Bloom filters, which were introduced in BIP 37, allow lightweight Bitcoin clients to provide a filter describing which transactions they care about. Full nodes then send back only matching transactions, so the SPV client doesn’t need to download every transaction.

However, this approach can leak user data: analyzing which transactions are returned can hint at the user’s addresses. Consequently, many nodes disabled or limited Bloom filter support. Later proposals, like BIP 158’s compact block filters, offer more privacy-friendly ways for lightweight wallets to fetch relevant data.