Powpeg

Building the Most Secure, Permissionless and Uncensorable Bitcoin Peg

RSK’s 2-way peg protocol, called “the Powpeg”, has matured from its inception in 2018 as a federation to now include many decentralized qualities. The new RSK Powpeg protects private keys stored in special purpose PowHSMs based on tamper-proof secure elements (SE). Each PowHSM runs an RSK node in SPV mode, and so signatures can only be commanded by chain cumulative proof of work. Security is established in the Powpeg through the simplicity of a layered design we refer to as defence-in-depth.

The History of the Powpeg

Two blockchains with distinct block formats can communicate in a fully decentralized manner if each one can evaluate the other blockchain’s consensus rules, and if cross-chain messages are not censored for long periods of time. Currently, only platforms with “Turing-complete” smart contracts can evaluate other blockchain consensus rules. Bitcoin, for better or for worse, lacks the ability to unlock coins over arbitrary predicates. Therefore, when RSK was created, it had to use the only existing technology in Bitcoin to distribute trust among parties: multi-signatures. With a multi-signature it is possible to give a group of notaries the task to protect locked bitcoins, tolerating a certain amount of malicious, hacked or unavailable parties.

When the RSK genesis block was mined, the RSK Federation, an autonomous set of functionaries aimed at protecting the multi-signature, was born. The federation was controlled by the RSK Bridge, an unstoppable smart-contract running on RSK, and has been successfully working since its creation. In 2020 the RSK community decided it was time for the RSK peg to grow, both in security and in censorship resistance, evolving from a federated system to the Powpeg. The Powpeg is a unique 2-way peg system that secures the locked bitcoins with the same Bitcoin hashrate that establishes consensus. The set of functionaries still exists, but their role is mainly to keep their hardware and nodes connected and alive at all times; they do not directly control the Bitcoin multisig private keys.

The Powpeg in RSK

The RSK researchers and developers strategy when designing the Powpeg differs from the one adopted by other teams that have built 2-way peg protocols. The RSK Powpeg is based on a layered security model, a practice we call “defence-in-depth”. Most other pegs rely on a single all-encompassing cryptographic protocol that solves a multi-party custody problem in an intricate way. These complex cryptographic protocols are delicate and very few entities can audit them thoroughly. Often these types of protocols become compromised, resulting in a sudden loss of security for users.

Other recent 2-way peg designs focus on crypto-economic incentives that take advantage of high collateralization in a new token. However, using a different token for the core sidechain functionality is not aligned with Bitcoin values. The RSK Powpeg bridge, instead, relies on multiple defences, or layers, with each layer relatively simple to understand and test. This defence-in-depth approach is what has allowed RSK to grow from genesis to the current state without major problems, and without downtime. Since there is no collateral, the RSK Powpeg members are incentivized to participate by receiving a small portion of RSK transaction fees that is automatically channeled to them. As seen in the Ethereum ecosystem, transaction fees can eventually provide a sustained income for miners and sometimes even higher than the blockchain subsidy.

Powpeg Functionaries

Functionaries participating in the RSK PowPeg keep specialized hardware called PowHSMs active and connected to special types of RSK full nodes (the “Powpeg Node”). A PowHSM is an external tamper-proof device that creates and protects one of the private keys required for the Bitcoin multi-signature protocol, only signing transactions proven valid by enough cumulative work. The Powpeg node is designed to have maximal connectivity and to communicate information about the RSK blockchain, specifically cumulative work, to the PowHSM.

The functionary’s role is to ensure that only valid multi-signature transactions are signed by the PowHSM through auditing changes in the PowHSM, the Powpeg node and the communication between them. Functionaries themselves are not actively involved in the signing of transactions in any way, and do not participate in the production of blocks on the RSK blockchain.

Merged-miners and the Armadillo Monitor

A large portion of Bitcoin miners participate in RSK merge-mining, providing the persistence and liveness blockchain properties required for effectively securing the RSK network. The role of merged-miners in the Powpeg protocol is the largest and most crucial layer of our defence-in-depth approach in securing the bridge between RSK and Bitcoin. Functionaries rely on the stability of merge-mining to ensure valid multi-signature transactions are signed and validated in a secure and timely manner.

Economic Actors and the Bridge Contract

Economic actors such as merchants and exchanges, interact with the RSK 2-way peg by sending and receiving peg-in and peg-out transactions (described in more detail below) to the Bridge smart contract through the RSK network. The Bridge is a pre-compiled smart contract living in the RSK blockchain. The role of the Bridge is to maintain an up-to-date view of the Bitcoin blockchain, verify peg-in requests and command peg-outs. To achieve this functionality, the Bridge contract manages a Bitcoin wallet in SPV (Simple Payment Verification) mode. In this mode, transactions are confirmed by block headers and block headers are minimally validated, but the validation includes the expected proof of work. These validations ensure the Bridge wallet follows the Bitcoin chain which has the highest chain work, but does not check that the chain is valid.

