BTCPay Server Grin Plugin

Accept Grin payments in your BTCPay Server store.

Grin is a privacy-preserving cryptocurrency using MimbleWimble. Unlike Bitcoin, Grin transactions are interactive — both sender and receiver must exchange data (slatepacks) to build a transaction. This plugin manages that exchange through a checkout UI.

How It Works

1. Merchant configures their grin-wallet connection in BTCPay store settings
2. Customer creates an invoice (via API or storefront integration)
3. Checkout page shows a slatepack message (with QR code) for the customer to process in their wallet
4. Customer pastes their wallet's response slatepack back into the checkout page
5. Plugin finalizes and broadcasts the transaction
6. Background service monitors confirmations and marks the invoice complete

Customer                    BTCPay (this plugin)              grin-wallet
   |                              |                               |
   |  --- create invoice -------> |  --- issue_invoice_tx ------> |
   |  <-- slatepack S1 ---------  |  <-- slatepack S1 ----------- |
   |                              |                               |
   |  (process S1 in wallet)      |                               |
   |                              |                               |
   |  --- paste response S2 ----> |  --- finalize_tx (S2) ------> |
   |                              |  --- post_tx ---------------> |
   |  <-- "payment broadcast" --- |                               |
   |                              |                               |
   |                            [monitor service polls every 30s] |
   |                              |  --- retrieve_txs ----------> |
   |                              |  --- node_height -----------> |
   |  <-- "confirmed" ----------- |  (confirmations >= threshold) |

Requirements

• BTCPay Server v1.12+ (.NET 8)
• PostgreSQL (used by BTCPay)
• A running grin-wallet instance with the Owner API enabled
• A synced Grin node (the wallet connects to it)

grin-wallet Setup

The plugin communicates with grin-wallet's Owner API using v3 encrypted JSON-RPC (ECDH key exchange + AES-256-GCM).

# Start the owner API (default port 3420)
grin-wallet owner_api

You'll need three things from your wallet for plugin configuration:

• Owner API URL: default http://127.0.0.1:3420
• Wallet password: the password used to open/unlock the wallet
• API secret: contents of .owner_api_secret in your grin-wallet data directory

Installation

From BTCPay Plugin Builder (recommended)

Coming soon — the plugin will be published to the BTCPay plugin directory.

Manual / Development

git clone https://github.com/Such-Software/btcpayserver-grin-plugin.git
cd btcpayserver-grin-plugin
git submodule update --init --recursive

# Build the plugin
dotnet build BTCPayServer.Plugins.Grin/BTCPayServer.Plugins.Grin.csproj

# Run BTCPay with the plugin loaded
export BTCPAY_DEBUG_PLUGINS="$(pwd)/BTCPayServer.Plugins.Grin/bin/Debug/net8.0/BTCPayServer.Plugins.Grin.dll"
dotnet run --project btcpayserver/BTCPayServer --no-launch-profile

Set your standard BTCPay environment variables (BTCPAY_NETWORK, BTCPAY_POSTGRES, BTCPAY_BTCEXPLORERURL, etc.) as needed.

Configuration

1. In BTCPay, go to your store's settings
2. Click Grin in the left sidebar
3. Enter your wallet's Owner API URL, password, and API secret
4. Optionally enter your Grin node's API URL for detailed sync status monitoring (shows sync percentage and phase in the BTCPay footer panel)
5. Set minimum confirmations (default: 10, roughly 10 minutes)
6. Check Enable Grin Payments and save
7. Click Test Connection to verify

See SETUP.md for detailed deployment instructions including Docker networking with socat proxies.

Creating Invoices

Invoices are created via the plugin's REST API:

curl -X POST "http://localhost:23000/stores/{storeId}/plugins/grin/invoices" \
  -H "Content-Type: application/json" \
  -d '{"amount": 1.5, "orderId": "order-123"}'

Response:

{
  "invoiceId": "a1b2c3d4e5f6",
  "checkoutUrl": "http://localhost:23000/stores/{storeId}/plugins/grin/checkout/a1b2c3d4e5f6",
  "amount": 1.5,
  "amountNanogrin": 1500000000,
  "txSlateId": "...",
  "slatepackAddress": "grin1...",
  "status": "Pending"
}

Direct the customer to the checkoutUrl to complete payment.

