[experimental][threshold_ecdsa] Add threshold ECDSA module with native Paillier and YACL-style cleanup

[jjllzhang]

Summary

This PR adds yacl/crypto/experimental/threshold_ecdsa and finalizes the staged integration work.

• Added threshold ECDSA module structure: common/, crypto/, net/, protocol/, tests/
• Replaced direct secp256k1 usage with YACL ECC (EcGroup, OpenSSL backend)
• Replaced libhcs dependency with native Paillier implementation
• Aligned error handling to YACL style via common/errors.h (TECDSA_THROW*)
• Aligned Bazel build style (yacl_cc_library / yacl_cc_binary)
• Removed bench/docs artifacts from this module (bench/, docs/)

Motivation

• Remove third-party crypto dependency coupling in this module
• Keep protocol flow and interfaces stable while improving maintainability
• Align implementation style with YACL conventions

Main Changes

• Build integration:
  • yacl/crypto/experimental/threshold_ecdsa/BUILD.bazel
  • yacl/crypto/experimental/threshold_ecdsa/CMakeLists.txt
• Core crypto/protocol implementation:
  • ECC wrapper and math verify path
  • Native Paillier keygen/encrypt/decrypt/homomorphic ops
  • Keygen/sign session flow and strict-proof checks
• Tests:
  • crypto_primitives_tests
  • protocol_infrastructure_tests
  • keygen_flow_tests
  • sign_flow_tests
• Docs:
  • Updated README.md to match current module scope (no bench/docs section)

Compatibility

• Module public behavior remains consistent with current flow expectations
• No extra external repo/file changes required outside this module

Validation

Commands executed:

bazelisk build //yacl/crypto/experimental/threshold_ecdsa:tecdsa_core \
  --extra_toolchains=@rules_foreign_cc//toolchains:preinstalled_make_toolchain,@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain

bazelisk run //yacl/crypto/experimental/threshold_ecdsa:crypto_primitives_tests \
  --extra_toolchains=@rules_foreign_cc//toolchains:preinstalled_make_toolchain,@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain

bazelisk run //yacl/crypto/experimental/threshold_ecdsa:protocol_infrastructure_tests \
  --extra_toolchains=@rules_foreign_cc//toolchains:preinstalled_make_toolchain,@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain

bazelisk run //yacl/crypto/experimental/threshold_ecdsa:keygen_flow_tests \
  --extra_toolchains=@rules_foreign_cc//toolchains:preinstalled_make_toolchain,@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain

bazelisk run //yacl/crypto/experimental/threshold_ecdsa:sign_flow_tests \
  --extra_toolchains=@rules_foreign_cc//toolchains:preinstalled_make_toolchain,@rules_foreign_cc//toolchains:preinstalled_pkgconfig_toolchain

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request adds an experimental threshold ECDSA module to YACL, enhancing its cryptographic capabilities. It focuses on improving maintainability and reducing external dependencies by replacing libhcs with a native Paillier implementation and aligning the code with YACL's coding conventions. The module provides threshold key generation and signing functionalities, supported by a comprehensive test suite and updated documentation.

Highlights

• Threshold ECDSA Module: This PR introduces a new threshold_ecdsa module within the YACL crypto library, providing a C++20 implementation of the GG2019 threshold ECDSA protocol.
• Dependency Removal: The PR replaces the libhcs dependency with a native Paillier implementation, reducing external dependencies.
• YACL Alignment: The code has been refactored to align with YACL conventions, including error handling and build style.
• Comprehensive Testing: The PR includes a suite of tests covering crypto primitives, protocol infrastructure, key generation flow, and signing flow.
• Documentation Update: The README.md file has been updated to reflect the current scope of the module.

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Activity

• The PR introduces a new module with core crypto/protocol implementation.
• Includes ECC wrapper, native Paillier, and keygen/sign session flow.
• Adds tests for crypto primitives, protocol infrastructure, keygen, and sign flows.
• Updates documentation to reflect the module's scope.

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[gemini-code-assist]

Code Review

This pull request introduces a new experimental module for threshold ECDSA, which is a substantial and well-structured contribution. The implementation aligns with the GG2019 paper and includes a native Paillier implementation, replacing previous dependencies. The code quality is high, with strong error handling, good security practices like secure zeroization of sensitive data, and a comprehensive test suite covering happy paths and adversarial scenarios. My main feedback concerns the maintainability of several very large files. Splitting them into smaller, more focused modules would significantly improve readability and future development. I've also identified a minor potential integer overflow issue in the random number generation utility.

_{Note: Security Review did not run due to the size of the PR.}