Citrea: Peter Thiel's Key Deployment on the Bitcoin L2 Network

Author: Ryan Yoon, Source: Tiger Research, Translated by: Shaw Jinse Finance

Key Takeaways

• Most Bitcoin L2 security solutions don't actually utilize Bitcoin's security mechanisms. Citrea achieves this through zero-knowledge proofs and BitVM, allowing Bitcoin to directly verify its state.

• Founders Fund has a strong track record of discovering unique technologies (e.g., SpaceX and Palantir). Choosing Citrea when Bitcoin L2 prices were at their lowest point demonstrates their unwavering belief in the technology.

• Founders Fund has a proven track record of discovering unique technologies (e.g., SpaceX and Palantir). Their choice of Citrea when Bitcoin L2 prices were at their lowest point also reflects their strong belief in the technology.

The Citrea team doesn't have any particularly illustrious credentials. However, they were the first to validate zero-knowledge proof technology in a Bitcoin production environment and launched the mainnet before most Bitcoin secondary payment systems had even gone live. The technology validation is complete. The remaining challenge is to give EVM ecosystem users a reason to use the same services on Bitcoin. The next step is to actively expand the user base. 1. L2 Skepticism and Citrea: Why Thiel Chose the latter Bitcoin's market capitalization has surpassed $2 trillion. However, when asked about the actual use of this money, the answer is unclear. "Make this money work. Utilize Bitcoin's security." This is the core logic behind the surge in Bitcoin L2 prices. But of the dozens of projects born from this logic, very few have actually been effective. The need was clear, but the results were less than satisfactory. Against this backdrop, Peter Thiel's Founders Fund invested in Citrea, choosing a team capable of building the first zero-knowledge base aggregation on Bitcoin, rather than another marketing gimmick. This was a high-conviction bet on the forefront of Bitcoin's L2 technology, made precisely at a time when the overall market was most skeptical. 2. Why Thiel chose Citrea? Founders Fund has a history of investing in unique technologies that other companies dare not venture into. Limited partners have called the investment in SpaceX "crazy." One partner at Kleiner Perkins even walked out midway through a Palantir roadshow. The investment in Citrea follows the same technological conviction. While most companies in the Bitcoin L2 space are focused on marketing over-engineered sidechains, Citrea focuses on solving the "from zero to one" problem, truly bringing zero-knowledge technology to Bitcoin. Founders Fund stepped in as market confidence in Bitcoin L2 plummeted. Partner Joey Krug believes Citrea represents "the strongest team and technical architecture in the Bitcoin L2 space." As the company's first investment in the Bitcoin ecosystem, this move is not about following trends; it's about supporting the only design that can ultimately make Bitcoin's $2 trillion capital a programmable design. 2.1. A Team That Proved Its Zero-Knowledge Capabilities Before developing based on Bitcoin, the team released a "Proof of Innocence." In 2022, the U.S. Treasury Department sanctioned Tornado Cash for money laundering. The collateral damage was immediate. Innocent users with no connection to the illegal activities were also treated the same way. The team utilizes zero-knowledge proofs, allowing users to prove their wallets are not associated with sanctioned addresses without revealing their identity. RAILGUN DAO integrates this protocol. Vitalik Buterin has also publicly cited it. None of the founders come from large tech companies or well-known protocol fields. Yet, they have successfully launched a zero-knowledge sharing product that is already in production and has received external validation. Founders Fund's assessment of them as a "best team" is not unfounded.

2.2. Solving the Core Problem of Bitcoin L2 with Zero-Knowledge Proofs

Citrea overcomes the core obstacle of Bitcoin L2 using zero-knowledge proof technology, leveraging Bitcoin's actual security to run services on it.



Most Bitcoin L2 security mechanisms assign security permissions to a small group of signers. This can be imagined as a vault shared by five key holders, where any three can authorize withdrawals. This model is quick and practical, but it doesn't fully inherit Bitcoin's security because it relies on the integrity of a few people. Most Bitcoin Level 2 (L2) security layers are not significantly different in structure. Design details may vary, but the reliance on a few signers remains consistent across the domain. Citrea built a 1/N system where only one honest participant is needed. For example, when a user attempts to withdraw Bitcoin from Citrea, the process is controlled by four roles: Signer: Cryptographically locks the withdrawal conditions before any request is made. Once set, the conditions cannot be changed, preventing operators from manipulating withdrawal operations. Operator: Can only process withdrawals within the limits set by the signer. Collateral must be provided in advance, and cryptographic proof of its validity must be submitted to retrieve it. **Watchtower:** Simultaneously monitors Citrea and Bitcoin. If the operator attempts a fraudulent withdrawal, the Watchtower collects evidence and passes it to the challenger. **Challenger:** Uses the evidence provided by Watchtower to invalidate the fraudulent withdrawal on Bitcoin. Once fraud is confirmed, the operator's collateral is forfeited. The system can continue to operate if any one of these four participants remains honest. However, there is a challenge: Bitcoin's base layer is not designed for complex computations. How can cryptographic proof verification and dispute resolution mechanisms operate on top of it? ZK technology and BitVM make this possible. They can implement cryptographic proof verification and dispute resolution directly on Bitcoin. Founders Fund is optimistic about this unique technology. 3. How Citrea Actually Works The following is an application of the above architecture in practice. The system is driven by three components.

