# Tap-to-Pay (NFC / HCE)

BML supports contactless NFC payments via Host Card Emulation (HCE). The app fetches single-use payment tokens from the server, then emulates an EMV mag-stripe contactless card using Android's `HostApduService`.

---

## Overview

```
1. Fetch tokens  →  POST /api/mobile/walletpayments/gettoken  (TOTP-authenticated)
2. HCE exchange  →  Android NFC subsystem drives the APDU exchange with the POS terminal
```

---

## Step 1 — Fetch Payment Tokens

### Endpoint

```
POST https://www.bankofmaldives.com.mv/internetbanking/api/mobile/walletpayments/gettoken
```

### Headers

| Header | Value |
|---|---|
| `Authorization` | `Bearer <access_token>` |
| `User-Agent` | `bml-mobile-banking/348 ({manufacturer}; Android {version}; {model})` |
| `x-app-version` | `2.1.44.348` |
| `Content-Type` | `application/json` |

### Three-Step OTP Flow

Token retrieval requires TOTP verification and completes in three POSTs to the same endpoint.

#### Step 1a — Initiate

```json
{
  "type": "track2",
  "cardid": "<cardId>",
  "quantity": 3
}
```

Expected response: `{ "code": 99 }` (OTP required)

If `"code": 0` is returned directly the payload contains tokens immediately (skip to parsing).

#### Step 1b — Request OTP Channel

```json
{
  "type": "track2",
  "cardid": "<cardId>",
  "quantity": 3,
  "channel": "token"
}
```

Expected response: `{ "code": 22 }` (OTP generated on BML side; TOTP is used locally)

#### Step 1c — Submit TOTP

```json
{
  "type": "track2",
  "cardid": "<cardId>",
  "quantity": 3,
  "channel": "token",
  "otp": "<TOTP>"
}
```

Expected response: `{ "code": 0, "payload": [...] }`

The OTP is a standard TOTP (RFC 6238, SHA-1, 30-second window, 6 digits) derived from the stored BML authenticator seed.

### Token Response

```json
{
  "code": 0,
  "payload": [
    {
      "token": "4761360000000000",
      "expiry": "2512",
      "app_code": "A0000000031010",
      "service_code": "000",
      "data": "0960919802623742",
      "valid_until": "2025-12-01 12:00:00.000"
    }
  ]
}
```

### Token Fields

| Field | Description |
|---|---|
| `token` | PAN-equivalent single-use token (used as Track 2 primary account number) |
| `expiry` | Expiry in `YYMM` format (e.g. `"2512"` = December 2025) |
| `app_code` | AID (Application Identifier) hex string — identifies the card network |
| `service_code` | 3-digit service code for Track 2 |
| `data` | Discretionary data appended to Track 2 |
| `valid_until` | Server-side expiry timestamp for the token |

### AID to Card Network Mapping

| AID prefix | Network |
|---|---|
| `A0000000031010` | Visa |
| `A0000000041010` | Mastercard |
| `A000000025...` | Amex |
| (other) | BML |

---

## Step 2 — HCE APDU Exchange

Once a token is set, Android's NFC subsystem routes contactless commands to the app's `HostApduService`. The flow follows the EMV mag-stripe contactless profile.

### APDU Exchange Flow

```
POS Terminal                              Android HCE
     |                                        |
     |  SELECT PPSE (INS=A4)                  |
     |--------------------------------------->|
     |  FCI Template (6F) + 9000              |
     |<---------------------------------------|
     |                                        |
     |  SELECT AID (INS=A4)                   |
     |--------------------------------------->|
     |  FCI Template (6F) + 9000              |
     |<---------------------------------------|
     |                                        |
     |  GET PROCESSING OPTIONS (INS=A8)       |
     |--------------------------------------->|
     |  Response Message Template (80) + 9000 |
     |<---------------------------------------|
     |                                        |
     |  READ RECORD (INS=B2)                  |
     |--------------------------------------->|
     |  Record Template (70) + 9000           |
     |<---------------------------------------|
```

### APDU Command Bytes

| INS | Hex | Command |
|---|---|---|
| `SELECT` | `0xA4` | Select PPSE or AID |
| `GET PROCESSING OPTIONS` | `0xA8` | Request AIP + AFL |
| `READ RECORD` | `0xB2` | Read Track 2 data |

### SELECT PPSE Response

PPSE AID: `2PAY.SYS.DDF01` = `325041592E5359532E4444463031`

```
6F <len>
  84 <len> 325041592E5359532E4444463031    ← DF Name (PPSE)
  A5 <len>
    BF0C <len>
      61 <len>
        4F <len> <AID>                     ← ADF Name
        87 01 01                           ← Application Priority Indicator
9000
```

### SELECT AID Response

```
6F <len>
  84 <len> <AID>                           ← Dedicated File Name
  A5 <len>
    50 <len> <label-ascii-as-hex>          ← Application Label (e.g. "VISA")
    9F38 02 9F6602                         ← PDOL: TTQ (2 bytes)
9000
```

The application label is derived from the AID prefix (see mapping table above).

### GET PROCESSING OPTIONS Response

```
80 06 0080 08010100
9000
```

| Field | Value | Meaning |
|---|---|---|
| Tag `80` | — | Response Message Template 1 |
| AIP | `0080` | Mag-stripe mode |
| AFL | `08010100` | SFI=1, records 1–1, 0 offline auth records |

### READ RECORD Response

```
70 <len>
  57 <len> <track2-data>                   ← Track 2 Equivalent Data
9000
```

Track 2 format:
```
{token} D {expiry} {serviceCode} {data} [F]
```

The trailing `F` nibble is appended when the total length is odd (standard Track 2 padding).

Example from a real token:
```
4761360000000000 D 2512 000 0960919802623742
→ 4761360000000000D2512000096091980262374 2F  (padded)
```

### Status Words

| SW | Meaning |
|---|---|
| `9000` | Success |
| `6F00` | Generic / unknown error |
| `6D00` | Instruction not supported |

---

## TLV Encoding

All APDU responses use BER-TLV encoding. Tags are 1 or 2 bytes (hex string). Length follows DER short/long form:

| Length range | Encoding |
|---|---|
| 0–127 bytes | `LL` (1 byte) |
| 128–255 bytes | `81 LL` (2 bytes) |
| 256–65535 bytes | `82 HH LL` (3 bytes) |

---

## Prerequisites

- Valid `access_token` from [OAuth Token Exchange](03-oauth-token.md)
- TOTP seed enrolled via BML app (same seed used for login 2FA)
- `cardId` from the dashboard — see [Dashboard](04-dashboard.md)

---

&nbsp;

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