Normally the chain with the highest chain work is the network’s best chain. In the history of Bitcoin there was only a single unintended network fork where one branch was invalid according to pre-established consensus rules. The fork length was 24 blocks. Therefore, in order to prevent intended or unintended invalid forks, the Bridge is designed to wait for 100 confirmations before confirming a peg-in transaction.

Peg-in/Peg-out and Other Properties of RSK Powpeg

We use the now standardized terms peg-in for the process that transfers bitcoins to the sidechain, and peg-out to the process that returns them back to Bitcoin. Performing a peg-in is as easy as sending the bitcoins to the Powpeg address and informing the Bridge about the Bitcoin transaction. The Powpeg functionaries provide a “watch tower” service on behalf of users and inform the Bridge of any peg-in as well.

The RSK Powpeg is an asset migration protocol and cannot abort a peg-in in case of network delays. The inability to abort a peg-in during network delays is what generally distinguishes asset migration protocols from exchange protocols. In exchange protocols, there is always a risk that the counterparty fails to unlock funds, and a user is forced to inform this failure within a bounded delay. Only in a special case does RSK refund the bitcoins of a peg-in operation, and this is when a cap, which gradually increases over time, is surpassed.

Technically, the RSK Powpeg is a hybrid peg. Peg-ins work in a fully decentralized manner using SPV proofs with the Powpeg members acting only as watchtowers to make sure bitcoin deposits are correctly informed to RSK. The user issuing the peg-in transaction can inform RSK if the Powpeg members fail to, assuming a worst-case scenario where the user is eventually online to inform RSK of the transaction. Since RSK assumes a user is the sender and receiver of a 2-way peg transaction, it is highly advised that users inform the RSK network.

To perform peg-outs, the Bridge accepts requests from RSK accounts, and after thousands of confirmation blocks, the Bridge builds a Bitcoin peg-out transaction commanding the PowHSMs to sign this transaction. The Bridge selects the transaction inputs (or UTXOs) to include in the peg-out transactions, preventing selective censorship of UTXOs of any kind. The Bridge also coordinates and applies forced-delays to all treasury operations required when the Powpeg composition changes. Finally the Bridge serves as an Oracle to expose the Bitcoin blockchain to RSK smart-contracts. RSK peg-outs rely on the participation of the PowHSMs and collaboration of the majority of Powpeg members, as the PowHSMs need to sign every peg-out transaction. Assuming the practical security provided by PowHSMs, Powpeg peg-outs are also trustless.

RSK Powpeg Security

RSK peg is becoming one of the most secure multi-signature systems in existence. Technically, the security of the Powpeg relies on several concurrent strategies: Defence-in-depth, coordination transparency, and public attestation, but a peg’s security does not only rely on its technical features. The real-world security must be analysed from several points of view: technical, operational and reputational. In the following, we focus on the Powpeg technical design decisions.

Defence-in-Depth

Defence-in-depth is realized by a careful separation of responsibilities so that compromising the system requires more than just compromising one element or one actor. The miners alone cannot steal the funds of the peg, neither can the functionaries, nor the PowHSM manufacturer, nor the developers. The peg process is governed by consensus rules enforced in software and firmware, each protecting the other from bugs and vulnerabilities. Furthermore, the RSK community protects the code from mistakes. The community goal is to improve the Powpeg by adding more protective layers, each layer adding more security.

As described above, each functionary not only runs a Powpeg node, but also a PowHSM. In the coming months, all existing Powpeg members will have finished upgrading to the PowHSM version 2.0. As explained before, each PowHSM runs a consensus node in SPV mode, so commands need to be backed-up by real hashrate. Cheating the PowHSM becomes too difficult if not impossible without hacking several Bitcoin mining pools.

The term “vetocracy” is very useful in this context. A vetocracy is a system of governance whereby no single entity can acquire enough power to make decisions and take effective charge. Our defence-in-depth approach to security of the RSK Powpeg follows such an ideology, rendering attacks ineffective. A good question to ask when designing a 2-way peg system should be: “how closely does our protocol resemble a vetocracy”, saving many from endless religious debates over federated vs. decentralized systems.

Coordination Transparency

All communications between functionaries occur over the RSK blockchain. There are no hidden messages between functionaries and there is no pre-established subsystem that allows them to communicate secretly. All exchanged messages are public. While we can’t prevent hidden communication by hypothetical attackers in full control of the Powpeg node executable code, we do prevent hidden collusion for long periods. As coordination is carried out over the public network, the system forces the PowHSMs to be exposed to the blockchain honest best chain, and allows all network participants to periodically know the PowHSM internal state. As for external hackers, the existence of a pre-established system for hidden coordination would be a powerful tool for privilege escalation as it can be used to to obtain functionaries IPs and attempt targeted attacks. Powpeg functionaries could connect to the network over Tor, or change their IPs daily without problem.

