Light Client

A light client is bitcoin software that verifies payments without downloading the entire blockchain, storing only block headers of 80 bytes each rather than the several hundred gigabytes a full node requires. The approach, called simplified payment verification or SPV, was described in section 8 of the original bitcoin whitepaper in 2008.

Why it matters

Full nodes offer the strongest security model, but they demand storage, bandwidth, and sync time that many users cannot spare, especially on phones. Light clients make self-custody practical on constrained devices by letting a wallet confirm that a transaction is buried in the chain with the most accumulated proof of work, without handing any single server full authority over its view of the ledger.

The tradeoff is real. A light client cannot check that every consensus rule was followed, so it inherits weaker assurances than a full node, and depending on its design it can leak information about which addresses it is watching to the servers it queries.

How it works

The client downloads the header chain, roughly 80 bytes per block, and requests Merkle proofs showing that specific transactions are included in specific blocks. Older designs used BIP 37 bloom filters, which proved poor for privacy. Newer wallets use BIP 157 and BIP 158 compact block filters, where the client downloads a small filter for each block and checks it locally, revealing far less about the wallet it serves.