Commit-Reveal Scheme

A commit-reveal scheme is a two-phase cryptographic protocol designed to conceal sensitive information, such as a bid, vote, or random number, during a commitment phase before it is publicly disclosed in a subsequent reveal phase. In the first phase, a user generates a cryptographic commitment—typically a hash like keccak256(value, salt)—and broadcasts this hash to the network. This hash acts as a sealed, binding promise of the hidden data. The crucial property is that the commitment reveals nothing about the underlying value, yet it is cryptographically binding; the user cannot later reveal a different value than the one originally committed to without detection.

The protocol's second phase, the reveal, occurs after all participants have submitted their commitments. Here, users must publicly broadcast the original plaintext value and the secret salt used to create the hash. The network can then verify the reveal by re-hashing the submitted data and checking it matches the earlier commitment. This mechanism ensures fairness and prevents strategic manipulation. For example, in a blockchain auction, it stops participants from seeing others' bids and simply bidding slightly higher, a form of front-running. The scheme enforces that all decisions are made based on hidden information, which is only exposed when it can no longer influence the outcome.

Commit-reveal schemes are fundamental to many blockchain applications beyond auctions. They are used in random number generation (RNG) for smart contracts, where multiple parties commit random seeds to create a provably fair collective randomness. They also underpin private voting mechanisms in decentralized autonomous organizations (DAOs) and protect against transaction front-running on decentralized exchanges by hiding trade details in mempools. The security relies on the cryptographic properties of the hash function: its pre-image resistance ensures the value cannot be deduced from the hash, and its collision resistance guarantees a user cannot find two different inputs that produce the same commitment hash.

While highly effective, basic commit-reveal schemes have limitations. They require all participants to submit two transactions (commit and reveal), doubling gas costs and blockchain footprint. A significant drawback is participant dropout; if a user commits but fails to reveal, the process can stall, potentially requiring complex timeout mechanisms or slashing deposits to penalize non-revealers. More advanced variants, like timelock puzzles or schemes using verifiable delay functions (VDFs), address some of these issues. Despite its constraints, the commit-reveal pattern remains a cornerstone protocol for achieving fairness and privacy in transparent, adversarial environments like public blockchains.

A commit-reveal scheme is a two-phase cryptographic protocol designed to hide sensitive information, such as a bid, vote, or random number, until a predetermined time, preventing front-running and manipulation. In the commit phase, a participant generates a secret value, creates a cryptographic commitment (typically a hash like keccak256), and publishes only this commitment to the network. This commitment acts as a sealed envelope—it binds the participant to their secret without revealing it. The reveal phase occurs later, where the participant discloses the original secret value, allowing anyone to verify that it matches the earlier commitment by re-hashing it.

The core mechanism relies on the properties of cryptographic hash functions: pre-image resistance ensures the secret cannot be deduced from the commitment, and binding ensures the participant cannot reveal a different secret that produces the same hash. This makes the scheme cryptographically secure. Common implementations involve a simple structure: commitment = hash(secret, salt), where a random salt (nonce) prevents brute-force attacks against low-entropy secrets. Once the reveal transaction is broadcast, network nodes or a smart contract can execute the verification function verify(commitment, secret, salt) to validate the participant's honesty.

In blockchain contexts, commit-reveal is critical for applications requiring fairness and secrecy in open, transparent systems. Key use cases include on-chain voting (to prevent influence from early results), random number generation (where multiple participants commit values later combined for randomness), and auction mechanisms (to stop bidders from adjusting based on others' bids). For example, an English auction smart contract would require all bids as commitments in phase one, then reveal them in phase two, ensuring the highest committed bid wins without last-second sniping.

While effective, the scheme has operational trade-offs. It requires two transactions (commit and reveal), doubling gas costs and complexity. Participants must remain online to submit the reveal transaction, risking forfeiture if they fail. Furthermore, the need for a secure, random salt and the management of multiple commitment periods adds implementation overhead. These constraints make it suitable for high-stakes, batch-processed operations rather than real-time, frequent interactions, leading to hybrid models or alternative privacy solutions like zk-SNARKs for more complex state hiding.

