AlgoZero: Zero-Connectivity Crypto-UPI Offline Algorand Payment Protocol

Abstract

Current digital payment infrastructures, ranging from traditional fiat-based Unified Payments Interface (UPI) systems to decentralized cryptocurrency wallets, share a critical vulnerability: an absolute dependency on continuous internet connectivity. In regions with constrained telecommunications or during disaster recovery scenarios, these systems fail entirely. AlgoZero addresses this limitation by introducing a "Governed Offline" architecture on the Algorand blockchain. By utilizing advanced smart contract escrow mechanisms and Bluetooth Low Energy (BLE), this protocol enables secure, peer-to-peer value transfers that settle deterministically once network connectivity is restored.

1. The Problem: Offline Double-Spending

To successfully replicate the reliability of physical cash, a digital payment framework must function without immediate network validation. However, offline digital currencies face the fundamental challenge of the "double-spending" problem. In a partitioned network (where devices cannot reach the main ledger), a malicious actor could theoretically replicate a digital signature and spend the identical funds with multiple offline merchants.

To resolve this localized constraint of the CAP theorem (Consistency, Availability, Partition Tolerance), the system must prioritize local availability while guaranteeing eventual consistency.

Mitigation Strategies in Offline Payment Architectures

There are three primary approaches to enabling secure offline digital transactions:

Trusted Execution Environment (TEE)

A hardware-based approach that leverages secure, isolated regions within mobile devices to manage and lock funds off-chain. Offers high security but requires deep hardware and OS-level integration.

Host Card Emulation (HCE)

A software-driven method that allows mobile devices to emulate physical smart cards for NFC-based transactions. Easier to deploy but more vulnerable to system-level attacks.

Smart Contract Escrow with IOU-based Exchange (AlgoZero)

Funds are locked on-chain in advance. Offline, users exchange cryptographically signed IOUs. Once connectivity is restored, claims are verified and settled on-chain. This ensures high security, low deployment complexity, and scalability.

AlgoZero utilizes the Smart Contract Escrow (IOU) architecture.

2. Technical Architecture

The Algorand network enforces a strict validity window for standard transactions. AlgoZero bypasses this constraint using a decoupled cryptographic IOU mechanism.

2.1 Smart Contract Infrastructure (Puya & AlgoKit 3.0)

Funds locked in escrow with incremental nonce
Replay protection via strictly increasing nonce
Domain separation using custom prefix (AlgoZeroV1)
Raw signature verification using ed25519verify_bare

import algopy 
from algopy import ARC4Contract, UInt64, Account, Bytes, op

class OfflineEscrow(ARC4Contract):
    def __init__(self) -> None:
        self.owner = Account()
        self.current_nonce = UInt64(0)

    @algopy.arc4.abimethod
    def settle_offline_iou(self, payee: Account, amount: UInt64, nonce: UInt64, signature: Bytes) -> None:
        assert nonce > self.current_nonce

        prefix = Bytes(b"AlgoZeroV1")
        payload = op.concat(prefix, op.concat(payee.bytes, op.itob(amount)))
        payload = op.concat(payload, op.itob(nonce))

        is_valid = op.ed25519verify_bare(payload, signature, self.owner.bytes)
        assert is_valid

        self.current_nonce = nonce

        algopy.itxn.Payment(
            receiver=payee,
            amount=amount,
            fee=0
        ).submit()


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2.2 Mobile Transport Layer (BLE GATT Server)

160-byte payload split into 20-byte BLE chunks

Sequential transmission ensures reliability

Mutex-based locking resolves GATT race condition


fun sendClaimInChunks(claim: ByteArray, characteristic: BluetoothGattCharacteristic) {
    val chunkSize = 20
    val totalChunks = Math.ceil(claim.size.toDouble() / chunkSize).toInt()

    for (i in 0 until totalChunks) {
        val start = i * chunkSize
        val end = minOf(start + chunkSize, claim.size)
        val chunk = claim.copyOfRange(start, end)

        gattMutex.withLock {
            characteristic.value = chunk
            gatt.writeCharacteristic(characteristic)
        }
    }
}


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3. Security and Settlement

Fully collateralized transactions via escrow

Deterministic on-chain settlement

Cryptographic verification ensures integrity

No dependency on hardware trust



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4. UI/UX

Neo-banking minimalist design

Space Grotesk typography with tabular figures

Built using Jetpack Compose

Optimized for clarity in outdoor environments



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5. Technology Stack

Blockchain: Algorand Layer-1

Smart Contracts: Python (Puya), AlgoKit 3.0

Mobile: Kotlin, Jetpack Compose, BLE GATT

Cryptography: Ed25519, SHA-512/256



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Links

GitHub: https://github.com/HarshPrajapati7/AlgoZero

Demo: https://youtu.be/9LPLokChwE0?si=ADb-qqz-JWoQNbSK

Build (APK): https://drive.google.com/file/d/1lBj48e-8xpPcbSOysMAkgVFXh0fYsh0-/view?usp=drivesdk