1. ZK Rollup:Compresses transaction data and publishes it to Bitcoin

2. BitVM:Supports optimistic Bitcoin native verification

3. Clementine:Citrea's Bitcoin bridge based on BitVM

3.1. ZK Proof: Thousands of transactions, one proof

Take a notary office as an example. Notarizing thousands of contracts one by one is time-consuming and expensive. A lawyer (Citrea) only needs to review all the contracts and submit a notarized document proving that "all these contracts are valid."

The notary office (Bitcoin) only needs to review this one document. It's quick, cheap, and the final notarization record is still kept by the notary office. This certification is a zero-knowledge proof. Citrea processes thousands of transactions off-chain, generates zero-knowledge proofs using zkEVM, and records them on the Bitcoin underlying layer. Anyone running Citrea and a Bitcoin node can verify whether Citrea's state is correct. 3.2. BitVM: No Rule Changes Required One problem remains. Bitcoin was not designed to verify complex zero-knowledge proofs. Its scripting language can only handle basic transaction conditions. Running complex calculations directly on Bitcoin is virtually impossible. Citrea solves this problem using BitVM. You can think of it as a notary office's appeal window. Under normal circumstances, zero-knowledge proofs are accepted according to their submission status. If someone questions the validity of a proof, Bitcoin intervenes and verifies it directly. The computation only runs when a dispute arises; it does not run by default. Bitcoin's consensus rules remain unchanged. 3.3. **Clementine: One Honest Actor Is Enough** In any Bitcoin secondary payment system, the bridge is the weakest link. If funds are stolen during a transfer between Bitcoin and Citrea, the entire architecture fails. Citrea's bridge, Clementine, takes a different approach. This security model requires only one honest participant out of N participants. If the operator attempts fraud, the challenger uses the chainhead proof issued by Watchtower to initiate a challenge transaction directly on the Bitcoin network, forfeiting the operator's collateral. Funds cannot be stolen unless all participants are compromised simultaneously. Standard multi-signature bridges transfer funds only when two out of three people agree, while Clementine ensures fund security as long as even one honest participant is present. The result is that attacking Citrea is tantamount to attacking Bitcoin itself. This requires controlling more than half of the total computing power to block challenges during the disputed window, which is fundamentally different from threatening a small group of signers. 4. The Current State of Citrea Most projects conduct token generation (TGE) before launching the mainnet. Testnets allow failures without consequences. This model is common: sell the vision first, then postpone the technology launch. But the mainnet is different. It involves real funds, real transactions, and real accountability. Citrea did the opposite. The mainnet launched before the token generation exchange (TGE), along with cBTC (bridged to Citrea via BTC) and ctUSD (Citrea's native stablecoin), with over 40 applications already running at launch. Applications such as Satsuma, Signals, Zentra, Crest, and CellFi all belong to the Citrea Origins project, which is directly supported by the Citrea team. Unlike most Bitcoin Level 2 (L2) cryptocurrencies that focus on staking and yield, Citrea directly supports developers in the DEX, prediction market, money market, privacy trading, and payments sectors. The intention is clear: to expand the practical functionality of Bitcoin L2 and build genuine usability. Despite this, Citrea is still in its early stages. The TVL is approximately $6 million, and while the testnet event attracted 33,000 participants, this momentum has not yet carried over to the mainnet, even after the launch of the new mainnet event. The network has been online for less than three months, and key services, including the Morpho Vault, were only recently introduced. Citrea's long-term vision is to become a hub for Bitcoin applications, building an ecosystem that goes far beyond staking and yielding. The goal is clear, but user acquisition is still underway. 5. Design vs. Reality Web3 is rife with projects peddling ideas: decentralization, censorship resistance, financial sovereignty. These ideas sound easy, but actually getting funding is difficult. Citrea's technical solution is sound. Bitcoin L2 should truly utilize Bitcoin's security mechanisms. Two trillion dollars of idle funds. Zero-knowledge proofs are the only way to achieve this. The Founders Fund agrees, and Citrea has successfully launched its mainnet. This distinguishes it from projects that only talk the talk. Next comes a more thorny issue. Most Bitcoin holders are not uneasy. Holding Bitcoin is perfectly normal. The idea of ​​"building on Bitcoin" doesn't appeal to ordinary users. Secure infrastructure is a necessary, but not sufficient, condition. What Citrea needs now is not more technical proofs, but rather to make the operations users perform on Arbitrum and Base work better on Bitcoin. This is precisely the mission of the Origins project (decentralized exchanges, money markets, privacy trading, prediction markets). If these applications cannot attract users, then the infrastructure will remain just infrastructure. The foundation is laid, the highway is built, and the traffic lights are installed. Now all that's left is to wait for the cars. No matter how well the road is paved, without traffic, it's just an empty asphalt road. The next challenge for Citrea isn't building a better road, but finding drivers.