Input-triggered transaction expiry

The concept of "Input-triggered transaction expiry" has been developed to address the issue of free relay in blockchain transactions, specifically aiming to refine the handling and expression of "now" within the protocol. This idea was advanced further by incorporating feedback which suggested that the delay associated with this mechanism could be minimized, leading to a more refined definition of the maximum "height" an input "coin" can achieve. This not only contributes to the existing understanding but also opens up comparisons to previous ideas like coin-height introspection, albeit implemented at a different level of the system architecture.

Different stakeholders in the blockchain network perceive "now" distinctively based on their roles and the information available to them. For nodes, it is about the local time and the status of the blockchain's tip and confirmations, while for wallets, it includes the state of transactions signed and potentially their confirmations. Economic agents view "now" through the lens of current local time and the transactions pending before them. This multifaceted perception influences the implementation and expectations from the blockchain protocol regarding state-contingent claims, which are essential for setting conditions on virtual coins held in transactions.

The proposal introduces a new type of state that could conflict with what the witness sees, going beyond the current operational norm where only one type of conflicting state exists. By conceptualizing the "expiry" through 'OP_EXPIRE' and similar mechanisms, the aim is to enhance the protocol's ability to accurately reflect "now." This change proposes to secure the blockchain against deep chain reorganizations by ensuring that each transaction commits to the state of prior transactions as closely as possible to the actual confirmation time. Such a mechanism would inherently increase the cost and complexity for malicious entities attempting to alter the blockchain's history, thereby enhancing overall network security.

In conclusion, refining the primitive of "now" in blockchain protocols not only aids in better transaction verification but also has significant implications for the stability and security of the network post-subsidy phase. This approach encourages a broader discussion and further exploration into how enhancing the expressiveness of temporal states within blockchain transactions can preemptively mitigate risks associated with large-scale reorganizations. The detailed examination of this proposal can be accessed and discussed further through the provided Expiring HTLCs without free relay.