Light (SPV) clients download only block headers and check merkle proofs, trusting the most-work chain instead of verifying every rule. Learn the trade-offs.
Headers only
A light (SPV) client downloads just the 80-byte headers, about 4 MB per 50,000 blocks, and checks their Proof of work. To confirm a payment, it asks a full node for a merkle proof: a few hashes showing the transaction is in a block.
What it trusts
A light client checks that a transaction is included in a block with lots of work. It doesn't check that the block follows every rule. It assumes miners wouldn't spend that much work on an invalid block, which is true unless miners collude.
Privacy
Early SPV wallets (BIP 37 bloom filters) leaked which addresses belonged to the user. Newer compact block filters (BIPs 157/158) let the client download a filter per block and check privately whether it needs that block.
Other "light" wallets
Many mobile wallets don't use SPV at all: they ask a company's server for balances. That's the most trust and the least privacy. The best upgrade is to point your wallet at your own node.
Full node vs SPV vs server wallet
| Full node | SPV client | Server wallet | |
|---|---|---|---|
| Checks every rule | Yes | No | No |
| Verifies proof of work | Yes | Yes | No |
| Download size | Hundreds of GB | A few hundred MB | Tiny |
| Privacy | Best | Good with filters | Weak |
Connect your light wallet to your own node
Many mobile wallets can point to your own node or Electrum server. You keep the convenience of a phone wallet with the verification of your own node.
Key takeaways
- SPV clients verify headers and merkle proofs, not every rule.
- They trust that the most-work chain is valid.
- Compact block filters improve light-client privacy.
Common questions
Is an SPV wallet safe?
For everyday amounts it's widely used. For large amounts, verifying with your own full node is stronger.
Did Satoshi describe SPV?
Yes, section 8 of the whitepaper describes simplified payment verification.
Is an SPV wallet safe for daily use?
For everyday amounts it is widely used. For large savings, verify with your own full node.
What are compact block filters?
Small per-block summaries (BIPs 157/158) that let light clients find relevant blocks without revealing their addresses.