Cypherock COT

Correlated Oblivious Transfer (COT) based 2-party multiplication over secp256k1.

This project contains:

• Alice / Server: C++ (Boost.Asio + trezor-crypto)
• Bob / Client: TypeScript (Node.js + protobufjs)

The protocol computes additive shares $(U, V)$ of the product of private multiplicative shares $(x, y)$:

$$ U + V \equiv x \cdot y \pmod n $$

where $n$ is the secp256k1 curve order.

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Why this repository exists

This codebase is a practical, readable implementation of COT-style share conversion from multiplicative shares to additive shares. It is designed for:

• protocol experimentation,
• interoperability between languages (C++ and TypeScript),
• and correctness verification in a local/dev environment.

⚠️ Security notice

This repository is currently focused on protocol correctness and developer testing. It is not production-hardened: no authenticated transport, no TLS, and no advanced session hardening.

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Protocol summary

For each bit $y_i$ of the client scalar $y$, one OT round is executed (256 rounds total):

1. Server samples round scalar $a_i$ and sends $A_i = a_iG$.
2. Client uses choice bit $c_i = y_i$ and replies with:
   • $B_i = b_iG$ if $c_i = 0$
   • $B_i = b_iG + A_i$ if $c_i = 1$
3. Server derives two keys from EC shared points and encrypts:
   • $m_{0,i} = U_i$
   • $m_{1,i} = U_i + x$
4. Client decrypts only $m_{c_i,i}$.

After all rounds:

$$ V = \sum_{i=0}^{255} m_{c_i,i} \cdot 2^i \pmod n $$

$$ U = -\sum_{i=0}^{255} U_i \cdot 2^i \pmod n $$

and both parties verify:

$$ U + V \stackrel{?}{=} x \cdot y \pmod n $$

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Project architecture

Server (server/)

• src/main.cpp — creates random server share x, starts TCP server on port 12345
• src/server.cpp — protocol orchestration, message exchange, final verification
• src/ot_session.cpp — OT round state, encrypted pair generation, additive share computation
• src/crypto_utils.cpp — secp256k1 operations, hashing, AES wrappers, scalar math
• src/proto_utils.cpp — protobuf-compatible framing and message encode/decode

Client (client/)

• src/index.ts — end-to-end protocol execution and verification
• src/client.ts — framed TCP socket helper
• src/ot_session.ts — choice-bit logic, decrypt path, additive share accumulation
• src/crypto_utils.ts — secp256k1, AES, scalar arithmetic utilities
• src/proto_utils.ts — protobufjs message encoding/decoding helpers

Shared schema (proto/)

• proto/cot.proto — message schema and payload definitions

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Message transport and wire format

• Envelope type: CotMessage
• Discriminator field: message_type
• Payload: oneof payload
• TCP framing:
  1. 4-byte big-endian length prefix
  2. protobuf-encoded message bytes

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Cryptographic building blocks used in code

• Curve: secp256k1
• Shared secret: x-coordinate of EC point multiplication
• KDF: SHA-256
• Symmetric encryption: AES-256-CBC (IV prepended in ciphertext blob)
• Scalar arithmetic: modulo secp256k1 order $n$

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Prerequisites

Common

• Git
• Internet access during first CMake configure (to fetch trezor-crypto)

Server (C++)

• CMake >= 3.16
• C++17 compiler (MSVC is expected on Windows)
• Boost headers (>= 1.70)

Top-level CMakeLists.txt sets:

• BOOST_ROOT = C:/boost_1_82_0

If your Boost path differs, override BOOST_ROOT during configure.

Client (TypeScript)

• Node.js >= 20
• npm

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Build and run (Windows)

Use two terminals.

1) Build C++ server

From repository root:

1. Configure CMake (source . and build dir build)
2. Build target cot_server in your preferred configuration (Debug/Release)

Expected executable (Visual Studio multi-config):

• build/server/Release/cot_server.exe

2) Build TypeScript client

From client/:

1. Install dependencies (npm install)
2. Compile (npm run build)

Expected output:

• client/dist/index.js

3) Start protocol

• Terminal A: run cot_server.exe
• Terminal B (in client/): run npm start (or npm run dev)

Expected logs include:

• both sides print their multiplicative shares
• progress every 64 rounds
• verification section
• Result : PASS ✓ when successful

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Development workflow

• Keep protocol message compatibility between:
  • proto/cot.proto
  • server/src/proto_utils.cpp
  • client/src/proto_utils.ts
• If changing round logic, update both:
  • server/src/ot_session.cpp
  • client/src/ot_session.ts
• Rebuild both server and client after protocol changes.

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Troubleshooting

• Boost not found during configure
  • set BOOST_ROOT to your local Boost installation path.
• Port 12345 already in use
  • stop the previous process, or change host/port on both sides.
• Client cannot load protobuf schema
  • verify proto/cot.proto exists at expected runtime path relative to client build output.
• Verification fails (FAIL ✗)
  • clean and rebuild both sides to avoid stale mixed binaries.

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Open-source project files

For maintainers and contributors, see:

• CONTRIBUTING.md for contribution workflow
• CODE_OF_CONDUCT.md for community standards
• SECURITY.md for vulnerability reporting
• LICENSE for Apache 2.0 licensing terms

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License

Licensed under the Apache License 2.0. See LICENSE.