Demo, Testing & Limitations
This page walks through end-to-end validation of the USSD-RSK bridge using the Africa's Talking sandbox simulator, raw curl commands, and on-chain verification via the RSK Testnet Explorer. It also documents the current known limitations of the proof-of-concept and recommended improvements before any production deployment.
Live Demo Overview
The following demo shows the complete USSD flow running against the deployed InclusiveDeFi contract on RSK Testnet. A user on a feature phone dials a shortcode, navigates the text menu, and triggers real blockchain transactions without a smartphone or internet connection.
Main Menu
When a user dials the shortcode (e.g., *384#), the relay server responds with the main menu. The phone number is echoed back to confirm the active session.
Balance Check
Selecting option 1 triggers a read call to getBalance(wallet.address) on the RSK node — no transaction is broadcast, no gas is consumed. The result is returned immediately and the session ends.
P2P Transfer Flow
Selecting option 2 opens a three-round collection flow. The user enters the recipient address in round 2 and the amount in round 3. The relay server signs and broadcasts the transaction on round 4.
Micro-Loan
Selecting option 3 immediately calls applyForLoan(). If the address has no existing active loan, the contract credits 0.01 tRBTC to its internal balance and emits a LoanIssued event.
On-Chain Transaction Confirmation
Every write operation (transfer, loan) produces a transaction that can be verified on the RSK Testnet Explorer. The screenshot below shows a confirmed applyForLoan transaction.
Testing Without a Phone curl
You can simulate the full USSD session lifecycle against your local relay server using curl. This allows you to test all branches of the state machine without the Africa's Talking gateway or a physical device.
Ensure your relay server is running (npm run start-bridge) before executing these commands.
Main Menu
curl -X POST http://localhost:3000/ussd \
-d "text=&phoneNumber=%2B2348012345678&sessionId=test001"
Expected response:
CON Rootstock DeFi (+2348012345678)
1. My Balance
2. Send Money (P2P)
3. Request Micro-Loan
Balance Check
curl -X POST http://localhost:3000/ussd \
-d "text=1&phoneNumber=%2B2348012345678&sessionId=test001"
Expected response:
END Your Balance: 0.01 tRBTC
Transfer Recipient Prompt
curl -X POST http://localhost:3000/ussd \
-d "text=2&phoneNumber=%2B2348012345678&sessionId=test002"
Expected response:
CON Enter Recipient Address:_________
Transfer Amount Prompt
curl -X POST http://localhost:3000/ussd \
-d "text=2*0xSomeRecipientAddress&phoneNumber=%2B2348012345678&sessionId=test002"
Expected response:
CON Enter Amount:_____
Transfer Execute
Replace 0xSomeRecipientAddress with a valid RSK address and 0.005 with your desired amount:
curl -X POST http://localhost:3000/ussd \
-d "text=2*0xSomeRecipientAddress*0.005&phoneNumber=%2B2348012345678&sessionId=test002"
Expected response (address and hash will differ):
END Transfer Submitted
To: 0xSomeRecip...
Tx: 0xa924c40ed1...
Funds will arrive shortly.
The transaction enters the RSK mempool immediately and the session ends. The server terminal will log Transfer confirmed: 0x... once the block is mined. The full transaction hash is available in your server logs if you need to look it up on the explorer.
Micro Loan
curl -X POST http://localhost:3000/ussd \
-d "text=3&phoneNumber=%2B2348012345678&sessionId=test003"
Expected response (hash will differ):
END Loan Requested
Tx: 0xa924c40ed1...
0.01 tRBTC credited shortly.
Like the transfer, the loan transaction is submitted to the mempool and the USSD session ends immediately. Watch your server terminal for the background confirmation log Loan confirmed: 0x.... If the contract reverts on chain (e.g., because a loan is already active), it will appear as Loan failed on-chain: Existing loan active in the server logs the user will not see this since the session is already closed.
Verifying Transactions On-Chain
After any write operation through the relay server, verify the transaction on the RSK Testnet Explorer.
Search by your deployed contract address.
https://explorer.testnet.rootstock.io/address/0xYourDeployedContractAddress
Or search by transaction hash the truncated hash is returned in the END response string from the relay server (e.g., Tx: 0xa924c40ed1...). Copy it from your server terminal logs for the full hash. A successful transaction will show.
- Status: Success
- To: Your deployed contract address
- Input Data: The encoded function call (
applyForLoan,transfer, etc.) - Block: The block number it was included in
You can also inspect ignition/deployments/chain-31/journal.jsonl to review all deployment transactions including their block hashes and receipt status.
Africa's Talking Simulator
For end-to-end testing with the real USSD protocol (including session ID management and proper CON/END handling), use the built-in USSD Simulator in the Africa's Talking sandbox dashboard.
- Open your sandbox dashboard at account.africastalking.com/apps/sandbox
- Navigate to USSD → Simulator
- Enter a test phone number and dial your shortcode
- Navigate the menu and observe the responses
The ngrok inspector at http://localhost:4040 will show you every raw HTTP POST from Africa's Talking and every response your server returns essential for debugging unexpected session terminations or malformed responses.
Known Limitations
The current implementation is a working proof of concept. The following limitations must be addressed before any production or mainnet deployment.
Single relayer wallet for all users. The getBalance() call always returns the relay wallet's balance, not the individual user's, because all transactions are signed by the same wallet. In production, map each phoneNumber to a unique wallet address stored in a database (e.g., PostgreSQL or Redis), and derive or assign wallets per user at first dial.
In-memory session guard. The processedSessions Set that prevents duplicate transactions resets on every server restart. Under horizontal scaling, two server instances will have separate Sets and the duplicate-prevention guarantee breaks. Move session state to a shared Redis store with a matching TTL.
No input validation. Recipient addresses and amounts from the USSD text field are passed to ethers.js without explicit format checks. A non-numeric amount string passed to ethers.parseEther() will throw with a non-descriptive error. Add address and numeric format validation before calling contract functions.
No loan repayment mechanism. applyForLoan() issues a loan with no way to repay it. The require(loans[msg.sender] == 0) guard prevents a second loan but there is no repayLoan() function. Add a repayment flow to InclusiveDeFi.sol that accepts tRBTC and clears the loans mapping entry, then expose it as option 4 in the USSD menu.
Post-broadcast reverts are silent to the user. Because tx.wait() runs in the background after the USSD session has ended, any on-chain revert that occurs after broadcast cannot be communicated back to the user. They will see the END Transfer Submitted screen regardless of the final on-chain outcome. Monitor server logs actively and consider a secondary USSD option to check transaction status by hash.
No formal meta-transaction standard. The relayer pattern here is informal. The smart contract has no way to distinguish between the relay wallet acting on behalf of different users all calls appear to come from the same msg.sender. Implement EIP-2771 trusted forwarder to allow the contract to identify the true originating user by their phone-mapped wallet address.
Private key in .env. The relayer private key is stored in a plaintext .env file. For any deployment beyond local development, move secrets to a secrets manager (AWS Secrets Manager, HashiCorp Vault) or an HSM-backed signing service.