Neutrino

Neutrino is a privacy-preserving light client protocol for Bitcoin, specified in BIPs 157 and 158 in 2017, that lets resource-constrained wallets verify payments without downloading the full blockchain or disclosing their addresses. Full nodes publish a compact filter summarizing each block's contents, and the client scans those filters locally.

Why it matters

The older light client approach, bloom filters under BIP 37, sent a fingerprint of the wallet's addresses to remote servers, leaking the entire wallet to whoever operated them. Neutrino reverses the flow so the wallet reveals nothing. It underpins mobile Lightning wallets, giving phones a trust-reduced view of the chain, and it widens the population able to use bitcoin without intermediaries. There is no lightweight way to audit a gold vault from a phone; verifying metal always ends in trusting someone's attestation. The protocol shipped in 2019 and remains the standard approach for non-custodial mobile bitcoin wallets.

How it works

For each block, a node builds a Golomb-coded set, a compressed probabilistic structure containing the block's output scripts. A wallet downloads only headers and filters, tests its own addresses against each filter, and fetches complete blocks solely when a filter indicates a possible match. False positives occur occasionally by design, but false negatives cannot, so no payment is missed.