- Are technically complex to fully implement
- Create unstable network scale incentive structures
- Require consistent and high level cooperation between stakeholders
Simple Payment Verification (SPV) is a generic term for a range of different methodologies used by light clients on most major blockchain networks to verify aspects of the network state without the burden of fully storing and maintaining the chain itself. In most cases, this means relying on a form of hash tree to supply a proof of the presence of a given transaction in a certain block by comparing against a root hash in that block’s header or equivalent. This allows a light client or wallet to reach a probabilistic level of certainty about on-chain events by itself with a minimum of trust required with regard to network nodes.
Traditionally the process of assembling and validating these proofs is carried out off chain by nodes, wallets, or other clients, but it also offers a potential mechanism for inter-chain state verification. However, by moving the capability to validate SPV proofs on-chain as a smart contract while leveraging the archival properties inherent to the blockchain, it is possible to construct a system for programmatically detecting and verifying transactions on other networks without the involvement of any type of trusted oracle or complex multi-stage consensus mechanism. This concept is broadly generalisable to any network with an SPV mechanism and can even be operated bilaterally on other smart contract platforms, opening up the possibility of cheap, fast, inter-chain transfer of value without relying on collateral, hashlocks, or trusted intermediaries.
Opting to take advantage of well established and developmentally stable mechanisms already common to all major blockchains allows SPV based interoperability solutions to be dramatically simpler than orchestrated multi-stage approaches. As part of this, they dispense with the need for widely agreed upon cross chain communication standards and the large multi-party organizations that write them in favor of a set of discrete contract-based services that can be easily utilized by caller contracts through a common abstraction format. This will set the groundwork for a broad range of applications and contracts able to interoperate across the variegated and every growing platform ecosystem.
SPV Program - Client-facing interface for the inter-chain SPV system, manages participant roles. SPV Engine - Validates transaction proofs, subset of the SPV Program. Client - The caller to the SPV Program, typically another solana contract. Prover - Party who generates proofs for transactions and submits them to the SPV Program. Transaction Proof - Created by Provers, contains a merkle proof, transaction, and blockheader reference. Merkle Proof - Basic SPV proof that validates the presence of a transaction in a certain block. Block Header - Represents the basic parameters and relative position of a given block. Proof Request - An order placed by a client for verification of transaction(s) by provers. Header Store - A data structure for storing and referencing ranges of block headers in proofs. Client Request - Transaction from the client to the SPV Program to trigger creation of a Proof Request. Sub-account - A Solana account owned by another contract account, without its own private key.
For more information on the Inter-chain SPV system, see the book section at: https://docs.solana.com/book/v/master/proposals/interchain-transaction-verification