FOSS Digital Wallet

[Quemuel-Nassor]

Hello everyone,

First of all, thank you for all the FOSS tools! I really appreciate all the work you are doing.
I’d like to suggest the idea of a FOSS Digital Wallet as an open-source alternative to Google Wallet for payments.
I’m a .NET developer and I’m not very familiar with Kotlin, but I have experience with .NET MAUI and integrating payment gateway APIs.
If you find this idea interesting and there’s any way I could help with the project, please let me know. I’d be happy to contribute!

Open-Source Privacy-First NFC Wallet for Android

Contactless payments without Google Wallet or proprietary wallet lock-in

---

1. Summary

This ideia explores a privacy-first, open-source NFC payment wallet for Android, designed for Google-free environments (e.g., LineageOS).

Key idea:

Android already supports NFC card emulation. Payment networks already support tokenization. Banks already implement native wallets. The missing piece is an open, provider-agnostic wallet layer.

Proposed model

Open-source Wallet
   ↓
Provider Adapter Layer (banks / schemes)
   ↓
Issuer + Tokenization (MDES / VTS / etc.)
   ↓
Secure Payment Environment (SE / vTEE / SDK)
   ↓
NFC Terminal

Privacy model

• No mandatory cloud backend
• No centralized card database
• No transaction syncing service
• Local-first wallet state only

---

2. Motivation

2.1 Google-free Android (LineageOS)

Current stack:

Android → Google Play Services → Google Wallet → Payment Network

Target stack:

Android → Open Wallet → Payment Providers → NFC → Terminal

Goal: remove Google as a required intermediary for NFC payments.

---

2.2 Privacy requirement

The wallet must avoid storing or syncing:

• PANs (card numbers)
• payment tokens
• transaction history
• user profiles
• device identifiers

Only local encrypted metadata is allowed.

External communication is limited to:

• card provisioning
• authentication
• token lifecycle (suspend / resume / update)

---

3. Nubank as reference case

3.1 Key observation

Nubank already implements a native Android contactless wallet experience:

• Tap-to-pay flow inside the app
• automatic virtual card creation
• tokenization before NFC usage
• can be set as default Android wallet

Source:
https://blog.nubank.com.br/como-pagar-por-aproximacao-com-o-celular/

3.2 Business account limitation

For business accounts, Nubank currently exposes only:

• Google Pay
• Apple Pay
• Samsung Pay

No public API for third-party wallet provisioning is available.

---

4. NuPay API (what it is / is not)

NuPay for Business API provides:

• payment sessions
• authentication flows (OAuth / CIBA)
• checkout redirection into Nubank app
• payment status and refunds

Docs:
https://docs.nupaybusiness.com.br/checkout/docs/openapi/index.html

Important limitation

No evidence of:

• card tokenization API
• NFC wallet provisioning API
• MDES / Visa token provisioning endpoints

Conclusion:

NuPay is a payment orchestration API, not a wallet provisioning API.

---

5. Mastercard ecosystem

5.1 MDES (tokenization layer)

Mastercard Digital Enablement Service (MDES):

• handles card tokenization
• manages token lifecycle
• acts as issuer ↔ wallet bridge

Docs:
https://developer.mastercard.com/apis
https://developer.mastercard.com/product/mdes/

---

5.2 Mastercard Wallet Services (MWS)

Introduced as a wallet SDK + Secure Element applet:

• Android & iOS support
• contactless payment enablement
• reduces need for custom EMV integration

Docs:
https://www.mastercard.com/br/en/news-and-trends/stories/2026/digital-wallet-innovation.html
https://developer.mastercard.com/product/mastercard-wallet-services/

Key distinction:

• MDES = tokenization layer
• MWS = wallet execution layer (SDK + Secure Element integration)

---

5.3 Security validation

MWS SDK v1.0.0 is EMVCo certified:

https://www.emvco.com/wp-content/uploads/loa/EMVCo_SECN0142_N_11_2025.pdf

Implication:

Payment cryptography is handled by certified components, not the application itself.

---

6. Android NFC capabilities

Android supports:

Host-based Card Emulation (HCE)

NFC Terminal → Android OS → App (APDU handling)

Docs:
https://developer.android.com/develop/connectivity/nfc/hce

Secure Element model

NFC Terminal → NFC Controller → Secure Element

In this model, the app does not directly handle transaction logic.

---

7. Security principle

HCE should NOT be used to implement payment cryptography.

Preferred model:

Wallet App → Payment SDK → Secure Element / vTEE → NFC

Avoid:

Wallet App → HCE → Cryptographic payment logic

---

8. Proposed architecture

┌──────────────────────────────┐
│ Open-source Wallet           │
│ - UI                        │
│ - Local encrypted state     │
│ - Wallet policies           │
└──────────────┬───────────────┘
               │
┌──────────────▼───────────────┐
│ Payment Abstraction Layer    │
│ - PaymentCredential          │
│ - Provider interface         │
└───────┬───────────┬──────────┘
        │           │
   Mastercard     Visa      Banks
   (MDES/MWS)     (VTS)     APIs
        │           │
        └──────┬────┘
               ▼
     Secure Payment Layer
               ▼
              NFC

---

9. Provider abstraction

Core interface:

interface PaymentProvider {
    val id: ProviderId
    val schemes: Set<PaymentScheme>

    suspend fun discover(): List<PaymentCredential>
    suspend fun provision(request: ProvisioningRequest): ProvisioningResult
    suspend fun suspend(credential: PaymentCredential)
    suspend fun resume(credential: PaymentCredential)
    suspend fun remove(credential: PaymentCredential)
}

Goal:

• no direct dependency on banks or schemes
• pluggable providers (Mastercard, Visa, issuer-specific)

---

10. Privacy model

Local-only data

• wallet configuration
• UI preferences
• display metadata
• provider settings

Never stored by wallet core

• card numbers
• payment tokens
• cryptographic material
• transaction history (if any exists, handled by provider layer)

---

11. References

• Android HCE
  https://developer.android.com/develop/connectivity/nfc/hce

• Nubank NFC payments
  https://blog.nubank.com.br/como-pagar-por-aproximacao-com-o-celular/

• NuPay API
  https://docs.nupaybusiness.com.br/checkout/docs/openapi/index.html

• Mastercard APIs
  https://developer.mastercard.com/apis
  https://developer.mastercard.com/product/mdes/
  https://developer.mastercard.com/product/mastercard-wallet-services/

• Mastercard Wallet Services
  https://www.mastercard.com/br/en/news-and-trends/stories/2026/digital-wallet-innovation.html

• EMVCo certificate (MWS SDK)
  https://www.emvco.com/wp-content/uploads/loa/EMVCo_SECN0142_N_11_2025.pdf

[naveensingh]

Thanks for the idea, but a digital wallet app is not planned as of now.