Blockchain applications increasingly demand not only data integrity, but also efficient, verifiable interactions with off-chain systems. Nexa’s new OP_MERKLEROOT opcode meets this need by embedding Merkle-proof verification directly into script execution. This innovation bridges on-chain consensus and external verifiable databases, enabling smart contracts to confirm data operations, such as insertions, deletions and updates, without trusting third parties or incurring prohibitive resources.
By bringing Merkle-proof logic into the scripting layer, OP_MERKLEROOT transforms how developers architect blockchain-backed services. Registries, supply-chain tracking, decentralized identity systems and any application requiring cryptographic attestations of external state can now leverage a compact, efficient primitive. The result is both higher throughput, because proofs remain logarithmic in size and stronger security guarantees, since every step of the proof is enforced by the consensus rules themselves.
What is OP_MERKLEROOT?
OP_MERKLEROOT is a new instruction (or “opcode”) in Nexa’s scripting language that allows smart contracts to verify data stored in external databases using a cryptographic structure called a Merkle tree. A Merkle tree organizes data into a compact, verifiable format, ensuring that operations like adding, removing or updating data can be checked efficiently without trusting intermediaries.
Imagine a library catalog, instead of checking every book to confirm one title exists, you use a single, unique “fingerprint” (a Merkle root) that summarizes the entire collection. OP_MERKLEROOT lets Nexa smart contracts check this fingerprint to confirm data operations, such as verifying a book’s presence, without scanning the whole library. Process is fast, secure and uses minimal blockchain resources (or “gas”), making it ideal for real-world applications (RWA).
For Those Technically Minded…
Such a breakthrough in blockchain efficiency deserves an explanation which gives a more thorough treatment.
Definition
OP_MERKLEROOT calculates a merkle root given a list of elements and a merkle proof. This instruction can be used to efficiently verify a variety of operations in Nexa script on verifiable databases.
proof elementsArray N option OP_MERKLEROOT → [N adjacency] [N depth] [N index] merkleRoot
Parameters: - proof: A bytearray merkle proof as defined in the “Merkle Proof Definition” below - elementsArray: N bytearrays individually pushed onto the stack, where each push corresponds to either the raw element or the element hash depending on the selected options. Must be <= 256. (REQ1) - N: Integer number of elements to be proven. Must be > 0 (REQ 2) - option: Merkle root algorithm selection (see Specifications).
How OP_MERKLEROOT Works
OP_MERKLEROOT allows a smart contract to calculate a Merkle root, a unique hash summarizing a dataset, based on a provided list of data elements and a Merkle proof. The proof is a compact set of instructions that confirms whether specific data (e.g., a transaction or record) belongs to the dataset. A simplified breakdown:
Smart contract receives:
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A list of data elements (e.g., transaction records).
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A Merkle proof (instructions to build the Merkle tree).
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An option to select a hashing algorithm (e.g., SHA256 for security).
Contract processes the proof, hashing elements step-by-step to compute the Merkle root. Computed Merkle root is compared to the expected root. If they match, the data is verified as authentic.
For example, a supply chain app might use OP_MERKLEROOT to verify that a shipment record exists in a database of 10,000 entries. Instead of checking all 10,000 records, the contract processes a small proof (a few kilobytes) in seconds, saving time and resources.
External Verifiable Databases
An external verifiable database is a database that is not part of the current system (Nexa blockchain), yet memory and time efficient methods exist to verify the correct behavior of many database operations, such as insertion/removal and proof of existence/non-existence.
An external verifiable database publishes a succinct probabilistically unique fingerprint (AKA cryptographic hash) of the data in the database and transactions can verify database operations, each operation producing a new fingerprint reflecting the changed database state.
Note About The Block’s Merkle Tree
Because of Nexa derivation from the Bitcoin source code, Nexa represents “empty” subtrees without an explicit empty element. Instead, blocks define Merkle trees in which all leaves reside at the same depth, and any absent (i.e. “empty”) subtrees occur on the right-hand side. When a subtree is empty, the parent’s hash is computed by duplicating the non-empty child’s hash, this condition is referred to as a “mutation” within the full-node implementation.
The new opcode may be employed to validate such “mutated” Merkle proofs. In these cases, the proof must omit any use of the xxx_IS_EMPTY instruction; rather, it should utilize MULTIPROOF_RIGHT_SIBLING, supplying the current node’s hash as the sibling. (Proofs could alternatively employ MULTIPROOF_LEFT_SIBLING with the current node’s hash, since multiple valid proofs exist corresponding to different assumed leaf positions).
Note that having a smart contract itself verify a transaction’s inclusion in a block via a Merkle proof is generally inadvisable, one would also need to establish that the block is part of the canonical chain. A more practical pattern is to leverage consensus directly by importing an existing UTXO into the contract call as a “read-only” input, thereby ensuring the output remains unspent (read more Read-only inputs).
Conclusion and Implications
Nexa’s OP_MERKLEROOT enables hybrid workflows, combining off-chain efficiency with on-chain security. Integrating OP_MERKLEROOT into Nexa script, developers can now embed cryptographic attestations of off-chain state directly within smart contracts. This capability unlocks a new class of decentralized applications:
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Decentralized Data Markets: Buyers can verify data provenance without trusting oracles, sellers can cryptographically prove dataset integrity.
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Hybrid On-chain/Off-chain Workflows: Systems that combine high-speed off-chain databases with on-chain settlement gain succinct, verifiable bridges, minimizing resources and maximizing throughput.
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Composability with DeFi and NFTs: Tokenized assets can carry verifiable proofs of real-world events (e.g. supply-chain milestones, identity credentials), all enforced by consensus.
Going forward, developers should prototype common database operations, such as batch updates, append-only logs and membership/non-membership checks, using OP_MERKLEROOT. Early integration into SDKs and wallet-level libraries will accelerate adoption by abstracting proof-generation details away from end-users. Ultimately, this feature positions Nexa as a leader in verifiable computation, marrying the efficiency of external databases with the security of blockchain consensus.
References and Further Reading
For technical details and specifications:
Nexa OP_MERKLEROOT Specifications