A comprehensive review of the Bitcoin protocol layer in 2025

Author: Pan Zhixiong

Bitcoin Optech's annual summary has always been regarded as a technological barometer of the Bitcoin ecosystem. It does not focus on price fluctuations, but only records the most authentic pulse of the Bitcoin protocol and key infrastructure.

The 2025 report reveals a clear trend: Bitcoin is undergoing a paradigm shift from "passive defense" to "active evolution".

Over the past year, the community has moved beyond simply patching vulnerabilities and has begun systematically addressing survival-level threats (such as quantum computing), aggressively exploring the boundaries of scalability and programmability without sacrificing decentralization. This report is not only a developer's memo but also a key index for understanding the asset properties, cybersecurity, and governance logic of Bitcoin over the next five to ten years.

Key conclusions

Looking back at 2025, Bitcoin's technological evolution exhibited three core characteristics, which are also the keys to understanding the following 10 major events:

1. Proactive defense: For the first time, the defense roadmap against quantum threats has become clear and practical, extending security thinking from the "present" to the "post-quantum era".
2. Functional layering: The high-density discussion of soft fork proposals and the "hot-swappable" evolution of the Lightning network demonstrate that Bitcoin is achieving its architectural goal of "stable bottom layer and flexible top layer" through a layered protocol.
3. Decentralized infrastructure: From mining protocols (Stratum v2) to node verification (Utreexo/SwiftSync), a large amount of engineering resources have been invested in lowering the barriers to participation and improving censorship resistance, aiming to counter the centralized gravity of the physical world.

Bitcoin Optech's annual report covers hundreds of code commits, mailing list debates, and BIP proposals over the past year. To extract the real signals from the technical noise, I removed updates that were merely "local optimizations" and selected the following 10 events that have a structural impact on the ecosystem.

1. Systemic defense and "hardening roadmap" against quantum threats

[Status: Research and Long-Term Proposal]

2025 marked a significant shift in the Bitcoin community's attitude towards the threat of quantum computing, moving from theoretical discussions to engineering preparations. BIP360 was assigned a number and renamed P2TSH (Pay to Tapscript Hash). This was seen as both an important stepping stone to the quantum hardening roadmap and a more general service for certain Taproot use cases (such as commitment structures that do not require an internal key).

At the same time, the community explored more specific quantum-safe signature verification schemes, including constructing Winternitz signatures with OP_CAT under the premise of introducing corresponding script capabilities in the future (such as reintroducing OP_CAT or adding new signature verification opcodes), discussing STARK verification as a native script capability, and optimizing the on-chain cost of hash signature schemes (such as SLH-DSA / SPHINCS+).

This issue ranks first because it touches upon the mathematical foundations of Bitcoin. If quantum computing does indeed weaken the elliptic curve discrete logarithm assumption in the future (thus threatening the security of ECDSA/Schnorr signatures), it will trigger systemic migration pressures and a layering of security for historical outputs. This forces Bitcoin to prepare upgrade paths in advance at the protocol and wallet levels. For long-term holders, choosing a custody solution with an upgrade roadmap and a culture of security auditing, and paying attention to potential future migration windows, will become essential lessons for asset preservation.

2. A surge in soft fork proposals: the cornerstone of building a "programmable vault"

[Status: Intensive Discussion / Draft Stage]

This year saw a high volume of discussions on soft fork proposals, with a core focus on how to unleash the expressive power of scripts while maintaining minimalism. Contract-based proposals such as CTV (BIP119) and CSFS (BIP348), as well as technologies like LNHANCE and OP_TEMPLATEHASH, all attempted to introduce more secure "restrictive clauses" for Bitcoin. Furthermore, OP_CHECKCONTRACTVERIFY (CCV) became BIP443, and various arithmetic opcode and script recovery proposals were also queuing up for consensus.

