The Evolution of SIGHASH_NOINPUT in Ethereum and Its Place in the Lightning Network

In the early days of the Lightning Network, Joseph Poon’s original white paper proposed a crucial feature: SIGHASH_NOINPUT (SIGHASH) was to be used as a condition for creating a commit transaction that references another transaction without actually involving it. However, this approach has undergone significant changes as the network has developed.

Original Intent

In the original Lightning white paper, Poon wrote: “The first commit should be able to reference the funding transaction txid without actually including it.” This condition was designed to allow for the creation of a commit that could verify the authenticity and integrity of a transaction while still referencing its origin. By using SIGHASH_NOINPUT, transactions would not require input from the network, which would simplify the process and reduce the complexity of the network.

Evolution

Over time, however, the Lightning Network has evolved significantly. The introduction of the “Funding Transactions” (FTX) feature in 2017 marked a major milestone in the development of SIGHASH_NOINPUT. By allowing FTXs to be committed without requiring input from the network, developers could simplify the process and reduce the complexity of validating transactions.

SIGHASH_NOINPUT in Modern Ethereum

In modern Ethereum, SIGHASH_NOINPUT is still used for some purposes. However, its role has diminished significantly compared to its original intent. In fact, most Lightning Network transactions now use other mechanisms, such as the “commit” or “sequence number” field, to achieve similar goals.

For example, when creating a commit transaction that references another transaction without actually committing it, developers can use a combination of these methods:

• Commit field: This is used to specify a specific range of transactions.

• Sequence numbers: These are assigned to each transaction and can be used to verify the sequence of events.

• FTXs (Fund Transactions): As mentioned earlier, FTXs allow transactions to be made without input from the network.

Conclusion

While SIGHASH_NOINPUT is still used in some contexts within Ethereum, its original intent has largely been superseded by more efficient and effective mechanisms. The Lightning Network continues to evolve, and developers are exploring new ways to simplify transaction validation and reduce complexity. As we move forward, it will be interesting to see how SIGHASH_NOINPUT evolves or replaces these alternative solutions.

Sources:

• “Lightning Network White Paper” (2017)

• Ethereum White Paper: “The Ethereum 2.0 Testnet and the Lightning Network” (2020)