Invoice Lifecycle

Status	Description
Pending	Invoice created, slatepack issued, awaiting customer response
AwaitingResponse	Reserved for future use
Broadcast	Transaction finalized and broadcast to the network
Confirmed	Transaction has enough confirmations (per store setting)
Expired	No response within 24 hours; wallet transaction cancelled automatically

The background monitor service polls every 30 seconds to update confirmation counts and expire stale invoices.

USD Pricing

The checkout page shows an approximate USD value alongside the Grin amount. The price is fetched from Gate.io's spot API and cached for 2 minutes. Sub-cent amounts display as "< $0.01".

A GrinRateProvider is also registered with BTCPay's rate engine, making GRIN pairs available for rate rules and scripting.

Architecture

BTCPayServer.Plugins.Grin/
├── Plugin.cs                          # Service registration, nav extension
├── PluginMigrationRunner.cs           # EF Core migrations on startup
├── GrinRPCClient.cs                   # v3 encrypted Owner API client
├── Controllers/
│   ├── UIGrinController.cs            # Settings UI (store admin)
│   └── GrinCheckoutController.cs      # Checkout flow + REST API
├── Data/
│   ├── GrinDbContext.cs               # Plugin's own DB context
│   ├── GrinInvoice.cs                 # Invoice model
│   └── GrinStoreSettings.cs           # Per-store wallet config
├── Services/
│   ├── GrinService.cs                 # Business logic, price fetching
│   ├── GrinRPCProvider.cs             # Per-store RPC client cache
│   ├── GrinRateProvider.cs            # BTCPay rate engine integration
│   ├── GrinPaymentMonitorService.cs   # Background confirmation tracker
│   ├── GrinSyncService.cs            # Node sync status polling (30s)
│   ├── GrinSyncSummaryProvider.cs    # BTCPay footer panel integration
│   └── GrinDbContextFactory.cs        # DB context factory
├── Views/
│   ├── UIGrin/Settings.cshtml         # Store settings + invoice list
│   ├── GrinCheckout/Checkout.cshtml   # Payment page
│   ├── GrinCheckout/CheckoutComplete.cshtml
│   ├── GrinCheckout/CheckoutExpired.cshtml
│   ├── Shared/GrinNav.cshtml          # Sidebar nav item
│   └── Shared/Grin/GrinSyncSummary.cshtml  # Footer sync panel
├── Migrations/                        # EF Core migrations
└── Resources/img/                     # Grin logo

Each BTCPay store connects to its own grin-wallet instance, similar to how Lightning works — the plugin doesn't hold keys, the merchant runs their own wallet.

Trust Model

Like Lightning in BTCPay, this plugin requires the merchant to run their own grin-wallet. The wallet password and API secret are stored in the BTCPay database. This means:

• Self-hosted BTCPay: Fully self-custodial. You control both the server and the wallet.
• Third-party BTCPay: The server operator has access to your wallet credentials. This is the same trust model as Lightning — if you don't run the server, you're trusting the operator.

Grin's interactive transaction model requires private keys for receiving, so there's no equivalent to Bitcoin's xpub-based watch-only wallets. True non-custodial operation requires self-hosting.

Security Notes

• Wallet credentials (password, API secret) are stored in the plugin's PostgreSQL tables, not in files
• The Owner API connection uses v3 encrypted JSON-RPC (ECDH + AES-256-GCM) — even over plaintext HTTP, the RPC payload is encrypted
• The plugin never holds private keys; all signing happens in grin-wallet
• Invoice slatepack exchange happens over HTTPS (or whatever your BTCPay instance uses)
• Error messages shown to customers are generic — internal RPC errors are logged server-side only

Running grin-wallet as a Service

See contrib/systemd/ for example systemd unit files for running the Grin node and wallet as background services.

License

MIT