These seemingly obscure upgrades are actually adding new "physical laws" to the global value network. They are expected to make the original "vaults" type of construction simpler, more secure, and more standardized, allowing users to set mechanisms such as delayed withdrawals and cancellation windows, achieving "programmable self-protection" at the level of protocol expressibility. At the same time, these capabilities are expected to significantly reduce the interaction costs and complexity of layer 2 protocols such as the Lightning Network and DLCs (Discrete Logarithmic Contracts).

3. "Censor-resistant" reconstruction of mining infrastructure

[Status: Experimental Implementation / Protocol Evolution]

The decentralization of the mining layer directly determines Bitcoin's censorship resistance. In 2025, Bitcoin Core 30.0 introduced an experimental IPC interface, which significantly optimized the interaction efficiency between mining pool software/Stratum v2 services and Bitcoin Core verification logic, reduced reliance on inefficient JSON-RPC, and paved the way for the integration of Stratum v2.

One of Stratum v2's key capabilities (when mechanisms like Job Negotiation are enabled) is to further decentralize transaction selection from mining pools to more decentralized miners, thereby enhancing censorship resistance. Meanwhile, the emergence of MEVpool attempts to address the MEV problem through blinded templates and market competition: ideally, multiple marketplaces should coexist to prevent a single market from becoming a new centralized hub. This directly relates to whether ordinary users' transactions can still be fairly packaged in extreme environments.

4. Immune System Upgrade: Vulnerability Disclosure and Differential Fuzzing Testing

[Status: Ongoing engineering operation]

Bitcoin's security relies on self-checks before real attacks. In 2025, Optech documented numerous vulnerability disclosures targeting Bitcoin Core and Lightning implementations (such as LDK/LND/Eclair), ranging from fund freezes and privacy deanonymization to even serious theft risks. That same year, Bitcoinfuzz used differential fuzzing to identify over 35 deep-seated bugs by comparing how different software responded to the same data.

This high-intensity "stress test" is a sign of a mature ecosystem. Like a vaccine, it may expose weaknesses in the short term, but in the long run, it significantly enhances the system's immunity. For users who rely on privacy tools or the Lightning Network, this also serves as a wake-up call: no software is absolutely perfect, and keeping critical components updated is the most basic rule for ensuring the safety of your deposits.

5. Lightning Network Splicing: "Hot Update" of Channel Funds

[Status: Experimental support across implementations]

The Lightning Network saw a major breakthrough in usability in 2025 with Splicing (hot channel updates). This technology allows users to dynamically adjust funds (deposits or withdrawals) without closing channels, and is currently experimentally supported in the three major implementations: LDK, Eclair, and Core Lightning. While the relevant BOLTs specifications are still being refined, significant progress has been made in cross-implementation compatibility testing.

Splicing is a key capability that allows users to add or remove funds without closing the channel. It is expected to reduce payment failures and operational friction caused by the inconvenience of adjusting channel funds. Future wallets are expected to significantly reduce the learning cost of channel engineering, allowing more users to use LN as a payment layer similar to a "balance account," which is a crucial piece of the puzzle for Bitcoin payments to become widely used in daily life.

6. A Revolution in Verification Costs: Enabling Full Nodes to Run on "Common Devices"

[Status: Prototype Implementation (SwiftSync) / BIP Draft (Utreexo)]

The moat of decentralization lies in verification costs. In 2025, SwiftSync and Utreexo launched a direct attack on the "full node threshold." SwiftSync optimizes the UTXO set write path during IBD (Initial Block Download): it only adds an output to the chainstate when it is confirmed that the output has not been spent by the end of IBD, and uses a "least trusted" hints file to accelerate the IBD process by more than 5 times in the sample implementation, while opening up space for parallel verification. Utreexo (BIP181-183), on the other hand, allows nodes to verify transactions without storing the complete UTXO set locally through a Merkle forest accumulator.

The advancement of these two technologies means that running full nodes on resource-constrained devices will become feasible, increasing the number of independent validators in the network.

