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Identity state replication

Most self-sovereign identity protocols rely on zero-knowledge proofs when interacting with smart contracts. This approach typically requires some stateful metadata (let's call it the state) available on-chain.

Generally, this metadata falls into two categories:

  • Merkle tree roots
  • Nullifiers

Iden3 protocol uses three sparse Merkle trees to track all the issued and revoked credentials.

Galxe Protocol uses nullifiers for sybil resistance and Merkle trees for revocable credentials (they are irrevocable by default).

WorldCoin keeps a Merkle tree of all identities and allows using nullifiers for sybil resistance.

Cross-chain identity challenges​

The stateful nature of the on-chain identity protocols hinders the portability of credentials. The state must be replicated when submitting a zero-knowledge proof to any chain other than the primary one.

The state replication process introduces a few challenges:

  1. It must be protected from spoofing of the metadata
  2. It must ensure that the replicas are always reasonably up-to-date
  3. It should be decentralized so there's no single point of failure
  4. It should minimize the gas fees on the target chains

Rarimo's cross-chain state replication on-demand​

Rarimo has faced the same challenges when implementing its cross-chain identity protocol. As a result, an on-demand cross-chain state replication protocol was created.

In a nutshell, it's a hub-and-spoke where the source chain is the single source of truth. Each target chain has a lightweight state contract updated on-demand when a user wants to submit proof. The state updates are protected from spoofing by a decentralized network of oracles observing the state updates on the source chain and a decentralized network of validators that secures these updates with a threshold signature (TSS).

Let's look at its inner workings in more detail by following the state replication from Ethereum to Polygon:

The key benefits of this solution are:

  • Decentralization and resilience. There's not a single point of centralization in the entire flow.
  • Security. The destination chains are protected from state spoofing by a decentralized network of oracles and validators with material incentives (PoS).
  • Cost efficiency. The solution can be scaled to any number of destination chains because the lightweight state contracts are lazily updated. The use of TSS makes the state updates as cheap as checking a single ECDSA signature. The fees can be paid either by the user or a relayer service.
  • State relevancy. The on-demand approach ensures that the state is always recent on any target chain.
  • Support for any EVM-compatible destination chains. WASM-based options can be reasonably easily added upon a request.