Question

What specific data structure is used by a light client to verify the inclusion of a transaction in a block without downloading the entire blockchain history?

Accepted Answer

The specific data structure used by a light client to verify transaction inclusion is the Merkle tree. A Merkle tree is a mathematical structure where every transaction in a block is hashed, and these hashes are paired and hashed repeatedly until a single value remains, known as the Merkle root. This root is stored in the block header. To verify a transaction, a light client only needs the Merkle root from the block header and a Merkle path. A Merkle path is the set of sibling hashes required to reconstruct the root from the specific transaction being verified. The light client hashes the target transaction and combines it with the provided sibling hashes in the path, moving upward through the tree levels. If the final computed value matches the Merkle root stored in the block header, the client has cryptographic proof that the transaction is part of that specific block. This process allows the client to confirm data existence without needing to download or process any other transactions or the full blockchain history.

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What specific data structure is used by a light client to verify the inclusion of a transaction in a block without downloading the entire blockchain history?