🔐 Google Pay Payload Decryption & Formatter

This project provides tools for working with Google Pay encrypted payloads:

• Decrypt payloads using Google's ECv2 protocol in Java.
• Format payloads into Omise-style JSON using a Go helper.
• Extract the base64 public key derived from your Google Pay private key.

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📁 Project Structure

.
├── cmd/
│   └── google_pay_public_key_helper/
│       └── main.go                # Go helper to derive the public key from GOOGLE_PAY_PRIVATE_KEY
├── internal/
│   └── googlepay/
│       ├── keyutil.go             # Shared EC key conversion utilities
│       └── keyutil_test.go        # Tests for public-key derivation
├── src/
│   └── main/
│       └── java/
│           └── DecryptApp.java    # Java decryption entrypoint
├── google_pay_json_helper.go      # Go helper to format Google Pay JSON payload
├── go.mod                         # Go module definition file
├── go.sum                         # Go dependency checksum file
├── pom.xml                        # Maven build configuration file
├── Makefile                       # Makefile to simplify build and run commands
├── .env                           # Environment variables for configuration
└── README.md                      # Project overview and usage instructions

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🔧 Setup

1. Install dependencies

# Java / Maven
brew install maven   # or use apt/choco depending on your OS

# Go (if not installed)
brew install go

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2. Environment Variables

Create a .env file in the project root with the following:

GOOGLE_PAY_ENV=test # or prod
GOOGLE_PAY_MERCHANT_ID=gateway:your_gateway_name
GOOGLE_PAY_PRIVATE_KEY=YOUR_PKCS8_BASE64_KEY
GOOGLE_PAY_PAYLOAD={"signature":"...","protocolVersion":"ECv2",...}

GOOGLE_PAY_PRIVATE_KEY is expected to be a base64-encoded PKCS#8 EC private key. The Go helpers derive the matching public key from this value automatically in Google Pay's uncompressed point format.

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▶️ Usage

Decrypt payload (Java)

make decrypt

Format payload as Omise JSON

make payload

Output:

{
  "public_key": "<base64 uncompressed EC public key derived from GOOGLE_PAY_PRIVATE_KEY>",
  "data": "{...original payload JSON...}"
}

Extract the derived public key

make public-key

Output:

MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE...

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🛠 Development Notes

• Decryption uses Tink (Google's crypto library).
• The Go helpers expect GOOGLE_PAY_PRIVATE_KEY to be a base64-encoded PKCS#8 EC private key.
• The Go formatter pulls the payload from the GOOGLE_PAY_PAYLOAD env variable and derives public_key from the private key in uncompressed point format.