protocols beyond the semi-honest setting, without the overhead of cryptographic proofs.

Teaching an Old Dog New Tricks: Verifiable FHE Using Commodity Hardware

TL;DR

Argos tackles verifiability in fully homomorphic encryption by relocating attestation secrets to a discrete TPM, yielding an integrity-only enclave design that mitigates microarchitectural side channels. It formalizes circuit-level verifiable FHE (vFHE) and demonstrates how to compose standard FHE with TPM-based remote attestation to achieve malicious security with minimal overhead, including extensions to authenticated PIR and PSI. The implementation on commodity hardware shows low overhead for FHE evaluation and modest costs for complex protocols, aided by batching to amortize TPM usage, and an intentionally small TCB built around a minimal security monitor. By combining cryptography with trusted hardware, Argos enables practical deployment of FHE-based protocols beyond semi-honest settings while avoiding the prohibitive proof overhead of prior approaches.

Abstract

We present Argos, a simple approach for adding verifiability to fully homomorphic encryption (FHE) schemes using trusted hardware. Traditional approaches to verifiable FHE require expensive cryptographic proofs, which incur an overhead of up to seven orders of magnitude on top of FHE, making them impractical. With Argos, we show that trusted hardware can be securely used to provide verifiability for FHE computations, with minimal overhead relative to the baseline FHE computation. An important contribution of Argos is showing that the major security pitfall associated with trusted hardware, microarchitectural side channels, can be completely mitigated by excluding any secrets from the CPU and the memory hierarchy. This is made possible by focusing on building a platform that only enforces program and data integrity and not confidentiality. All secrets related to the attestation mechanism are kept in a separate coprocessor (e.g., a TPM)-inaccessible to any software-based attacker. Relying on a discrete TPM typically incurs significant performance overhead, which is why (insecure) software-based TPMs are used in practice. As a second contribution, we show that for FHE applications, the attestation protocol can be adapted to only incur a fixed cost. Argos requires no dedicated hardware extensions and is supported on commodity processors from 2008 onward. Our prototype implementation introduces 3% overhead for FHE evaluation, and 8% for more complex protocols. In particular, we show that Argos can be used for real-world applications of FHE, such as private information retrieval (PIR) and private set intersection (PSI), where providing verifiability is imperative. By demonstrating how to combine cryptography with trusted hardware, Argos paves the way for widespread deployment of FHE-based protocols beyond the semi-honest setting, without the overhead of cryptographic proofs.

Paper Structure

This paper contains 57 sections, 13 equations, 2 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Threat model
  3. Background & Motivation
  4. FHE Schemes And Client-Server Setup
  5. Semi-Honest vs. Malicious Security
  6. Why Does FHE Need Verifiability?
  7. Why Do We Need a New TEE Platform?
  8. Insights & Design Principles
  9. FHE Applications Do Not Expose Secrets
  10. Eliminating Microarchitectural Side Channels Using A Physical TPM
  11. System Overview
  12. The Argos Attestation Scheme
  13. Existing Attestation Protocols
  14. Overhead of Using a Discrete TPM
  15. Attested Transcript for FHE Applications
  16. ...and 42 more sections

Figures (2)

  • Figure 1: Evolution of the attack surface on TEE platforms. Enc: enclave program, Att: attestation enclave.
  • Figure 2: Evolution of remote attestation protocols. The naive approach with a secure co-processor is inefficient as it requires several back and forth communications with the discrete chip. The virtual TPM approach is insecure as it manipulates sensitive keys in the CPU, exposing them to microarchitectural side channels. The Argos protocol is both secure and efficient.