We are beginning to witness the emergence of second-layer potential stemming from the foundational layer innovations that have been introduced or refined over the past decade. The Lightning Network, despite facing significant limitations, is increasingly flourishing. This is merely the initial version that has been specified and implemented. Additionally, various sidechains have been launched, including Liquid, RSK, and token chains associated with Bitcoin, developed by Commerceblock. This is just the beginning.
**Schnorr and Taproot**
On the horizon, we have the promising combination of Schnorr signatures and Taproot. Schnorr introduces a more cost-effective signature verification method, particularly when processing batches, and represents a significant advancement in optimizing multi-signature scripts within Bitcoin. Initially, multi-signature setups involved including all public keys and scripts in a transaction output, which had to be replicated in the input for spending. P2SH improved this by incorporating a fixed-length hash of the public keys and scripts, reducing fees for transactions directed to multi-signature addresses and increasing costs only for the sender. SegWit further enhanced this by lowering the costs associated with spending multi-signature UTXOs through the witness discount. Schnorr takes these incremental improvements to the next level by allowing the combination of individual public keys into a single key, enabling all parties to collaborate on a single signature, which can then be verified collectively. This results in substantial cost savings for multi-signature usage, including second layers like Lightning and federated sidechains, while also enhancing privacy by making multi-signature UTXOs indistinguishable from single-signature ones.
However, this does not automatically ensure complete privacy. Lightning channel states (transactions) still necessitate separate key paths for penalty transactions to respond to the submission of outdated states, which must be included in the output scripts, creating a recognizable fingerprint. Taproot addresses this issue with its innovative cryptographic approach, allowing the commitment of a Merkle tree of various spending conditions. This requires only the condition used and a Merkle proof to the Merkle root for spending, all tied to a standard-looking Schnorr public key. Consequently, Taproot enables the concealment of penalty script paths and any conditional script paths beneath a seemingly ordinary Schnorr key, facilitating agreement among participants and enabling the execution of standard-looking transactions.
**SIGHASH_ANYPREVOUTPUT**
SIGHASH_ANYPREVOUTPUT (formerly known as SIGHASH_NOINPUT) is anticipated to be the next significant primitive introduced. This new public key format and sighash flag upgrade enhances transaction flexibility. Sighash flags determine which components of a transaction a signature is committing to, allowing for scenarios where an individual can sign their inputs and outputs while permitting others to add their own inputs and outputs without invalidating the transaction.