[ETHCon Korea 2019] Jung woohyun 정우현

Multi-sig vs Threshold-sig
정우현
May 28. 2019, Ethcon Korea

1 © Copyright Atomrigs Consulting Inc. 2018
Private Key in Blockchain
image source:https://blockchainhub.net/blog/blog/cryptography-blockchain-bitcoin/

Private Key in Blockchain
● Single point of failure
● Recovery problem
● UX issue
● Governance

Classic Valut: One Lock, One Sig, One Key
Private Key Signature
Vault
On-chain

Multi-sig Vault: Two Locks, Two Sig, Two Keys
Private Key Signature Vault
SignaturePrivate Key
On-chain

Threshold Sig: One Lock, One Sig, Two Key Shares
Private Key Share
Signature
Vault
Private Key Share
On-Chain

Naive Secret Sharing
Private Key Share
Private Key Share
Private Key Private Key
Key Generation Key Storing Signing

sMPC Secret Sharing
Private Key Share
Private Key Share
Private Key Share
Private Key Share
Private Key Share
Private Key Share

sMPC Secret Sharing
Private Key Share
Private Key Share
Private Key Share
Private Key Share
Private Key Share
Private Key Share
Never generate a full private key!

sMPC - Secret Share Rotation
Same Lock = Same Public Key
Rotation Rotation

Threshold Signature
Threshold Signature based on secure MPC
● 프라이빗키를 multiparty가
참여해서 생성하는데, 이때 각
개별주체 누구도 full key를 알
수가 없음.
● n 명이 프라이빗키를 나누어
가지고 있고, 이중 t 명 (t of n)이
참여해야만 서명을 만들 수
있음.
● 공개된 퍼블릭키로 이 서명을
검증할 수 있음.

Multi-sig 와 Threshold-sig 비교
Multi-Sig Threshold-Sig
개별 블록체인의 특성에 의존 - 비트코인 멀티시그와
이더리움 스마트 컨트랙 멀티시그는 다른구조
범용적 동일 솔루션 - 체인의 특성에 의존하지 않음.
체인상에서 싱글 사인 트랜잭션과 구분됨 체인상에서 싱글 사인과 구분이 안됨 - 전체 몇명이
참여하는지, 몇개의 사인이 필요한지 알 수 없음
높은 트랜잭션 코스트 - 개별 사인이 모두 온체인에
등록되고 검증되어야 함
싱글 사인 트랜잭션과 동일한 비용 - 개별 사인과정은
오프체인이고 온체인에는 한번만 등록
유연성이 없거나, 컨트랙에 한번 올라간 비지니스
로직을 수정하기 힘들고, 수정하기 위한 비용이 비쌈
참여자들의 동의에 의해 자유롭게 새로운 비지니스
로직을 설정할 수 있음.
사용된 프라이빗 키 분실시 복구가 불가능하거나,
새로운 키를 등록하는데 비용 발생
오프체인상에서 시크릿 쉐어를 쉽게 갱신할 수 있음.
각 개별주체의 시크릿 쉐어는 바뀌어도 퍼블릭 키는
동일

Bitcoin - P2SH Multi-sig
Creating a 2-of-3 P2SH multisig address
bitcoin-cli -testnet createmultisig 2 '''
[
"mjbLRSidW1MY8oubvs4SMEnHNFXxCcoehQ",
"02ecd2d250a76d204011de6bc365a56033b9b3a149f679bc17205555d3c2b2854f",
"mt17cV37fBqZsnMmrHnGCm9pM28R1kQdMG"
]'''
Result
{
"address" : "2MyVxxgNBk5zHRPRY2iVjGRJHYZEp1pMCSq",
"redeemScript" :
"522103ede722780d27b05f0b1169efc90fa15a601a32fc6c3295114500c586831b6aaf2102e
cd2d250a76d204011de6bc365a56033b9b3a149f679bc17205555d3c2b2854f21022d609d2f
0d359e5bc0e5d0ea20ff9f5d3396cb5b1906aa9c56a0e7b5edc0c5d553ae"
}
source: https://bitcoin.org/en/developer-reference#createmultisig

Bitcoin - Multi-sig 2 of 3
bitcoin-cli -testnet decodescript 522103ede722780d27b05f0b1169ef
c90fa15a601a32fc6c3295114500c586831b6aaf2102ecd2d250a76d204011de
6bc365a56033b9b3a149f679bc17205555d3c2b2854f21022d609d2f0d359e5b
c0e5d0ea20ff9f5d3396cb5b1906aa9c56a0e7b5edc0c5d553ae
{OP_CHECKMULTISIG",
"reqSigs" : 2,
"type" : "multisig",
"addresses" : [
"mjbLRSidW1MY8oubvs4SMEnHNFXxCcoehQ",
"mo1vzGwCzWqteip29vGWWW6MsEBREuzW94",
"mt17cV37fBqZsnMmrHnGCm9pM28R1kQdMG"
],
"p2sh" : "2MyVxxgNBk5zHRPRY2iVjGRJHYZEp1pMCSq"
}

Bitcoin - Multi-sig wallets
● bitcoind
● Armory
● Electrum
● Copay
● BitGo
● CoinBase
● GreenAddress

Bitcoin Multi sig wallet examples

Bitcoin Multi-Sig Limitations
● Each public key being used for the multi-sig is visible to the
public
● Size limit of number of keys
● Can not update the security conditions and logics
● Higher transaction cost

Ethereum Multi-Sig Wallets
● ConsenSys multisig wallet
● Gnosis multisig wallet
● BitGo multisig wallet
● Mist multisig wallet
● Parity multisig wallet (2017 bug)

