Zephyr Bridge — Private Cross-Chain Bridge

The privacy gap

Normal bridges leave a trail. Zephyr doesn't.

On a standard bridge, anyone watching can correlate the address that locks funds with the address that claims them. Zephyr uses ZK proofs so the deposit and claim are mathematically decoupled.

Normal bridge

Alice locks $100 on ETH
Alice claims $100 on Stellar

→ Anyone correlates the two addresses. Privacy broken.

Zephyr bridge

Alice locks $100 on ETH
SOMEONE claims $100 on Stellar

→ No link between deposit & claim. Proven by ZK.

How it works

Four steps, fully private

The same ZK flow powers both directions. Your secret never leaves your browser — the proof is generated client-side and verified on-chain.

1

Lock

Lock USDC or ZUSDC on the source chain. A Poseidon commitment Poseidon(amount, secret, recipient) is recorded — no plaintext address stored.

2

Merkle sync

The relayer monitors events, builds a Poseidon Merkle tree, and posts the root to the destination chain. The frontend auto-detects when it's ready.

3

Generate proof

Your browser runs the Groth16 prover (snarkjs WASM) to build a zero-knowledge proof of membership, amount, and nullifier — all in ~30–60 seconds.

4

Claim privately

Submit the proof on-chain. The contract verifies it via native BN254 pairing, checks the nullifier, and releases net funds. No link to the original deposit.

Bidirectional

Private in both directions

ZK proofs are load-bearing in both directions — a valid proof is strictly required to claim bridged funds, no matter which way you're going.

→

ETH → Stellar

1 Lock USDC on Ethereum. commitment = Poseidon(netAmount, secret, recipient) recorded, 0.1% fee collected.

2 Relayer builds Poseidon Merkle tree from Locked events, posts root to Stellar via set_src_root.

3 Auto-detect: frontend polls Stellar contract root and enables Claim when it matches local tree.

4 Claim via Groth16 proof + claim(proof, pub, recipient, commitment) on Soroban. Native BN254 verifies.

→

Stellar → ETH

1 Lock ZUSDC via lock_stellar_private(amount, commitment). ETH recipient is completely hidden on-chain.

2 Relayer accumulates all historical commitments (survives Soroban pruning), posts root to Ethereum via setStellarRoot.

3 Auto-detect: frontend fetches full commitment list from relayer API, confirms yours is in the tree.

4 Claim via claimFromStellar(proof, ...) on EVM. Native alt_bn128 precompiles verify.

Zero-knowledge

One circuit, two chains

A single shared Circom circuit powers both directions. The exact same Groth16 proof generated in your browser verifies on both Ethereum's alt_bn128 and Stellar's native BN254 host functions.

Circuit metrics

Groth16 over BN254, Poseidon hashes, depth-15 Merkle tree.

Curve

BN254

Proof system

Groth16

Constraints

9,337 NL

Tree depth

15

Privacy set

32,768

Ceremony

pot14_final

What the proof guarantees

Five statements proven in a single Groth16 proof.

1commitment = Poseidon(amount, secret, recipient)

2leaf = commitment (exists in tree)

3nullifierHash = Poseidon(secret, leaf)

4leaf ∈ Merkle tree (path matches root)

5amountCommitment = Poseidon(amount)

Fee economy

Symmetric, transparent fees

A flat 0.1% is collected on each side (lock + claim) to fund operations and maintain liquidity. Fees accumulate on-chain and are withdrawable by admin only.

Direction	Source fee	Destination fee	Total
ETH → Stellar	0.1% on lock	0.1% on claim	~0.2%
Stellar → ETH (ZK)	0.1% on lock_private	0.1% on claimFromStellar	~0.2%
Stellar → ETH (release)	0.1% on lock_stellar	0.1% on claimBurned	~0.2%

Deployed

Live on testnet

Depth-15 deployment with burn-proof release path and VK-init bypass. Click any address to view on the explorer.

Ethereum Sepolia

MockUSDC

0x82eDeE25b30DA4629FAa73b58880569184eaF55B

BridgeEscrow

0xE2E882f8b74032CF56739003Ae2c21CC3Bdcb351

Groth16Verifier

0xAaA8A825778E5ff8ccD9cE69952b1D1AdebA6fbC

Stellar Testnet

ZephyrBridge

CALV7CZLDXJKHJOXKHMA4KOTKS7BZVBQACP7MCPZI5RTGQBUFQMNGFUN

ZUSDC Token (SAC)

CB42VBDIGTKDSHG43335RSYU3VUE3VVVBA7Q6NA575TZOUAQC4DGPFSD

ZUSDC Issuer

GBCWIZI5TMATWJ6WCF2SUM6GF7OBDPTOIFDUTWVGZ5BZEANUINILQJRW

Tech stack

Built on solid primitives

Every layer chosen for soundness and auditability — from the ZK proving toolkit down to the smart contract languages.

ZK Proving

Circom 2.2 + snarkjs

ZK circuit DSL and the in-browser WASM prover.

Proof System

Groth16 / BN254

Economical on-chain verification on both chains.

Hash Function

Poseidon

ZK-friendly, native in Stellar Protocol 25/26.

Ethereum

Solidity 0.8.28 + Hardhat

Escrow, verifier, and burn-proof registry.

Stellar

Soroban SDK 26 (Rust)

Native BN254 pairing and commitment state.

Frontend

React 19 + Vite + wagmi

Wallet integration and client-side Merkle + prover.

Bridge privately between Ethereum & Stellar