Finally the bridge smart-contract builds the peg-out transaction and won’t let any of the PowHSMs pick anything related to the transaction to sign. The whole transaction content is decided by RSK consensus.

Firmware Attestation

RSK PowHSM firmwares, as well the full node and Powpeg nodes, are generated using deterministic builds, yet currently the firmware installation on PowHSMs cannot be fully trust-free. An auditing group must attest for the correctness of the process of firmware installation on each new device or batch of devices. But we’re improving this area with a new defence: the next iteration of the PowHSM firmware (version 2.1) is capable of providing firmware attestation using security features provided by the device. Therefore, our next objective is to include firmware attestation as part of our deployment procedures, or even periodically as keepalive messages. Soon attestation messages will be stored in the blockchain and every member of the community will be able to validate PowHSM firmwares.

Proof of Work is Proof of Time

The cumulative work required by the PowHSM also works as a rate limiter or forced time delay for any attack: Given the fact that RSK has a large portion of the Bitcoin hashrate through merge-mining, the amount of cumulative difficulty required to “cheat” the PowHSM into confirming a peg-out over a malicious forked branch implies a large scale collusion by some of the major Bitcoin mining pools for a duration of multiple days. Such an attack would be transparent and visible to both the Bitcoin and RSK communities. As in banking vault opening procedures, the PowHSM is actually enforcing a time-delay that lets humans enter the loop if an attack is suspected.

Peg-in and Peg-out Finality

Since the Bitcoin blockchain and the RSK sidechain are not entangled in a single blockchain or in a parent-child relation as in a syncchain, the transfers of bitcoins between them must at some point in time be considered final. If not, bitcoins locked on one side would never be able to be safely unlocked on the other. Therefore, peg-in and peg-out transactions require a high number of block confirmations. Peg-ins require 100 Bitcoin blocks (approximately 2000 RSK blocks), and peg-outs require 4000 RSK blocks (approximately 200 Bitcoin blocks). Transactions signed by federation nodes are considered final by RSK: these transactions are broadcast and assumed to be included sooner or later in the Bitcoin blockchain. Due to the need for finality, RSK consensus does not attempt to recover from an attack that manages to revert the blockchain deep enough to revert a final peg-in or peg-out transaction. If a huge reversal occurs, Powpeg nodes halt any future peg-out, and the malicious actors should not be able to double-spend the peg.

Decentralization - Building a Vetocracy

The use of PowHSMs in a federation is a step forward in decentralization, because a remotely compromised functionary does not compromise the main element for the security of the peg: a multisig private key. Since RSK has a large portion of the Bitcoin merge-mined hashrate, currently surpassing 51%, it seems extremely unlikely that a new group of merge-miners can hijack consensus long enough to force PowHSMs to perform a malicious peg-out. But the RSK community should never rest on its laurels. Instead, the RSK community is planning to apply once again a layered approach leading to more “additive security”.

The Powpeg Censorship-Resistance

The RSK Powpeg is also unique in the limited set of responsibilities delegated to each Powpeg node. In particular, Powpeg functionaries cannot apply selective censorship on peg-in and peg-out transactions. If one Powpeg functionary attempts to censor a particular transaction, the others functionaries sign and execute the peg-out transaction, causing the censorship to fail. If all functionaries attempt to censor a transaction, then the functionaries cannot continue to perform other peg-outs, as peg-outs are linked with UTXOs, and functionaries cannot choose the UTXOs for the peg-out transactions. The peg-out UTXOs, including “change” UTXOs, are selected by the Bridge contract, forming a consensus-enforced chain. Therefore, selectively banning a transaction leads eventually to a complete halt of the Powpeg, and that’s why selective censorship is not possible.

Regarding the complete shutdown of the Powpeg by a single government, it would be very difficult to pull off as the functionaries are geographically distributed all over the world. To protect from powerful worldwide coordinated attacks or attacks coming from three-letter agencies, RSK plans to add an emergency recovery multisig time-lock to activate one year after the Powpeg is proven dismantled. A shutdown attempt would only make RSK stronger and more resilient to subsequent attacks, as a new RSK Powpeg would rapidly expand and decentralize itself into a hundred individual users around the world, each running an PowHSM device and a Powpeg node over Tor.

Conclusion

The RSK peg has matured from a federation to a Powpeg. As the peg grows over time, more bitcoins are being moved into RSK.

Developers can find a unique opportunity to build their dApps on our secure and efficient money vault. Compared to alternatives, the Powpeg combines strong security based on layered protections, with maximum decentralization within the constraints established by the Bitcoin scripting system.