Compact Block Filter

Overview

Meaning and scope

A compact block filter is a small probabilistic summary enabling light wallets to identify blocks that may contain relevant scripts or transactions. Filters can produce false positives but should not produce false negatives when built correctly. Clients download matching blocks and perform normal verification, avoiding direct address queries that would reveal wallet interests to a server.

Mechanism and operation

Compact block filters summarize selected elements from a block in a space-efficient structure. In Bitcoin, BIP 157 and BIP 158 define client-server protocols and Golomb-coded sets that let lightweight clients test locally whether a block may match wallet scripts. Wallets should validate filter-header chains, obtain filters from suitable peers, and scan every relevant height. High-assurance systems should use full nodes or audited indexes for complete deposit coverage. Operational use of Compact Block Filter depends on node mode and data retention. Test fixtures for Compact Block Filter should cover normal blocks, boundary sizes, protocol upgrades, and invalid encodings.

Practical payment example

A lightweight Bitcoin wallet downloads a compact filter indicating possible activity for its addresses. A match prompts further inspection of block data; it does not by itself prove that the wallet was paid.

Operational checks

Operational takeaway: Compact block filters improve private light-wallet discovery, while matched blocks still require full transaction and chain validation.

Distinctions and limits

A match is only a prompt to inspect the block, not proof of payment. Verification of Compact Block Filter should begin with a canonical block identifier rather than a height alone.

Related reading

Related definitions: block filters, block.