Accessing Block Hash

Block hashes are the cryptographic representations of a block's content. They encapsulate the entirety of a block's information, primarily its block header. When it comes to storage proofs, accessing the relevant block hash is the critical first step in affirming the integrity and authenticity of any on-chain data. It sets the stage for all subsequent verification processes in the storage proof workflow.
Smart contracts can retrieve these block hashes in various ways, each with its own trade-offs and considerations. Let’s explore some common methods:

Direct Access using EVM opcode

Smart contracts can natively utilize the BLOCKHASH opcode to fetch hashes of the most recent 256 blocks in the Ethereum Virtual Machine (EVM). While straightforward, this method is restricted to a limited timeframe, and accessing older blocks becomes challenging.
This limitation prevents developers from injecting proofs of arbitrary on-chain data into their smart contracts, as they cannot easily verify inclusion proofs against historical block hashes.

Utilizing Third-Party Providers

Alternately, an off-chain actor could forward block hashes on-chain. However, this approach introduces latency and usually requires trusting a third party, a committee or similar. Additionally, it potentially introduces vulnerabilities tied to economic incentives or adversarial behaviours.
Merkle Mountain Range Accumulator Root
The Historical Block Hash Accumulator can be used to circumvent these issues. It enables on-chain smart contracts to derive older block hashes by validating them against a stored accumulator root (MMR Root). The methodology eliminates the need to trust external actors and widens the accessible range of block hashes without sacrificing security.