Simplified Payment Verification (SPV) is a mechanism described back in the original concept of bitcoin that allows a network participant to make sure that a particular transaction is indeed included in the blockchain without downloading the entire chain of blocks in full. A full node may store tens and hundreds of gigabytes of data, whereas for an SPV check it is enough to download only the block headers — compact records of about eighty bytes each.

The principle of operation is based on a Merkle tree. Each block contains the root of such a tree, summarizing all the transactions included in it. A lightweight client requests from full nodes a chain of hashes (the so-called Merkle path) proving that a specific transaction is part of a confirmed block. If the mathematical verification of the path matches the Merkle root in the header, the payment is considered confirmed.

Where SPV is used

• in lightweight (mobile and desktop) wallets with limited disk space;
• in devices with low computing power;
• where fast synchronization without fully downloading the blockchain is important.

The trade-off of SPV lies in the level of trust: a lightweight client relies on the honesty of full nodes and does not verify the entire history on its own, so it is inferior to a full node in autonomy and resistance to certain attacks. Nevertheless, for everyday payments this approach provides a reasonable balance between security and convenience.