Quantumroot vaults are now live on Bitcoin Cash’s 6-months-ahead preview network. Contracts, testing suite, and transaction generation code now available.

Nov 20, 2025 • 3 min read

Today I’m publishing an end-to-end implementation of Quantumroot, a post-quantum vault for CashVM – Bitcoin Cash’s restored Bitcoin Script language.

CashVM makes quantum readiness ultra-efficient: sweeps from quantum-ready addresses cut transaction sizes vs. P2PKH/P2WPKH by up to 10.9%, despite the increase from 20-byte hashes to 32-byte hashes (for highest-level, NIST Category 5 quantum security strength).

Users who regularly buy or earn Bitcoin Cash to a Quantumroot vault will save on fees after just 6 payments.

Left: a legacy P2PKH transaction, requiring several duplicated signatures. Right: a Quantumroot sweep transaction in which a single signature covers all matching inputs. (A future upgrade like TXv5 would enable deduplication of the preimages, too.)

Maximum Quantum Security

With NIST Post-Quantum Cryptography Category 5 security, Quantumroot is expected to remain secure for decades into the post-quantum era.

Quantumroot implements the standard LM-OTS signature scheme (RFC 8554), which itself relies only on SHA256 for security – no lattice-based or other relatively-experimental cryptography. From a cryptographic-security perspective, Quantumroot is maximally conservative.

Low Transaction Fees

Bitcoin Cash’s highly parallel architecture – the “UTXO model” – offers better performance and lower-level control than account-based systems, enabling Quantumroot to offer 100-1000× lower fees than equivalent vaults on Ethereum.

Privacy Nonces

Pre-quantum transactions do not expose associations between vault addresses: a 32-byte privacy nonce included in the hidden token-based spending path prevents even quantum attackers from connecting the address with authorized token(s) and/or recovery rules unless revealed by a post-quantum recovery.

Sweep-Free Upgrades

The hidden recovery path can be upgraded without sweeping the vault or revealing any association between vault addresses: only unassociated token UTXOs are moved on-chain. This simplifies user experiences and makes it easier for vaults to upgrade recovery, inheritance, or business continuity policies.

Deep Dive and Contract Walkthrough

For a deep dive and walkthrough of the CashVM contracts, see the August 20 tech talk:

Details & Example Transactions

Example transactions are now on-chain. Some extracted numbers:

• Pay to Public Key Hash (P2PKH):

• 34-byte UTXO, 141-byte inputs. Lifecycle total: 175 bytes.

• 6 inputs, 1 output (P2PKH): 890 bytes.

• Quantumroot, Pay to Script Hash, 32 Bytes (P2SH32):

• Schnorr spend (one per TX): 44-byte UTXO, 248-byte input. Total: 282 bytes.

• Introspection spend (all other inputs): 44-byte UTXOs, 112-byte inputs. Total: 156 bytes.

• 6 inputs, 1 output (P2SH32): 862 bytes.

• Note that a future upgrade like TXv5 would cut another 74 bytes per input. Introspection spend total: 82 bytes. Savings vs. P2PKH up to ~53.1%.

Post-Quantum Stats

• 1-input, 1-output post-quantum transaction: 2,613 bytes.

• For today’s most common transactions:

• 2-input, 2-output post-quantum transaction (including quantum-ready outputs): 2,923 bytes.

• 2-input (unique addresses), 2-output post-quantum: 3,169 bytes.

Given these stats, we can estimate that category 5 post-quantum activity on Bitcoin Cash will average ~1.5KB per payment. (With sufficient aggregation, ZK-STARK covenants/apps could improve this further.)

Comparing Large Sweeps

Note that post-quantum contract code adds zero bytes to pre-quantum spends.

• Pre-Quantum Sweeps (Schnorr signatures):

• P2PKH addresses can support 708 input sweeps per 100KB transaction; Quantumroot increases that to 891 inputs.

• P2PKH sweep (708 inputs)

• Quantumroot pre-quantum sweep (891 inputs)

• **Post-quantum Sweeps **(LM-OTS, RFC 8554):

• 868 inputs per 100KB transaction (one NFT input)

• 448 unique addresses per 100KB transaction (one NFT input)

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You can learn more about Quantumroot in the initial announcement:

Quantumroot: Quantum-Secure Vaults for Bitcoin CashA new contract design offers full 256-bit classical, 128-bit quantum security strength. Quantum spends are ~1.3KB per UTXO, and with cross-input and CashToken-based aggregation, quantum sweeps of 400+ unique addresses or 800+ inputs fit in a single transaction (100KB).bitjson’s blogJason Dreyzehner