A BUD You Can Count On: Efficient, Verifiable Membership Proofs

Abstract

A block typically changes only a small fraction of blockchain state, yet producing a state root makes validators update an authenticated tree spanning it all. We show how to support verifiable reads from the writes alone. We introduce Block Update Digests (BUDs), that commit to a block's writes and link each key touched in that block to its previous modification. An optional prior-value record lets one successor BUD authenticate the value between modifications, replacing the baseline's two BUD openings. SuperBUDs summarize those writes over longer intervals, letting a reader verify that a value remained unchanged without checking every intervening block. Thanks to BUDs and SuperBUDs, validators can just keep ordinary flat state while still serving proofs of current and historical values, including deletion. We prove soundness and completeness for reads at or after a key's first logged modification when the required proof data is available. In experiments with the baseline format, a 50x increase in state size raises the measured base-BUD cost by just 1.24x. In paired QMDB experiments, lowering its garbage-collection activation threshold raises process CPU time by 1.3-2.7x and device bytes written by 1.6-2.4x relative to its dormant configuration. BUD authentication requires no corresponding relocation of live entries. In the evaluated baseline, two-digest read-layer payloads stay below 800 bytes, and warm hash-path verification takes at most 146 μs at p99. Authenticated reads need not make validators maintain an authenticated tree over the entire state.