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Blockchain Mutability: Challenges and Proposed Solutions
Eugenia Politou, Fran Casino, Efthimios Alepis, Constantinos Patsakis
TL;DR
Blockchain’s immutability supports security and decentralised trust but conflicts with the GDPR Right to be Forgotten, which requires personal-data erasure under specified conditions. This paper reviews technical workarounds and mutable-blockchain techniques, assessing their applicability, constraints, and limitations across permissioned and permissionless settings.
Problem
Blockchain immutability prevents editing or deletion of stored data, conflicting with the GDPR Right to be Forgotten’s requirement for permanent personal-data erasure upon request.
Method
The paper comprehensively reviews technical methods, cryptographic techniques, and mutable-blockchain approaches proposed to reconcile data erasure with blockchain security, auditability, and transparency.
Results
The review identifies off-chaining, conditional immutability removal, consensus-based redaction, and mutable transactions as proposed routes toward blockchain compliance with the Right to be Forgotten.
Takeaways & Limitations
Resolving the immutability conflict is presented as relevant to blockchain adoption across a broad range of applications while preserving security, auditability, and transparency.
Takeaways & Limitations
Proposed mutable designs raise concerns about reliance on authorities or trapdoor keys, which may weaken decentralisation or expose systems to malicious attacks.
Abstract
from arXiv · showhide
Blockchain's evolution during the past decade is astonishing: from bitcoin to over 2.000 altcoins, and from decentralised electronic payments to transactions programmable by smart contracts and complex tokens governed by decentralised organisations. While the new generation of blockchain applications is still evolving, blockchain's technical characteristics are also advancing. Yet, immutability, a hitherto indisputable property according to which blockchain data cannot be edited nor deleted, remains the cornerstone of blockchain's security. Nevertheless, blockchain's immutability is being called into question lately in the light of the new erasing requirements imposed by the GDPR's ``\textit{Right to be Forgotten (RtbF)}'' provision. As the RtbF obliges blockchain data to be editable in order restricted content redactions, modifications or deletions to be applied when requested, blockchains compliance with the regulation is indeed challenging, if not impracticable. Towards resolving this contradiction, various methods and techniques for mutable blockchains have been proposed in an effort to satisfy regulatory erasing requirements while preserving blockchains' security. To this end, this work aims to provide a comprehensive review on the state-of-the-art research approaches, technical workarounds and advanced cryptographic techniques that have been put forward to resolve this conflict and to discuss their potentials, constraints and limitations when applied in the wild to either permissioned or permissionless blockchains.
I. INTRODUCTION
Blockchain evolved from bitcoin and decentralised payments into programmable applications, while immutability remains central to its security and auditability. This paper reviews ways to reconcile immutability with data-erasure requirements across blockchain settings.
- Blockchain expanded from bitcoin and cryptocurrencies toward programmable transactions, smart contracts, and complex tokens governed by decentralised organisations.The paper presents this evolution as part of a broader movement toward decentralised computing and applications.
- Immutability underpins blockchain’s security, transparency, transactional integrity, and auditability.The property ensures that stored transaction data remains unchanged, supporting trustless operation without third-party intermediation.
- The paper examines the contradiction between immutable blockchain records and the GDPR Right to be Forgotten’s erasure requirements.Its scope includes both modification or deletion methods and their applicability to permissioned and permissionless blockchains.
- The review covers state-of-the-art research approaches, technical workarounds, and advanced cryptographic techniques for resolving this conflict.It discusses their potentials, constraints, and limitations when applied in practice.
II. DECENTRALIZED ARCHITECTURES
Decentralised architectures distribute control and information across networked machines rather than relying on a central authority. Distributed ledger technologies combine cryptography, peer-to-peer networking, and consensus to operate in untrusted environments.
- Decentralisation describes the conceptual and logical model of control, whereas distribution describes the technical infrastructure supporting an information system.The paper notes that the terms are often used interchangeably despite this distinction.
- DLTs are distributed digital ledgers whose changes are reflected across ledger holders and authenticated with cryptographic signatures.They eliminate the need for a central authority or intermediary to process, validate, or authenticate transactions.
