Bug Report: Incomplete Fix for Issue #110 - HYBRID Public Keys Always Use Q×P Basis

Misc
1

Summary

The fix implemented in commit 64ed308 (September 2022) for Issue #110 never worked. All HYBRID key switching users that does not need PRE pay unnecessary storage and performance overhead on public keys because PREMode defaults to INDCPA instead of NOT_SET.

The Problem

Issue #110 identified that GetParamsPK() returns the extended Q×P basis for all HYBRID scenarios, even when PRE functionality isn’t needed. Commit 64ed308 attempted to fix this by adding a check in rns-cryptoparameters.h:

if ((m_ksTechnique == HYBRID) && (m_PREMode != NOT_SET))
    return m_paramsQP;

However, the default value for PREMode in gen-cryptocontext-params-defaults.h is INDCPA, not NOT_SET:

namespace CKKSRNS_SCHEME_DEFAULTS {
    constexpr ProxyReEncryptionMode PREMode = INDCPA;
};
// Same for BFVRNS_SCHEME_DEFAULTS and BGVRNS_SCHEME_DEFAULTS

Since INDCPA != NOT_SET is always true, GetParamsPK() always returns m_paramsQP for HYBRID - the fix never worked.

Timeline

Date Commit Change
Aug 3, 2022 8e1a341 PREMode = INDCPA added as default
Sep 6, 2022 64ed308 Fix attempted with m_PREMode != NOT_SET check
Present - Bug persists (fix was dead on arrival)

Impact

  • Larger public keys for all HYBRID users (overhead depends on parameters)
  • Slower key generation and encryption
  • No security impact (purely optimization issue)
  • Affected users: Everyone using HYBRID key switching (default for CKKS and BGV)

Proposed Fix

Change the default PREMode from INDCPA to NOT_SET in src/pke/include/scheme/gen-cryptocontext-params-defaults.h:

namespace CKKSRNS_SCHEME_DEFAULTS {
    constexpr ProxyReEncryptionMode PREMode = NOT_SET;  // was INDCPA
};

namespace BFVRNS_SCHEME_DEFAULTS {
    constexpr ProxyReEncryptionMode PREMode = NOT_SET;  // was INDCPA
};

namespace BGVRNS_SCHEME_DEFAULTS {
    constexpr ProxyReEncryptionMode PREMode = NOT_SET;  // was INDCPA
};

Also update pre-buffer.cpp to explicitly set PREMode = INDCPA since PRE now requires opt-in.

I Have a Fix Ready

I have implemented and tested this fix locally:

  • All unit tests pass (make testall)
  • PRE examples (pre-buffer, pre-hra-secure) work correctly

I tried to submit a PR but cannot create one directly. My fork with the fix is at:

How can I submit this fix?

Thanks!

Related

2

Thank you for spotting the bug. I’ve added you access to create issues and PRs. I am concerned about the side effects resulting from the proposed solution. For instance, some existing PRE-based applications, e.g., GitHub - openfheorg/openfhe-network-examples: OpenFHE Experiments in Encrypted Network Control and Secure Data Distribution with Proxy Re-Encryption, might be using the default. Please create an issue and let us continue the discussion there to figure out the right way to fix it.

3

Incomplete Fix for Issue #110 - PREMode default causes public keys to always use Q×P basis when using HYBRID key switching · Issue #1108 · openfheorg/openfhe-development

Done :+1: