Multibit spv Wallet – Simplified Payment Verification (SPV) security is good enough for most Bitcoin applications.Simplified Versus Full Validation. A full node validates block headers and complete block bodies (right). An SPV node, in contrast, validates only headers and transactions of interest (left).
SPV offers users a security tradeoff. In exchange for ignoring block bodies, an SPV node can operate at a fraction of the cost of a full node. Provided that the hash rate majority never tries to publish an invalid block body, an SPV node offers security that approaches that of a full node.
The Stealth Hard Fork
SPV security relies on the honesty of the hash rate majority. In this context, “honest” means that nodes follow the Bitcoin protocol. Specifically, an honest node can be counted on to neither publish nor relay an invalid block. However, this assumption falls apart in the event of a hard fork update.
A hard fork update causes nodes that implement it to publish and relay blocks that were previously considered invalid. Invalid block features can reside in the header, the body, or both. As described above, an SPV node can only detect blocks containing invalid headers. Therefore, blocks with invalid bodies will be accepted as valid.
Stealth Hard Fork. Ideally, a hard fork produces invalid headers and bodies (left). A stealth hard fork produces invalid block bodies only (right). Because SPV nodes ignore block bodies, they can reject blocks with invalid headers, but not those with invalid bodies.
Any hard fork update that produces valid block headers will be invisible to SPV nodes. As shown previously, block size isn’t one of the six values encoded by a block header. Rather, block size can only be computed from a header/body combination. Because SPV nodes specifically avoid requesting block bodies, they can never reject a header based on the size of the block it represents.
This means that in the absence of a signal encoded into the block header, an SPV node will blindly accept any block size limit increase, no matter how large, provided that it is sufficiently strong.