Address Reuse

Address reuse is receiving more than one payment at the same address. Nothing breaks when it happens — the funds arrive and can be spent — so the cost is not to security but to privacy, and it is cumulative. A public ledger records every transaction against an address permanently, so a single address used for a salary, a refund, a donation page and a purchase links all four to each other, and to whoever knows any one of them. Anyone who learns that address once can watch its balance and every future payment for as long as it is used. This is why hierarchical deterministic wallets generate a fresh receiving address for each payment: all of them belong to the same wallet and are recoverable from the same seed, so there is no bookkeeping cost to using a new one. Where reuse is unavoidable — a published donation address, a service that pins one address to an account — the trade-off is worth making deliberately rather than by default.