- DLTs combine public-key cryptography, distributed peer-to-peer networks, and consensus mechanisms for operation in untrusted decentralised environments.
B. Blockchains
A blockchain is one type of distributed ledger: a linked sequence of hash-connected blocks that append transactions and provide an immutable transaction history. Its applications extend beyond cryptocurrencies into programmable decentralised systems.
- Blockchains are linked lists of blocks in which each block references the previous block through hash codes.Blocks contain transactions that are continuously appended and accessible to network participants.
- Blockchain transaction histories are distributed and immutable, while smart contracts provide executable code residing on the blockchain.
- Cryptocurrencies are one application of blockchain, and blockchain systems can operate without a built-in cryptocurrency.Cryptocurrencies nevertheless underlie most public blockchain applications to facilitate and incentivise transactions.
- More than 2,000 alternative cryptocurrencies exist alongside bitcoin, which remains the dominant cryptocurrency for decentralised payments.
- Blockchain applications are moving from protocol-layer value generation toward decentralised applications with programmable transactions and complex tokens governed by DAOs.
A. Permissionless and permissioned blockchains
Permissionless blockchains allow anyone to participate in reading, writing, verification, mining, or smart-contract creation. Permissioned blockchains restrict participation and can be better suited to networks with defined participating parties.
- Permissionless blockchains: Permissionless blockchains allow anyone to become a node, submit transactions, verify transactions, mine blocks, create smart contracts, and read the chain.
- Permissioned blockchains: Permissioned blockchains limit which parties may transact and contribute to the blockchain’s state.
- Permissionless blockchains usually compensate network contributors with tokens, whereas permissioned blockchains generally do not require cryptocurrency or monetary tokens.The paper associates this difference with the respective public and business-network settings.
- Both blockchain types have advantages and disadvantages and may suit different situations.
B. Maintaining trust through consensus protocols
Consensus protocols let blockchain nodes agree on one transaction history without a trusted third party, with mechanisms differing between permissionless and permissioned settings.
- Consensus algorithms maintain data consistency by enabling nodes to agree on transactions and their order without a trusted third party.
- Proof of Work validates blocks through computationally difficult but easily verified hash puzzles, after which the new block is appended across network ledgers.
- Permissionless consensus promotes decentralised trust in non-trusted environments through incentive mechanisms and assumptions about miners’ profitable behaviour.
- Permissioned blockchains may use authority-based or uniquely designed consensus mechanisms and can operate without a built-in cryptocurrency.
C. Privacy and transparency
Blockchain transparency and permanence make transactional privacy difficult and discourage storing personal data, while cryptographic privacy techniques remain resource intensive.
- Transactions are visible to participating nodes for validation, making transactional privacy difficult to attain by design.
- Permanent, publicly available blockchain records have led to broad discouragement of storing personal data on blockchains.
- Sensitive personal information can be inferred from metadata and big-data analytics even when personal data are not directly exposed.
- Homomorphic encryption, zero-knowledge proofs, and secure MPC support computation without revealing inputs and outputs, but their resource demands hinder scale.
D. Blockchain immutability
Blockchain immutability makes verified transactions effectively irreversible through cryptographically linked blocks, supporting integrity and security while remaining an emergent property vulnerable to coordinated alteration.
- Transactions cannot be edited or deleted after verification and recording, defining blockchain immutability or irreversibility.
- Changing transaction data alters a block hash and triggers a cascade of reference changes through subsequent blocks.
- Immutability makes blockchains append-only and tamper-proof while assuring transactional integrity, security, trustlessness, and censorship-resistance.
- Immutability is emergent rather than intrinsic, so agents with sufficient computing power or permissioned majorities may modify ledger data.
IV. BLOCKCHAIN IMMUTABILITY AND THE RIGHT TO BE FORGOTTEN
The GDPR’s Right to be Forgotten conflicts with blockchain’s permanent records because requested personal-data erasure is difficult or technically impossible once data enters a blockchain. This motivates research into controlled mutability and cryptographic approaches across permissioned and permissionless settings.
- The GDPR does not specifically account for decentralised technologies, while blockchain applications are nevertheless urged to comply with its requirements.
