Union is introducing Private Bridging, a zero-knowledge protocol that enables private transfers across chains. The system allows users to bridge assets without revealing the source chain, source address, or transaction history. Activity remains confidential to the public while still being auditable for compliance.
Privacy is a requirement for broad adoption in decentralized finance. In public blockchain systems, all transfers, balances, and addresses are visible, which can expose operational activity. Current privacy solutions typically work only within a single chain and do not preserve confidentiality when assets are bridged to other networks. Multichain activity adds complexity, as bridging can reveal movements across networks, effectively linking activity across multiple chains. A strong privacy system should work across different blockchains, keeping data private while still allowing verification when needed.
Union addresses these challenges with a trust-minimized, ZK protocol for ERC-20 transfers across EVM chains. The design enables confidential crosschain transfers while remaining auditable for authorized parties. Tokens can be transferred without publicly exposing the sender, receiver, or the originating chain, providing a privacy framework that functions at scale across multiple networks.
Privacy Limitations in Multichain Systems
Current blockchain environments do not provide consistent privacy across chains. When assets move between networks, the linkage between source and destination is exposed, even if transfers were confidential on the original chain. Furthermore, confidentiality in many existing protocols depends on the number of participants. Networks with limited usage provide minimal privacy, reducing the effectiveness of the system. Most privacy tools are also concentrated on a few major chains, leaving activity on newer or smaller networks exposed. Without a crosschain approach, privacy is fragmented and incomplete.
Union’s Solution: Confidential Crosschain Transfers
Union leverages ZKPs to create a privacy-preserving system that functions across all connected chains. Activity from all participating networks contributes to a combined anonymity set, improving privacy and preventing the reconstruction of user activity from transaction data alone.
The protocol works in three stages. First, the user generates a secret and derives an unspendable address to send tokens to. From an external perspective, this appears identical to a standard ERC-20 transfer. Next, a designated attestor signs a commitment linking the locked tokens to a beneficiary, forming an auditable record for authorized parties. Finally, on the destination chain, the user generates a zero-knowledge proof demonstrating knowledge of the secret and possession of a valid attestation. The system verifies the proof against Union’s global state root, and the corresponding tokens are issued to the beneficiary. At no point is the relationship between sender and recipient observable on-chain.
The system also allows users to select a hidden subset of connected chains as potential sources for a transfer. Observers cannot determine the actual origin of the assets, and privacy increases as the aggregated activity across all chains grows. All of this is supported by Union’s global state root, which combines the state roots of all connected networks and cannot be modified, providing a secure foundation for crosschain verification that standard or intent-based bridges cannot replicate.
The Z-Asset Standard
Union introduces an additional transfer mechanism that extends existing ERC-20 tokens into Z-assets. This extra feature allows any ERC-20 token to support both transparent and private bridging while remaining fully fungible. The Z-asset standard provides a unified framework where tokens can continue to use Union’s standard transparent bridge, while also utilizing the confidential bridging mechanism. Tokens moved privately remain fully interchangeable with those transferred transparently, ensuring consistent behavior across all use cases.
Why Private Bridging Matters
Private bridging enables secure, confidential activity across chains. Institutions and asset managers can move funds or execute strategies without exposing operational data. Traders and market makers can rebalance positions or execute crosschain strategies without leaking alpha. Exchanges can offer private onramps and support compliant crosschain activity, while users gain stronger protection against front-running and MEV exposure. By aggregating activity across multiple chains and providing auditable attestation, private bridging delivers scalable privacy without sacrificing transparency where required.
Private bridging represents a rare and significant advancement in blockchain privacy. Unlike previous protocols, which maintain anonymity only within a single chain, Union combines all connected chains into a single, unified anonymity set. Activity on one chain, such as a transfer on Ethereum, contributes to the set for other chains, like Sei. This crosschain aggregation is unique in the industry and eliminates the delays seen in earlier systems during low-traffic periods, allowing confidential transfers to complete immediately. By combining this design with a fully auditable framework, Union establishes a new standard for confidential crosschain transfers and positions the protocol as a foundational layer for future applications, enabling institutions and users to operate at scale while retaining control over sensitive information.
What’s Next
Union is actively building and deploying a confidential bridging protocol. Integration work is underway to enable asset issuers, institutions, and ecosystem partners to adopt the system. As deployment progresses, additional updates and technical details will be shared to support broader adoption and practical use of confidential crosschain transfers.
For full technical details, refer to the whitepaper: Private Bridging Whitepaper
Follow @union_build for updates. zkgm.