Ethereum Multi sig example

Ethereum - Gnosis smart contract

function addOwner(address owner)
function removeOwner(address owner)
function replaceOwner(address owner, address newOwner)
function changeRequirement(uint _required)
function submitTransaction(address destination, uint value, bytes data)
function confirmTransaction(uint transactionId)
function revokeConfirmation(uint transactionId)
function executeTransaction(uint transactionId)
function isConfirmed(uint transactionId)
function addTransaction(address destination, uint value, bytes data)
function getConfirmationCount(uint transactionId)
function getTransactionCount(bool pending, bool executed)
function getOwners()
function getConfirmations(uint transactionId)
function getTransactionIds(uint from, uint to, bool pending, bool executed)

Ethereum Multi-sig
● Smart contract base
● Flexible than bitcoin
● Each public key being used for the multi-sig is visible to the
public
● Difficult to update the security conditions and logics
● High transaction cost

MPC and Cryptocurrencies
● Research on MPC since 1980’s
● Active research on cryptocurrencies and MPC since 2013
● 2013 Marcin Andrychowicz, et al, “Secure Multiparty Computations on Bitcoin”
(https://eprint.iacr.org/2013/784.pdf)
● 2017 Yehuda Lindell, “Fast Secure Two-Party ECDSA Signing”,
● 2018 Yehuda Lindell et al, “Fast Secure Multiparty ECDSA with Practical Distributed
Key Generation and Applications to Cryptocurrency Custody”,
● 2019 Rosario Gennaro and Steven Goldfeder, “Fast Multiparty Threshold ECDSA
with Fast Trustless Setup” (https://eprint.iacr.org/2019/114.pdf)

Secure Multipary Computation
source: https://www.slideshare.net/sdziembowski/lecture-10-multiparty-computation-protocols-1988894

MPC and Blockchain
● Mainnet - Wanchain, Enigma
● Key management and wallet
● Sepior
● Kzen
● Unbound Tech
● Curv
● Atomrigs

Atomrigs sMPC Wallet & Custody
2P Light Mode (hosted client option)
User
User shared server
(Desktop or Mobile)
User share vault
Custody Server
Custody share vault
User & Rules
DB
Backup Backup
sMPC
web
Personal
Full
Backup
User & Rules
DB
Presentation
web server
Restful API
node.js
Vue
Flask API

2P Normal Mode (local client option)
User
Local Client
(desktop / mobile)
User share vault
(user encryption)
Custody Server
Custody share vault
User & Rules
DB
Backup Backup
sMPC
local
Personal
Full
Backup

3P Light Mode (hosted client option)
User
sMPC client layer
(using class)
User & Rules
DB
User share vault
(user encryption) Custody Server
Custody share
vault
User &
Rules DB
Backup Backup
sMPC
User Backup
Share Storage
(USB/HSM/Cloud)
sMPC
sMPC
Gen & Recovery
Gen & Recovery
Gen, Sign & Recovery
Backup
Copies
web
Presentation layer
Restful api

N-parties Relay Mode ( local client option)
User 1
Local Client
(desktop / mobile)
User share vault
(user encryption)
Custody Server
Custody share
vault
User &
Rules DB
Backup
Backup
Backup Share
Storage
(USB/HSM/Cloud)
Backup
Copies
User n
Local Client
(desktop / mobile)
User share vault
(user encryption)
Backup
Relay Server
sMPC
sMPC
sM
PC
sMPC

Atomrigs sMPC ethereum base wallet

Atomrigs sMPC API

Atomrigs sMPC CLI client

Atomrigs sMPC VUE web client

Atomrigs sMPC
VUE web client

Why outsourcing sMPC?
● Interactive and online requirements for sMPC
● High computational resource requirement for each
participant
● High communication costs (bandwidth & latency)
● Limited number of concurrent parties
● Scaling issues
● Difficult to develop generalized patterns

Outsourced sMPC Network
● Independent network providing outsourced computation
power for sMPC
● Operating as a Layer-2 chain of Ethereum mainnet
● Players - Validator, Computation Provider, Service
Provider, User
● ERC20 token, sMPC on mainnet → controls on total
supplies
● Transferred sMPC on layer-2 chain → security deposit,
validator & computation provider rewards, service fees

Evolving Strategy for Network Expansion
● Deductive vs Inductive
● Developing a generalized sMPC platform for blockchain
as a base layer from scratch is almost impossible!
● Establishing abstract base infrastructure -> security
deposit, rewards and penalty mechanism
● Adding sMPC outsourcing models(or patterns) one by
one
● Adding service based on this outsourced sMPC

Development Phase 0
Key management (Custody & Wallet)
sMPC Computation Provider
Base Shared Services

Development Phase 1 (Adding outsourced computation layer)
Key management
(Custody & Wallet)
sMPC
Computation
Provider
sMPC
Computation
Provider
Outsourced sMPC
Providers
Ethereum
Layer 2
Chain

Development Phase 1 (Expanding horizontal base shared services)
Key management
(Custody & Wallet)
Identity Management
sMPC
Computation
Provider
sMPC
Computation
Provider
Outsourced sMPC Providers
Ethereum
Layer 2 Chain

Development Phase 2 (Adding value added service layer)
Key management
(Custody & Wallet)
Identity Management
sMPC
Computation
Provider
Auction OTC Voting
sMPC
Computation
Provider
New Service
Value Added Services
Outsourced sMPC Providers
Ethereum
Layer 2 Chain

Thank you
● LIME public demo version opens soon (2 and 3 party
versions)
● LIME & Outsourced sMPC Network white paper coming
soon
● Contact: 정우현(Brian Cheong)
a@atomrigs.io

[ETHCon Korea 2019] Jung woohyun 정우현

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