This chapter covers
- Improving spreadsheet security
- Lightweight (SPV) wallets
- Reducing wallet bandwidth requirements
In chapter 5, we discussed transactions that let anyone verify all transactions in the spreadsheet. But there are still things verifiers can’t verify—that Lisa doesn’t remove or censor transactions. We’ll handle censorship resistance in chapters 7 and 8. This chapter examines how to make it impossible for Lisa to remove or replace transactions without also making it obvious that she’s tampered with the transaction history.
Lisa does this by replacing the spreadsheet with a blockchain (figure 6.1). The blockchain contains transactions that are secured from tampering through hashing and signing the set of transactions in a clever way. This technique makes it easy to provide cryptographic proof of fraud if Lisa deletes or replaces transactions. All verifiers keep their own copies of the blockchain, and they can fully verify it to ensure that Lisa doesn’t remove already-confirmed transactions.
This chapter also introduces a lightweight wallet, or simplified payment verification (SPV) wallet, that will defer blockchain verification to someone else—a full node—to save bandwidth and storage space. This is possible thanks to the blockchain, but it comes at a cost.