7. Cluster Mempool: Reshaping the Underlying Scheduling of the Fee Market

[Status: Nearing Release (Staging)]

Among the anticipated features of Bitcoin Core 31.0, the implementation of Cluster Mempool is nearing completion. It introduces structures such as TxGraph to abstract complex transaction dependencies into an efficient "transaction cluster linearization/sorting" problem, making block template construction more systematic.

While this is an upgrade to the underlying scheduling system, it is expected to improve the stability and predictability of fee estimation. By eliminating abnormal packet ordering caused by algorithmic limitations, the future Bitcoin network will perform more rationally and smoothly during congestion, and users' accelerated transaction requests (CPFP/RBF) will be effective under more deterministic logic.

8. Refined Governance of the P2P Transmission Layer

[Status: Strategy Update / Continuous Optimization]

In response to the surge in low-fee transactions that occurred in 2025, the Bitcoin P2P network underwent a strategic inflection point. Bitcoin Core 29.1 lowered the default minimum relay fee to 0.1 sat/vB. Meanwhile, the Erlay protocol continued to advance to reduce node bandwidth consumption; in addition, the community proposed initiatives such as "block template sharing" and continued to optimize compact block reconstruction strategies to cope with the increasingly complex propagation environment.

With more consistent policies and lower default thresholds for nodes, the feasibility of low-fee transactions propagating in the network is expected to improve. These directions are likely to reduce the rigid bandwidth requirements for running nodes, further maintaining network fairness.

9. OP_RETURN and the "Tragedy of the Commons" Debate in Blockspace

[Status: Mempool Policy changed (Core 30.0)]

Core 30.0 relaxed the policy restrictions on OP_RETURN (allowing more outputs and removing some size caps), which sparked a heated philosophical debate about the uses of Bitcoin in 2025. Note that this is part of Bitcoin Core's Mempool Policy (default forwarding/standard policy), not a consensus rule; however, it significantly affects how easily transactions are propagated and seen by miners, thus substantially impacting the competitive landscape of block space.

Supporters argue this corrects incentive distortions, while opponents worry it could be seen as an endorsement of "on-chain data storage." This debate reminds us that blockchain space, as a scarce resource, has rules governing its use (even those not at the consensus level) that are the result of ongoing competition among various stakeholders.

10. Bitcoin Kernel: Component-based Refactoring of Core Code

[Status: Architecture Refactoring / API Release]

In 2025, Bitcoin Core took a crucial step towards architectural decoupling by introducing the Bitcoin Kernel C API. This marked the separation of "consensus verification logic" from the massive node program, making it an independent, reusable standard component. Currently, this kernel can support external projects reusing block verification and chain state logic.

"Kernelization" will bring structural security benefits to the ecosystem. It allows wallet backends, indexers, and analytics tools to directly call the official verification logic, avoiding the risk of consensus discrepancies caused by reinventing the wheel. This is like providing the Bitcoin ecosystem with a standardized "original factory engine," making various applications built on it more robust.

Appendix: Glossary (Mini-Glossary)

To aid reading, here are brief explanations of key terms used in the text:

• UTXO (Unspent Transaction Output): The basic unit of Bitcoin ledger status, recording who owns how many coins.
• IBD (Initial Block Download): The process of synchronizing historical data when a new node joins the network.
• CPFP / RBF: Two transaction acceleration mechanisms. CPFP (Child-for-Parent Compensation) uses new transactions to drive old transactions; RBF (Fee Replacement) directly replaces low-fee transactions with high-fee transactions.
• Mempool: A buffer used by nodes to store transactions that have been broadcast but not yet included in blocks.
• BOLTs: A set of technical specifications for the Lightning Network (Basis of Lightning Technology).
• MEV (Maximal Extractable Value): The maximum extractable value, which refers to the additional profit that miners can obtain by reordering or reviewing transactions.