Non-Repudiation

The primary mechanism for achieving non-repudiation in blockchain. A user signs a transaction with their private key, creating a unique cryptographic proof.

• The network verifies the signature using the user's public address.
• The signer cannot later deny authorizing the transaction, as the signature is mathematically linked to their key pair.
• This is the basis for all on-chain asset transfers and smart contract interactions.

Blockchain's append-only ledger provides an indisputable record of signed actions.

• Once a signed transaction is confirmed in a block and added to the chain, it cannot be altered or deleted.
• Any attempt to repudiate the transaction is countered by the cryptographic proof in the signature and the consensus-verified historical record.
• This creates a permanent, auditable trail of all actions taken by a public key.

Non-repudiation extends to automated agreements. When a user interacts with a smart contract, their signed transaction triggers a deterministic function.

• The contract's resulting state change and any emitted events are permanently recorded.
• The user cannot deny initiating the contract call or its agreed-upon outcome, as the execution is a direct, verifiable consequence of their signed input data.
• This is critical for DeFi loans, NFT minting, and DAO governance votes.

A broader cryptographic system where a trusted Certificate Authority (CA) issues digital certificates binding an identity to a public key.

• The certificate itself is signed by the CA, providing non-repudiation of the binding.
• While blockchains typically use decentralized identity (DIDs), traditional PKI exemplifies the non-repudiation concept for SSL/TLS websites and signed documents.
• It establishes a chain of trust for verifying the origin of data.

Blockchains can provide non-repudiable proof of existence for any digital asset at a specific time.

• By hashing a document or file and recording the hash in a transaction, one creates an immutable timestamp.
• The signer of that transaction cannot later deny possessing the data at the time of the block's creation.
• This is used for intellectual property registration, legal document notarization, and supply chain audit trails.

Understanding what lacks non-reudiation highlights its importance.

• Traditional Database Entries: A central admin can alter or delete records without a cryptographic trace, allowing repudiation.
• Shared Passwords or Symmetric Encryption: If multiple parties share a secret, the origin of an action cannot be uniquely proven.
• Unauthenticated APIs: Requests without digital signatures can be spoofed, making denial trivial.
• Blockchain's design explicitly solves these weaknesses.

Blockchains provide an immutable, timestamped audit trail of all signed transactions. This is critical for:

• Regulatory compliance (e.g., financial regulations, data provenance).
• Supply chain management, proving the origin and transfer of goods.
• Smart contract execution, where actions are triggered by verifiable, non-repudiable events. This creates a legally-admissible record where parties cannot dispute their recorded actions.

Every on-chain asset transfer relies on non-repudiation. When you send ETH, BTC, or an NFT, your wallet signs the transaction. The network nodes verify the signature against your public address before adding it to the ledger. This ensures:

• Senders cannot deny initiating the payment.
• Recipients have cryptographic proof of rightful ownership.
• Double-spending is prevented as the signed transaction is finalized on-chain.

Interacting with a smart contract—such as minting a token, voting in a DAO, or providing liquidity—requires a signed transaction. The contract's logic executes based on this verifiable input. Non-repudiation here is essential for:

• Decentralized finance (DeFi): Users cannot deny loan agreements, trades, or yield farming commitments.
• Decentralized Autonomous Organizations (DAOs): Members cannot dispute their governance votes or proposals.
• Oracles: Signed data feeds provide tamper-proof, attributable information to contracts.

While cryptographically strong, non-repudiation in practice depends on key management:

• Private key compromise: If a key is stolen, the thief can create valid, non-repudiable signatures. The security burden shifts to key custody.
• Social engineering: Signing a malicious transaction is still non-repudiable.
• Legal vs. Technical: A cryptographic proof may not resolve all legal disputes (e.g., claims of signing under duress).
• Quantum threat: Future quantum computers could break current signature schemes (e.g., ECDSA), prompting research into post-quantum cryptography.

Non-repudiation is enforced through digital signatures and public-key cryptography. When a user signs a transaction with their private key, it creates a unique, mathematically verifiable proof that only the holder of that key could have produced. This signature is broadcast to the network and can be independently verified by anyone using the signer's public address, creating an immutable audit trail.

