Cross-Chains: How Blockchains Communicate With Each Other

Recognized as an integral technology, cross-chains enable blockchains to interoperate and communicate securely. This article explores their mechanisms, types, and implementation methods—focusing on Cosmos, a leading cross-chain platform.

Why Cross-Chains Matter

Initially, blockchains aimed to be "one-size-fits-all" solutions. However, scalability limits and innovation constraints revealed the need for interoperability. Cross-chains solve this by allowing independent blockchains to exchange data and assets through standardized protocols. Key benefits include:

• Avoiding single-chain limitations (e.g., congestion).
• Enabling asset swaps/transfers across ecosystems.
• Fostering innovation through modular design.

Cross-chain interactions fall into two categories:

1. Isomorphic cross-chains: Chains with identical security, consensus, and network structures (e.g., Cosmos SDK-based chains).
2. Heterogeneous cross-chains: Chains with differing technologies (e.g., Bitcoin’s PoW vs. Tendermint’s PBFT).

Isomorphic Cross-Chains: Implementation in Cosmos

Chains built on Tendermint (PBFT+PoS consensus) leverage the Inter-Blockchain Communication (IBC) protocol for seamless asset transfers. Here’s how it works:

Step-by-Step Asset Transfer

1. Chain Registration:

   • Chains A and B exchange genesis blocks and ChainIDs.
   • Validator information is shared to verify block headers.

2. Transaction Initiation:

   • User sends a packageTx to Chain A, locking/destroying assets.
   • Transaction details are written to Chain A’s egress (outbound mailbox).

3. Relayer Role:

   • Relayer forwards packageTx and its Merkle proof to Chain B via IBCPacketPostTx.
   • Chain B verifies the proof against Chain A’s block header.

4. Asset Minting:

   • On successful verification, Chain B mints equivalent assets.

IBC Protocol Packages

| Package Type | Purpose |
|----------------------------|------------------------------------------|
| IBCRegisterChainTx | Initial chain registration. |
| IBCUpdateChainTx | Updates latest block headers. |
| IBCPacketCreateTx | Stores cross-chain tx in egress. |
| IBCPacketPostTx | Relays Merkle proofs to destination. |

👉 Explore Cosmos IBC in depth

Heterogeneous Cross-Chains: Bridging External Blockchains

For chains like Bitcoin (PoW) or Ethereum, Cosmos uses PegZones—proxy chains that mirror the state of the original chain.

PegZone Workflow

1. Smart Contracts: Custody assets on the external chain (e.g., Ethereum) via lock/unlock/mint/burn functions.
2. Witness Nodes: Monitor the external chain, confirm finality (e.g., 100 blocks for Ethereum), and submit proofs to PegZone.
3. Relayers: Forward signed transactions to the Hub for cross-chain settlement.

Example: Ethereum ↔ PegZone ↔ Cosmos Hub

The Role of Cosmos Hub

The Hub acts as a central router, reducing the complexity of direct Zone-to-Zone connections.

Key Functions:

• Zone Registration: Tracks all connected Zones.
• State Synchronization: Zones report block updates to the Hub.
• Indirect Communication: Zones interact via the Hub instead of direct links.

Advantage: Scalability. For 100 Zones, direct links would require 4,950 connections—a Hub reduces this to 100.

Future of Cross-Chains

Cross-chain technology is still evolving. Success depends on:

• Real-world adoption of blockchain applications.
• Asset interoperability beyond tokens (e.g., data, credentials).
• Standardized protocols like IBC.

👉 Join the PPIO community to discuss cross-chain use cases!

FAQs

Q1: What’s the difference between isomorphic and heterogeneous cross-chains?
A1: Isomorphic chains share identical tech stacks (e.g., consensus), making transfers simpler. Heterogeneous chains (e.g., Bitcoin ↔ Cosmos) require bridges like PegZones.

Q2: How does Cosmos ensure cross-chain security?
A2: Through Merkle proofs and validator verification. PegZones add finality checks for external chains.

Q3: Can any blockchain join Cosmos?
A3: Yes! SDK-based chains integrate natively; others use PegZones.

Q4: What’s the role of relayers?
A4: They pay gas fees to forward transactions between chains.

Q5: Why use a Hub instead of direct Zone links?
A5: Hubs prevent exponential growth in connections, improving scalability.