Study Guide for Rewritable Blockchains

Glossary

Term DefinitionsA blockchain is a tamper-proof digital ledger consisting of blocks containing data and integrity codes. Block A component of a blockchain that contains data and an integrity output that links to the previous block. Integrity code A function (e.g., hash function, chameleon hash function) used to generate an output that verifies the integrity of the block data. Trusted Party An authorized entity that has permission to modify the blockchain. Untrusted Party An entity that does not have permission to modify the blockchain. Key Secret Allows a trusted party to rewrite information on the blockchain in a non-tampering manner. Chameleon Hash A cryptographic hash function that uses a key secret to find different inputs that produce the same hash output (i.e., collisions). Rewrite-Allow Phase A phase in which a trusted party can modify the blockchain. Rewrite-Forbidden Phase A phase in which a trusted party is not allowed to modify the blockchain. Turn-taking A mechanism in which multiple trusted parties take turns to gain rewrite permission. Distributed Key Scheme A scheme in which a key secret is split into multiple parts held by different parties. Short Answer Questions

Please answer each of the following questions in 2-3 sentences.

How are blocks in a blockchain linked together?

How does a chameleon hash function differ from a traditional hash function?

How can a trusted party use a key secret to modify a blockchain without leaving traces of tampering?

In the context of a rewritable blockchain, what is the rewrite-permit phase?

What is the rewrite-forbidden phase and how is it initiated?

Describe a mechanism for managing access to key secrets in a rewritable blockchain.

What is rotational control of the key secret portion and how does it work?

Explain the concept of a hybrid blockchain.

In the context of a rewritable blockchain, how is the “right to be forgotten” supported?

Provide a real-world use case for a rewritable blockchain.

Short Answer Question

Each block contains an integrity output that is generated using the data and integrity code of the previous block. This integrity output is linked to the next block, forming a tamper-proof chain.

The Chameleon hash function uses a key secret to compute collisions. Someone who knows this secret can generate different inputs that produce the same hash output, allowing the blockchain to be modified in a non-tampering manner.

A trusted party uses the key secret to find new block data that produces the same integrity output as the original block. This allows the contents of a block to be modified without invalidating the integrity of the blockchain.

The rewrite-permit phase refers to a specific period of time or set of conditions during which a trusted party can modify the blockchain using its key secret.

The rewrite-prohibited phase is when the blockchain can no longer be modified by trusted parties. This can be achieved by deleting or invalidating the key secret, effectively making the blockchain immutable.

Distributed key schemes can be used to manage access to the key secret. The key secret is split into multiple parts, held by different parties. Only when a certain number (threshold) of authorized parties cooperate can the full key secret be reconstructed and rewrites enabled.

A turn-taking key secret part is a component of the key secret that is passed around in a loop between multiple trusted parties. The trusted party that holds this part is granted permission to modify the blockchain for a limited time, after which access is passed to the next trusted party.

A hybrid blockchain combines a rewritable part with an immutable part. Critical data is stored in immutable blocks, while less critical or updated data is stored in rewritable blocks that can be modified using key secrets.

A rewritable blockchain can support the “right to be forgotten” by allowing trusted parties to delete or modify blocks containing personal information. This ensures that individuals can remove their data from the blockchain when they no longer need or want to keep their data.

A practical use case for a rewritable blockchain is supply chain management. Trusted parties (such as suppliers, manufacturers, and logistics companies) can use it to track the movement of goods, update status information, and modify records when needed, while ensuring that all changes are verified and auditable.

Essay topic

Please answer each of the following questions in essay format.

Discuss the advantages and disadvantages of rewritable blockchains over traditional immutable blockchains.

Explain how different key secret management schemes affect the security of rewritable blockchains.

Explore the ethical implications of using rewritable blockchains in the real world, such as in healthcare, finance, and data privacy.

Compare and contrast rewritable blockchains based on Chameleon hashing with other rewritable blockchain technologies, highlighting their advantages and disadvantages.

Envision future directions and potential applications for rewritable blockchains, considering emerging trends and challenges.