CellFi

Problem Statement

CellFi is a groundbreaking mobile application that allows users to perform crypto transactions without internet access, using only SMS technology. Designed to make blockchain accessible to everyone, even in low-connectivity regions, CellFi enables users to transfer and swap tokens seamlessly on the Ethereum network.How It Works: The CellFi app generates a cryptographic signature offline on the user’s device. This signature is then sent via SMS to a designated mobile number. That number is linked to a Node.js backend, which receives and verifies the signature. Once validated, the backend executes the transaction on the blockchain.Current Implementation: CellFi currently operates on the Ethereum Sepolia testnet. A macro-based system continuously scans incoming SMS messages and triggers the appropriate blockchain actions. Vonage’s messaging service is integrated to handle SMS replies, allowing users to receive instant confirmations and status updates about their transactions.Future Enhancements: We continually improve CellFi to expand its functionality, enhance security, and deliver a smoother user experience. Upcoming updates aim to introduce support for more blockchain networks, additional transaction types, and advanced SMS-based interactions.

Solution

CellFi was built by combining blockchain technology with traditional SMS infrastructure to enable crypto transactions without internet access. The mobile app, developed usingReact NativeandEthers.js, generates and signs Ethereum transactions offline using securely stored private keys. These signed transactions are then sent viaSMSto a dedicated number connected to aNode.js backend, which listens for incoming messages using a custommacro-based scanning system. The backend verifies the signature and broadcasts the transaction to theEthereum Sepolia testnetthroughInfuraorAlchemyRPC endpoints. We integratedVonage’s SMS APIto handle reliable two-way messaging, allowing users to receive real-time transaction confirmations via text.During development, we faced several challenges — especially when connecting the mobile number to the backend and maintaining stable communication between the two. Since we’re based inIndia, obtaining and managing avirtual phone numberfor long-term use was particularly difficult due to regional restrictions and carrier limitations. We also encountered numerous bugs and technical hurdles throughout the build process, from message parsing issues to backend synchronization problems. Despite these challenges, we managed to create a fully functional prototype that successfully enables blockchain transactions entirely over SMS, making crypto accessible even without internet connectivity.