Recent Bitcoin Proposals Could Dramatically Cut Transaction Fees

Bitcoin Transaction Fee Reductions

Between November 2 and 4, Bitcoin's development community made significant strides towards reducing transaction fees. The recent integration of three proposals—BIP458, BIP459, and BIP460—aims to enhance the efficiency of payments being processed collectively by multiple users while optimizing the consolidation of various inputs into a single output. These new specifications focus on two types of signature aggregation: half-aggregation and full aggregation, fundamentally shifting how transaction signatures are handled.

Signature Aggregation Mechanisms

The merging of BIP458 on November 2, BIP459 on November 3, and BIP460 on November 4 introduces mechanisms that amalgamate the signatures of transaction inputs, which are the coins involved in the transaction. Half-aggregation allows for the combination of independently generated signatures without requiring an interactive signing session. As a result, ten 64-byte signatures can be reduced to a mere 32(n+1) bytes. In contrast, full aggregation necessitates coordinated signing, yielding a simplified single 64-byte signature for all signatures in the group.

Impact on Transaction Size and Fees

To put these reductions into perspective, if ten signatures were originally 640 bytes, they could shrink to 352 bytes under half-aggregation or down to 64 bytes with full aggregation, marking reductions of 45% and 90% in the signature size, respectively. It's important to note, however, that the overall decrease in the transaction fee will likely be less pronounced due to additional factors such as encoding overhead.

Typically, Bitcoin transaction fees are calculated based on virtual bytes (vB). Under the weight system of Bitcoin transactions, the virtual size is determined by dividing the transaction weight by four and rounding up. In theoretical scenarios involving ten inputs and outputs, a joint transaction could drop from 1,016 vB to around 944 vB with half-aggregation or to 872 vB with full aggregation. This translates to approximately 7.1% and 14.2% fee reductions respectively, which at a rate of 10 satoshis per vB, would result in savings of about 720 or 1,440 satoshis.

Consolidation Benefits

The benefits of consolidation become even more evident in a hypothetical case with 100 inputs being combined into a single output, where the size would decrease from 5,804 vB to 5,012 vB using half-aggregation, or to 4,220 vB with full aggregation. This situation could facilitate fee reductions around 13.6% or 27.3%, underscoring the potential for significant cost savings.

Considerations for Implementation

However, it's worth mentioning that BIP460 does not specify activation parameters, and its proposed witness version, now designated as v2, necessitates alignment with other proposals. Furthermore, wallets will need to upgrade to support this new output type, as current Taproot outputs do not inherently facilitate aggregation unless they engage in direct signature-based or key-path spends. Script-path spends, which involve executing scripts, cannot be integrated into aggregation groups.

Moreover, while eligible inputs have the option to opt out, full aggregation demands a more complex two-round signing protocol. Reusing secret nonces during this process could jeopardize the signing keys, making robust hardware and air-gapped solutions essential, whereas half-aggregation sidesteps the need for joint signing sessions, despite still requiring consensus on collaborative transaction structuring.