Ethereum transaction signing with Builder Vault + WalletTransact

sequenceDiagram
    autonumber
    actor client as client application
    participant Blockchain as Blockdaemon Wallet Transact<br> REST API
    box Builder Vault
      participant TSM1 as MPC Wallet <br>(private key share 1)
      participant TSM2 as MPC Wallet <br>(private key share 2)
      participant TSM3 as MPC Wallet <br>(private key share 3)
    end
    #note over client,TSM2: Create wallet
    opt
      client ->> TSM1: create master key
      client ->> TSM1: create wallet 
    end

    #note over client,TSM2: Create transations
    client ->> Blockchain: get unsigned tx<br>(from addr, to addr, amount)
    Blockchain -->> client: return unsigned tx<br>(nonce, gas)
    client ->> TSM1: request signature (unsigned tx hash)
    TSM1 -->> client: return partial signature
    client ->> TSM2: request signature (unsigned tx hash)
    TSM2 -->> client: return partial signature
    client ->> TSM3: request signature (unsigned tx hash)
    TSM3 -->> client: return partial signature
    client ->> client: combine partial signatures
    client ->> Blockchain: compile unsigned tx with signature and broadcast<br>(unsigned tx, signature, public key)

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Prerequisites

• Go https://go.dev/doc/install or use code-spaces
• Register for a demo Builder Vault tenant: https://www.blockdaemon.com/get-started/builder-vault-sandbox-registration
• Download SDK bundle provided in registration email (extract authentication certificates)
• Place Builder Vault authentication certificate key-pair in working directory
• Register for Blockdaemon API Suite Free key and set as environment variable ACCESS_TOKEN

Step 1. Create Wallet with Builder Vault and Ethereum chain path

go run 1-create-wallet/main.go

• note the new Ethereum Wallet public key (to be used for future signing)
• note the new Ethereum Wallet address and fund it with Sepolia ETH https://sepolia-faucet.pk910.de
• note the Builder Vault Master Key ID (to be used for future signing)

Step 2. Craft an unsigned transaction

• set the wallet address created in step 1
• set the destination address to send funds to

go run 2-craft-transaction/main.go

• check the wallet balance is not 0
• note the raw unsigned transaction (to be broadcast once signed)
• note the unsigned transaction hash (to be signed by Builder Vault)
• (optional) decode the raw unsigned transaction to inspect the Blockdaemon provided attributes (https://rawtxdecode.in)

Step 3. Sign transaction with BuilderVault wallet

• set the Builder Vault Master Key ID to be used for signing (created in step 1)
• set the unsigned transaction hash to be signed by Builder Vault (created in step 2)

go run 3-sign-transaction/main.go

• note the transaction signature

Step 4. Broadcast signed raw transaction

• set the wallet compressed public key (created in step 1)
• set the raw unsigned transaction (created in step 2)
• set the transaction signature hash (created in step 3)

go run 4-broadcast-signed-transaction/main.go

• check the transaction hash on a block explorer