At this Meetup, Destry discussed the newly open-sourced multisig contract for Unchained Capital's ethereum storage. There is a blog post that covers some of the same material: https://blog.unchained-capital.com/a-simple-safe-multisig-ethereum-smart-contract-for-hardware-wallets-a107bd90bb52

1. TUCSON BLOCKCHAIN DEVELOPERS
MEETUP #4
Agenda:
15min - Introductions
45min - Ethereum Trezor Multisig Vault Contract & dApp by Destry
15min - Discussion/Planning
Ground Rules:
1. No Prices. No Politics.
2. Don’t be an asshole.
This week’s sponsors:
Borderlands Brewery
Unchained-Capital.com

2. UNCHAINED-CAPITAL.COM
We safely hold customer
collateral during the term of the
loan.
Open Source Ethereum Contract and dApp - https://github.com/unchained-capital/ethereum-multisig
Hosted dApp - https://ethereum-multisig.unchained-capital.com/
Blog Post -
https://blog.unchained-capital.com/a-simple-safe-multisig-ethereum-smart-contract-for-hardware-wallets-a
107bd90bb52
Bounty - https://www.unchained-capital.com/bounties/

3. Our MultiSig ‘Account’ Requirements
1. Anyone can deposit
2. N-of-M signatures required for spending/withdrawal
3. The private keys for the M signatures are controlled by Hardware Wallets
4. Account can be created without access to hardware wallet - easy to create.
5. Private keys are not reused.
6. ‘Hot’ wallets have no privileges
7. Simplest code possible
Disclaimer: I might gloss some details for the sake of clarity.

4. Bitcoin Multisig
1. Create Multisig Address using Alice, Bob, and Carol’s public keys.
2. Anyone can fund.
3. Create Spending Transaction - no special information needed
4. Alice and Bob sign spending Tx asynchronously
5. Once signed, anyone can broadcast.

5. Accounts vs. Smart Contracts
With Bitcoin, accounts and smart contracts are the same thing.
In Ethereum, there are 2 distinct types of accounts:
1. User Accounts
a. Single Signature (no code)
b. Can initiate transactions
c. Pays Fees
2. Contract Accounts
a. Code-based
b. Cannot initiate transactions, but can send transactions when authorized
c. Does NOT pay Fees
d. Cannot be interacted with directly

6. Two Types of Ethereum Accounts-Owner & Contract
Owner
Account
Owner
Account
Contract
Account
Contract
Account

7. Our MultiSig ‘Account’ Requirements
1. Anyone can deposit
2. N-of-M signatures required for spending/withdrawal
3. The private keys for the M signatures are controlled by Hardware Wallets
4. Account can be created without access to hardware wallet - easy to create.
5. Private keys are not reused.
6. ‘Hot’ wallets have no privileges
7. Simplest code possible

8. Previous Ethereum Multisig Accounts
Ethereum Foundation -
https://github.com/ethereum/dapp-bin/blob/master/wallet/wallet.sol
ConsenSys - https://github.com/ConsenSys/MultiSigWallet
Gnosis - https://github.com/gnosis/MultiSigWallet
Parity - beware!
I chose not to use any of the above because they were too complex

9. https://www.coindesk.com/ethereums-wallets-evolving/
- Multiple Transactions required to execute a spend
- Ability to ‘revoke’ a half-signed spend
- Owners can be changed
- 300+ lines of code
- Signers need a balance to pay fees for each transaction
- You NEED this if you use tx.orgin or msg.sender for authentication
(https://github.com/ConsenSys/MultiSigWallet/blob/master/MultiSigWalletWithDailyLimit.sol#L199)

10. Our Solution
1. Owners are specified when contract is created
2. To authorize a spend, a message is created that contains:
a. Contract Address
b. Destination Address
c. Amount
d. Nonce
3. Owners sign the message and give signatures to one person (server)
4. A single transaction is sent to the contract with the signatures, destination,
and amount.

11. Aside: Public Key (Asymmetric) Cryptography
https://www.petri.com/public_key_infrastructure_pki

12. Our MultiSig ‘Account’ Requirements
1. Anyone can deposit
2. N-of-M signatures required for spending/withdrawal
3. The private keys for the M signatures are controlled by Hardware Wallets
4. Account can be created without access to hardware wallet - easy to create.
5. Private keys are not reused.
6. ‘Hot’ wallets have no privileges
7. Simplest code possible

13. CODE TIME! - Questions?

14. https://github.com/unchained-capital/ethereum-multisig
Used Truffle for Contract Development and Testing
https://github.com/trufflesuite/truffle

15. Contract Factory

16. Anyone Can Pay

17. Message to Sign

18. Verify Signature

19. Tests!
Emacs/Terminal

20. dApp!
Go to browser

21. Things I totally skimmed over
- Trezor’s signing algorithm
- Hierarchical Deterministic Keys
- dApp code
- Bounties!