- Article 17’s Right to be Forgotten allows individuals to request erasure of personal data when specified conditions are met.
- Blockchain immutability contradicts the RtbF’s requirement for permanent deletion of personal data from all places where it was disseminated.
- CNIL considers erasure technically impossible for data entered into blockchains, even when cryptography makes data almost inaccessible.
- Proposed responses include preapproved, limited, transparent alteration methods and cryptographic techniques, with permissioned mutability viewed as more feasible than permissionless mutability.
V. CURRENT EFFORTS FOR BALANCING IMMUTABILITY AND THE RTBF
Researchers have proposed approaches that either circumvent blockchain immutability or remove it conditionally to address the Right to be Forgotten.
- Proposed solutions target the conflict between blockchain immutability and the Right to be Forgotten.
- Some approaches circumvent immutability rather than changing blockchain data directly.
- Other approaches conditionally remove blockchain immutability to address privacy requirements.
A. Bypassing blockchain’s immutability
Workarounds preserve blockchain records indirectly or render stored data inaccessible, while pruning removes older blockchain content. These methods can support GDPR compliance but introduce operational, security, or enforceability limitations.
- Off-chain storage: Off-chain storage keeps only a timestamp and hash on-chain, allowing the referenced information to be amended or deleted externally.The hash alone cannot reconstruct the original content.
- Off-chain storage: Off-chaining can reduce blockchain storage requirements and scalability issues while supporting GDPR compliance.
- Off-chain storage: Off-chaining shifts robust distributed storage to other protocols, adds complexity and delays, and introduces additional attack vectors.
- Encryption: Encrypted storage can make personal data inaccessible by forgetting or deleting its encryption key.Whether inaccessibility equals deletion remains disputed, and key loss or disclosure can expose the data.
- Encryption: Quantum-computing advances limit encrypted-data approaches because current encryption schemes may become decryptable.
- Blockchain pruning: Pruning deletes old transactions and blocks while retaining hashed block headers to preserve integrity and security.Its weak enforceability follows because nodes are not guaranteed to discard full-chain copies; it may suit permissioned frameworks.
B. Removing blockchain’s immutability
Proposed mutable blockchains enable deletion or modification through cryptographic redaction, consensus-based versioning, or flexible transaction structures. These designs preserve selected integrity properties but introduce trust, performance, security, or deployment constraints.
- Motivation: Mutable blockchains challenge the append-only and censorship-resistant properties that underpin blockchain designs.
- Cryptographic redaction: Chameleon hashes allow authorized parties holding a trapdoor key to replace block content by generating valid hash collisions.The design permits blockchain redactions while preserving chain linkage.
- Cryptographic redaction: Sharing the editing key among finite trustees raises centralization and malicious-attack concerns, particularly in permissionless settings.
- Consensus-based mutation: Consensus-based mutable transactions use validator voting and policies to approve alternate versions of blockchain history.Encryption hides inactive versions, while secret sharing distributes transaction-specific keys among validators.
- Consensus-based mutation: The mutable-transaction scheme adds significant performance overhead and limits verification enforcement for some transaction properties.
- Memory-flexible frameworks: Memory-flexible blockchains can modify, compress, or remove IoT transactions by hashing transaction hashes rather than transaction contents.
- Deletion structures: A block-matrix structure and algorithm support arbitrary record deletion while preserving hash-based integrity, but the proposal targets permissioned blockchains.
VI. DISCUSSION AND CONCLUSIONS
GDPR’s Right to be Forgotten conflicts with blockchain immutability, which supports tamper resistance and a globally accepted transaction view. This motivates a review of compliance techniques and their practical limitations across blockchain settings.
- The Right to be Forgotten requires retroactive erasure of personal data, whereas immutability prevents tampering and supports blockchain security.
- Public authorities and enterprises are increasingly exploring blockchain despite its slow integration into real-life applications.
- The paper synthesizes technical, cryptographic, and other advanced methods for addressing the conflict and reviews their benefits and limitations in permissioned and permissionless frameworks.The authors position this review as valuable to both industry and academia.