Anchoring an SPV light client on another chain: checkpoint trust, retargeting and reorg handling

The initial design approach for the verification system was stateless, requiring users to supply a series of headers with each proof. This method was ultimately rejected due to vulnerabilities it presented; specifically, it was susceptible to manipulation by presenting a sequence of low-difficulty headers. A proposed alternative to maintain the simplicity of the stateless model involves setting a minimum acceptable bits value, based on historical data such as the lowest bits value recorded in 2022. This threshold could be updated every four years to reflect changes in the Bitcoin blockchain's mining difficulty.

However, if maintaining a stateless system is not preferred, another solution is to establish a canonical header chain starting from the genesis block. This approach does not require validating each block, thus simplifying the process. Regarding blockchain reorganizations (reorgs), the strategy to manage these should depend on the transaction values involved. For significant amounts, a longer chain of blocks might be considered canonical, whereas for smaller or more typical values, like those near typical Lightning channel transactions, fewer blocks may be required. The exact number of blocks considered could be determined using a scaling method, possibly logarithmic, between established upper and lower bounds such as 100 and 6 blocks respectively.