BTCPayServer.NTag424 1.0.2

BTCPayServer.BoltCardTools

Introduction

This repository hosts tools that help with the creation of Bolt Cards.

Content:

• BTCPayServer.NTag424 is the base library implementing the NTag424 protocol, this library is platform agnostic.
• BTCPayServer.NTag424.PCSC is a library implementing APDU transport via PCSC-Sharp for supporting Windows/Linux/Mac.

We tested the following smart card reader:

• identiv utrust 3700 f
• ACR122U-A9

Examples

How to read the UID of an NTag 424 smart card

Plug in a smart card reader, and place an NTag 424 smart card on it.

Reference the nuget package BTCPayServer.NTag424.PCSC in your project.

dotnet add package BTCPayServer.NTag424.PCSC

Then to use it:

using BTCPayServer.NTag424.PCSC;
using System;

using var ctx = PCSCContext.Create();
var ntag = ctx.CreateNTag424();
var key = new AESKey(new byte[16]);
await ntag.AuthenticateEV2First(0, key);

var id = await ntag.GetCardUID();
var idStr = Convert.ToHexString(id, 0, id.Length).ToLowerInvariant();
Console.WriteLine($"Card UID: {idStr}");

How to read the NDEF message of an NTag 424 smart card

using BTCPayServer.NTag424.PCSC;
using System;
using NdefLibrary.Ndef;

using var ctx = PCSCContext.Create();
var ntag = ctx.CreateNTag424();
var message = await ntag.ReadNDef();
var uri = new NdefUriRecord(message[0]).Uri;
Console.WriteLine($"Card URI: {uri}");

How to verify the signature of an NTag 424 smart card

BoltCards involve the cooperation of three types of agents:

• Card Issuer: This agent configures the cards for lightning payments. This includes setting up the card to use a specific LNUrl Withdraw Service and generating the access keys.
• Payment processor: This agent reads the card and forwards the payment request to the LNUrl Withdraw Service.
• LNUrl Withdraw Service: This service authenticates the card and completes the payment.

BoltCards setup involves three different type of access keys:

• The IssuerKey: Owned by the Card Issuer, this key is used to configure the card.
• The EncryptionKey: This key can either be unique to each card or shared among multiple cards. It must be known by the LNUrl Withdraw Service.
• The AuthenticationKey: This key should be unique and is used to authenticate the card. It must also be known by the LNUrl Withdraw Service.

If you are the LNURL Withdraw Service, here how to authenticate the card:

using BTCPayServer.NTag424;
using BTCPayServer.NTag424.PCSC;
using System;
using System.Collections;
using NdefLibrary.Ndef;

// Set keys have you have setup the card
var encryptionKey = AESKey.Default;

using var ctx = PCSCContext.Create();
var ntag = ctx.CreateNTag424();
var message = await ntag.ReadNDef();
var uri = new NdefUriRecord(message[0]).Uri;
var p = Regex.Match(uri, "p=(.*?)&").Groups[1].Value;
var c = Regex.Match(uri, "c=(.*)").Groups[1].Value;

var piccData = PICCData.Create(encryptionKey.Decrypt(p));

// Note that the `piccData.Uid` contains the UID of the card which can be used to fetch
// the proper real `authenticationKey` of the card.
var authenticationKey = AESKey.Default;

if (!authenticationKey.CheckSunMac(c, piccData))
    throw new Exception("Invalid card");

// The LNUrlw service should also check `piccData.Counter` is always increasing between payments to avoid replay attacks.

How to setup a bolt card

using BTCPayServer.NTag424;
using BTCPayServer.NTag424.PCSC;
using System;
using System.Collections;

using var ctx = PCSCContext.Create();
var ntag = ctx.CreateNTag424();

// Example with hard coded keys
var keys = new BoltcardKeys(
    AppMasterKey: new AESKey("00000000000000000000000000000001".HexToBytes()),
    EncryptionKey: new AESKey("00000000000000000000000000000002".HexToBytes()),
    AuthenticationKey: new AESKey("00000000000000000000000000000003".HexToBytes()),
    K3: new AESKey("00000000000000000000000000000004".HexToBytes()),
    K4: new AESKey("00000000000000000000000000000005".HexToBytes()));

var lnurlwService = "lnurlw://test.com";

// Note `BoltcardKeys.Default` assumes the card hasn't been setup yet.
// If it was not the case, you would need to provide the access keys you provided during the last setup.
await ntag.SetupBoltcard(lnurlwService, BoltcardKeys.Default, keys);

// You can reset the card to its factory state with `await ntag.ResetCard(keys);`

How to setup a bolt card with deterministic keys, and decrypt the PICCData

Deterministic keys are useful if you want to be able to recover the keys of the card from a seed.

• The issuer can recover the keys of any card, just with a batchId and the issuer key.
• The LNUrlw service can recover the keys of any card (except the issuer key), just with the encryption key.

Note that you can reset the card to its factory state by only knowing the issuerKey with await ntag.ResetCard(issuerKey);.

using BTCPayServer.NTag424;
using BTCPayServer.NTag424.PCSC;
using System;
using System.Collections;

using var ctx = PCSCContext.Create();
var ntag = ctx.CreateNTag424();

await ntag.AuthenticateEV2First(0, AESKey.Default);
var uid = await ntag.GetCardUID();

var issuerKey = new AESKey("00000000000000000000000000000001".HexToBytes());
var keys = BoltcardKeys.CreateDeterministicKeys(issuerKey, uid, batchId: 0);
var lnurlwService = "lnurlw://test.com";

var encryptionKey = keys.EncryptionKey;
var piccData = PICCData.BoltcardDecrypt(encryptionKey, p, c);

// If this method didn't throw an exception, it has been successfully decrypted and authenticated.

// You can reset the card with `await ntag.ResetCard(issuerKey);`.

License

MIT