Barter - Offline P2P Crypto Wallet

A Go-based CLI application for peer-to-peer cryptocurrency transactions over local networks without internet connectivity. Uses TCP for data transfer, UDP for peer discovery, and Ed25519 for cryptographic signatures.

Features

• Offline P2P transactions over Wi-Fi Direct, hotspot, or localhost
• Ed25519 cryptographic signatures with fingerprint-based identity
• UDP broadcast peer discovery with TCP block transfer
• Simple blockchain (one transaction = one block)
• JSON persistence for keys, state, and ledger
• Deterministic port assignment for multi-wallet support
• Real-time transaction validation and balance updates

Requirements

• Go 1.21 or higher
• Linux/macOS/Windows
• Network connectivity between peers (local network, no internet required)

Installation

# Clone the repository
git clone git@github.com:yourusername/barter.git
cd barter

# Build the binary
go build -o barter .

# Or use make
make build

Usage

Starting a Wallet

./barter --wallet <name>

On first run, the wallet generates an Ed25519 keypair and initializes with 100 tokens.

Interactive Menu

1. Send Money - Discover peers and send tokens
2. View Blockchain - Display transaction history
3. View Peers - Scan for active wallets on network
4. Exit - Shutdown wallet

Multi-Wallet Demo

Open two terminals and run:

# Terminal 1
./barter --wallet alice

# Terminal 2
./barter --wallet bob

Alice and Bob can now discover each other and exchange tokens.

Architecture

Wallet Structure

Each wallet consists of:

• Ed25519 key pair with 4-character fingerprint
• Persistent JSON storage (keys, state, ledger)
• TCP listener for incoming transactions
• UDP listener for peer discovery
• Deterministic port assignment based on wallet name

Network Protocol

Peer Discovery (UDP broadcast)

• Wallets broadcast discovery requests
• Peers respond with their identity and TCP port
• Each wallet uses a unique UDP port (TCP port + 1000)

Transaction Transfer (TCP)

• Sender creates signed block
• Block sent to receiver's TCP port
• Receiver validates signature and updates balance
• Both wallets persist the transaction

Blockchain

Each block contains:

• Index, sender/receiver public keys, amount
• Timestamp, nonce, previous hash
• Transaction ID (SHA-256 of block data)
• Ed25519 signature

Validation includes:

• Signature verification
• Timestamp tolerance check
• Previous hash chain verification
• Double-spend prevention

File Structure

wallets/
  <wallet-name>/
    keys.json       # Ed25519 keypair and fingerprint
    state.json      # Balance and last sender hashes
    ledger.json     # Full blockchain

Development

Build

make build

Clean

make clean

Project Structure

.
├── main.go         # Core wallet implementation
├── go.mod          # Go module definition
├── Makefile        # Build automation
└── README.md       # Documentation

Security Considerations

• Ed25519 signatures prevent transaction forgery
• Timestamp validation prevents replay attacks
• Previous hash linking prevents chain manipulation
• Private keys stored locally (never transmitted)

This is a demonstration project. For production use, additional security measures would be required including:

• Secure key storage (hardware security modules)
• Network encryption (TLS)
• Consensus mechanism for distributed ledger
• Rate limiting and DoS protection

License

MIT License - see LICENSE file for details.

Author

Built as a demonstration of P2P networking, cryptography, and blockchain concepts in Go.