The strength of non-repudiation is entirely dependent on the security of the private key. If a key is compromised through phishing, malware, or poor storage (e.g., a hot wallet), an attacker can create valid, non-repudiable signatures on behalf of the victim. The victim cannot later repudiate those fraudulent transactions, as the cryptographic proof is technically valid. This shifts the security burden to key custody solutions.

Non-repudiation applies to the transaction signature, not necessarily to the user's intent. A user may sign a transaction interacting with a malicious or buggy smart contract, resulting in an unintended outcome. While they cannot repudiate the act of signing, they may dispute the contract's execution logic. This creates a legal and procedural gray area where cryptographic proof and human intent diverge.

In extreme scenarios like a successful 51% attack on a Proof-of-Work chain, an attacker with majority hash power could theoretically reorganize the blockchain (reorg). This could erase blocks containing transactions, undermining the finality of the non-repudiable proof. While signatures remain cryptographically valid, the historical record of their acceptance can be altered, creating a repudiation scenario at the consensus layer.

Blockchain provides technical non-repudiation—cryptographic proof of action. However, legal non-repudiation requires this proof to be recognized and enforceable in a court of law. Challenges include:

• Pseudonymity: Linking a public key to a real-world identity.
• Jurisdiction: Determining which legal framework applies to a decentralized network.
• Smart Contract Code as Law: Whether unreadable bytecode constitutes a valid legal agreement.

Future quantum computers could break the Elliptic Curve Digital Signature Algorithm (ECDSA) used by Bitcoin and Ethereum. If a quantum computer can derive a private key from a public key, it could forge signatures, completely breaking the cryptographic foundation of non-repudiation. This is a long-term, existential risk driving research into post-quantum cryptography for blockchains.

The primary cryptographic mechanism for achieving non-repudiation. A digital signature is created using a signer's private key and can be verified by anyone with the corresponding public key. It provides proof of origin, integrity, and non-repudiation for a message or transaction.

• How it works: A hash of the data is signed, binding the signature to that specific content.
• Blockchain Example: Every on-chain transaction is signed, providing an immutable, non-repudiable record of the sender's intent.

A framework that binds public keys to identities via digital certificates issued by a trusted Certificate Authority (CA). PKI establishes trust and enables non-repudiation in client-server models (e.g., HTTPS).

• Role in Non-Repudiation: The CA's verification of an entity's identity provides third-party assurance that a signature originated from a specific, vetted party.
• Limitation: Relies on centralized trust in the CA, unlike decentralized blockchain models.

A one-way function that produces a fixed-size digest (hash) from input data of any size. It is a foundational component for creating digital signatures and ensuring data integrity.

• Link to Non-Repudiation: Signatures are applied to the hash of a message, not the message itself. This creates a unique, tamper-evident seal.
• Properties: Deterministic, pre-image resistant, and collision-resistant. Common functions include SHA-256 and Keccak-256 (used by Ethereum).

A secure, chronological record that provides documentary evidence of a sequence of activities. In blockchain, the immutable ledger itself is a canonical audit trail.

• Enforcing Non-Repudiation: The permanent, append-only record of signed transactions makes it computationally infeasible for a participant to deny their recorded actions.
• Key Feature: The trail is cryptographically verifiable by all network participants, removing reliance on a single trusted logger.

A security service that corroborates the source of a message or data. Non-repudiation is a stronger guarantee that builds upon proof of origin.

• Difference: Proof of origin confirms who sent data. Non-repudiation adds the legal or logical guarantee that the sender cannot later deny having sent it.
• Implementation: Achieved via asymmetric cryptography, where only the holder of the private key could have created a valid signature.

A cryptographic method where one party (the prover) can prove to another (the verifier) that a statement is true without revealing any information beyond the validity of the statement itself.

• Relation to Non-Repudiation: While ZKPs focus on privacy, certain constructions (like signatures of knowledge) can provide non-repudiation. A prover can demonstrate they possess a secret (e.g., a private key) and authorized an action, without exposing the secret.
• Use Case: zk-SNARKs in Zcash provide transaction validity proofs while preserving sender/receiver privacy.