A commit-reveal scheme is a cryptographic protocol that allows a participant to commit to a value (like a bid, vote, or random number) by publishing a cryptographic commitment, and later reveal the original value. The commitment, typically a hash digest like keccak256(value, salt), binds the participant to their initial choice without exposing it, preventing them from changing it after seeing others' commitments. This creates a fair, two-stage process essential for applications requiring secrecy with later verifiable disclosure.

The scheme's security relies on the hiding and binding properties of cryptographic hash functions. The hiding property ensures the original value cannot be deduced from the commitment hash. The binding property guarantees that once a commitment is published, the participant cannot find a different input (a different value or salt) that produces the same hash output, locking them into their initial choice. A cryptographically secure random salt (nonce) is crucial to prevent brute-force revelation of simple values.

In blockchain contexts, commit-reveal is a fundamental mechanism to mitigate front-running and information leakage. For example, in decentralized exchanges or auction smart contracts, users submit a hashed version of their trade or bid. Only after a commit phase concludes does a reveal phase begin, where users submit their original data. The contract verifies the hash matches the commitment, ensuring all actions were determined before any were revealed. This prevents adversaries from exploiting visible transaction data in the mempool.

Practical implementation requires careful design of phase timings and incentives. The commit phase must have a sufficient window for participation, while the reveal phase must allow committed participants time to submit their data, often with a penalty (like a locked bond) for those who commit but fail to reveal. This structure is used in ZK-rollup fraud proofs, random number generation (RNG), decentralized voting, and privacy-preserving transactions to coordinate actions without premature disclosure.

A commit-reveal scheme is a two-phase cryptographic protocol where a user first publishes a commitment—a hashed, encrypted, or otherwise obfuscated version of their data—and later reveals the original data, allowing anyone to verify it matches the initial commitment. This process ensures the data cannot be changed after the commitment is made, a property known as binding, while also keeping it secret until the reveal phase, a property called hiding. It is a cornerstone for creating fair, transparent, and manipulation-resistant processes on-chain.

The scheme's primary use case is preventing front-running and information-based advantages in decentralized applications. For example, in a blind auction or a voting system, participants submit a hash of their bid or vote. Since the hash reveals nothing about the underlying value, no participant can strategically adjust their submission based on others'. Only after all commitments are collected does the reveal phase begin, where participants submit their original data. The contract then validates each reveal against its stored commitment hash.

Implementing a commit-reveal scheme requires careful management of cryptographic primitives and state. Typically, the keccak256 hash function is used to generate the commitment, often by hashing the secret data concatenated with a random salt (a nonce) to prevent brute-force guessing of simple values. The contract must store the commitment, usually mapped to the sender's address, and enforce strict phase timings. A critical consideration is ensuring the reveal transaction includes the exact preimage (original data and salt) that produces the stored hash.

While powerful, the scheme has notable limitations. It requires users to send two transactions (commit and reveal), doubling gas costs and complexity. There is also a free option problem: a user can choose not to reveal, potentially stalling the process. Mitigations include requiring a staked bond forfeited on non-revelation and using timelocks to automatically progress phases. Furthermore, the need for a secure, off-chain generated salt is paramount, as a predictable salt can compromise the hiding property.

Beyond auctions and voting, commit-reveal schemes enable random number generation (where multiple participants commit to seeds), fair lotteries, and privacy-preserving computations. They represent a shift from synchronous, transparent execution to a phased approach that decouples the act of decision-making from its publication. This pattern is essential for any on-chain mechanic where the order or content of transactions should not influence or be influenced by other participants in the same round.

When designing a system, developers must choose between a commit-reveal scheme and alternative privacy solutions like zk-SNARKs or trusted execution environments. Commit-reveal is often simpler and more gas-efficient for applications requiring temporary secrecy rather than permanent anonymity. Its elegance lies in using fundamental cryptographic hashing, a core primitive of blockchain itself, to create a level playing field, embodying the principle that system rules should be enforced by code, not by the speed or knowledge of a participant.