This is a beta build and is currently under active development. Please be advised of the following:

• This code currently is not audited by an external security auditor, use it at your own risk
• The code has not been subjected to thorough review by engineers at the Electric Coin Company
• We are actively changing the codebase and adding features where/when needed

🔒 Security Warnings

• The Zcash Android Wallet SDK is experimental and a work in progress. Use it at your own risk.
• Developers using this SDK must familiarize themselves with the current threat model, especially the known weaknesses described there.

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Zcash Android SDK

This lightweight SDK connects Android to Zcash. It welds together Rust and Kotlin in a minimal way, allowing third-party Android apps to send and receive shielded transactions easily, securely and privately.

Contents

• Requirements
• Structure
• Overview
  • Components
• Quickstart
• Examples
• Compiling Sources
• Versioning
• Examples

Requirements

This SDK is designed to work with lightwalletd

Structure

From an app developer's perspective, this SDK will encapsulate the most complex aspects of using Zcash, freeing the developer to focus on UI and UX, rather than scanning blockchains and building commitment trees! Internally, the SDK is structured as follows:

Thankfully, the only thing an app developer has to be concerned with is the following:

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Overview

At a high level, this SDK simply helps native Android codebases connect to Zcash's Rust crypto libraries without needing to know Rust or be a Cryptographer. Think of it as welding. The SDK takes separate things and tightly bonds them together such that each can remain as idiomatic as possible. Its goal is to make it easy for an app to incorporate shielded transactions while remaining a good citizen on mobile devices.

Given all the moving parts, making things easy requires coordination. The Synchronizer provides that layer of abstraction so that the primary steps to make use of this SDK are simply:

1. Start the Synchronizer
2. Subscribe to wallet data

The Synchronizer takes care of

- Connecting to the light wallet server
- Downloading the latest compact blocks in a privacy-sensitive way
- Scanning and trial decrypting those blocks for shielded transactions related to the wallet
- Processing those related transactions into useful data for the UI
- Sending payments to a full node through [lightwalletd](https://github.com/zcash/lightwalletd)
- Monitoring sent payments for status updates

To accomplish this, these responsibilities of the SDK are divided into separate components. Each component is coordinated by the Synchronizer, which is the thread that ties it all together.

Components

Component	Summary
LightWalletService	Service used for requesting compact blocks
CompactBlockStore	Stores compact blocks that have been downloaded from the LightWalletService
CompactBlockProcessor	Validates and scans the compact blocks in the CompactBlockStore for transaction details
OutboundTransactionManager	Creates, Submits and manages transactions for spending funds
Initializer	Responsible for all setup that must happen before synchronization can begin. Loads the rust library and helps initialize databases.
DerivationTool, BirthdayTool	Utilities for deriving keys, addresses and loading wallet checkpoints, called "birthdays."
RustBackend	Wraps and simplifies the rust library and exposes its functionality to the Kotlin SDK

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Quickstart

Add flavors for testnet v mainnet. Since productFlavors cannot start with the word 'test' we recommend:

flavorDimensions 'network'
productFlavors {
    // would rather name them "testnet" and "mainnet" but product flavor names cannot start with the word "test"
    zcashtestnet {
        dimension 'network'
        matchingFallbacks = ['zcashtestnet', 'debug']
    }
    zcashmainnet {
        dimension 'network'
        matchingFallbacks = ['zcashmainnet', 'release']
    }
}

Add the SDK dependency:

implementation 'cash.z.ecc.android:zcash-android-sdk:1.3.0-beta20'

Start the Synchronizer

synchronizer.start(this)

Get the wallet's address

synchronizer.getAddress()

// or alternatively

DerivationTool.deriveShieldedAddress(viewingKey)

Send funds to another address

synchronizer.sendToAddress(spendingKey, zatoshi, address, memo)

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Examples

Full working examples can be found in the demo app, covering all major functionality of the SDK. Each demo strives to be self-contained so that a developer can understand everything required for it to work. Testnet builds of the demo app will soon be available to download as github releases.

Demos

Menu Item	Related Code	Description
Get Private Key	GetPrivateKeyFragment.kt	Given a seed, display its viewing key and spending key
Get Address	GetAddressFragment.kt	Given a seed, display its z-addr
Get Balance	GetBalanceFragment.kt	Display the balance
Get Latest Height	GetLatestHeightFragment.kt	Given a lightwalletd server, retrieve the latest block height
Get Block	GetBlockFragment.kt	Given a lightwalletd server, retrieve a compact block
Get Block Range	GetBlockRangeFragment.kt	Given a lightwalletd server, retrieve a range of compact blocks
List Transactions	ListTransactionsFragment.kt	Given a seed, list all related shielded transactions
Send	SendFragment.kt	Send and monitor a transaction, the most complex demo

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Compiling Sources

⚠️ Compilation is not required unless you plan to submit a patch or fork the code. Instead, it is recommended to simply add the SDK dependencies via Gradle.

In the event that you do want to compile the SDK from sources, follow these steps:

1. Install rust
   1. If you're a macOS user with homebrew already installed
      1. brew install rustup
      2. rustup-init
2. Then, add the android targets via:

rustup target add armv7-linux-androideabi aarch64-linux-android i686-linux-android x86_64-linux-android

3. Clone this repo
4. Install Android Studio and open this project via /your/path/to/zcash-android-wallet-sdk/build.gradle.kts
5. Open Android Studio’s SDK manager

6. Then, install NDK 20.0.5594570 22.1.7171670 (pro tip: build.gradle -> ndkVersion defines the actual required version. Use that because this README may get out-of-date. Also note that the "Show Package Details" box in the bottom right corner must be checked in order to install specific versions of the NDK.)

8. To build from the command line, run:

./gradlew assemble

// or to install in MavenLocal

./gradlew publishToMavenLocal

This creates a build of the SDK under build/outputs/aar/ that can be used to preview functionality. For more detailed examples, see the demo app.

Note that merely using the SDK does not require installing Rust or Cargo--that is only required when compiling from source. Also note that the Mozilla Rust Gradle plugin puts outputs under sdk-lib/targets, which has implications for manually testing build script changes. This is discussed further under docs/tests/Build.md.

The repo also contains a small demo application, to verify integration with the SDK. Note that by default, the demo application is configured to retrieve dependencies from artifact hosting and therefore does not rely on the local compilation of the SDK. This can be changed by publishing to maven local as described above, as local maven publications will take precedence over hosted publications in the demo app.

1. Create an emulator if you don’t already have one (recommended target: API 31)
2. Import the subdirectory samples/demo-app as a separate Android Studio project
3. Select your desired build variant. Currently, we recommend zcashmainnetDebug as the testnet variants are slower to sync to current height due to a lack of checkpoints.

4. Sync project with Gradle files, and build from the IDE. Alternatively, to build from the command line run:

./gradlew clean assembleZcashmainnetDebug

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Versioning

This project follows semantic versioning with pre-release versions. An example of a valid version number is 1.0.4-alpha11 denoting the 11th iteration of the alpha pre-release of version 1.0.4. Stable releases, such as 1.0.4 will not contain any pre-release identifiers. Pre-releases include the following, in order of stability: alpha, beta, rc. Version codes offer a numeric representation of the build name that always increases. The first six significant digits represent the major, minor and patch number (two digits each) and the last 3 significant digits represent the pre-release identifier. The first digit of the identifier signals the build type. Lastly, each new build has a higher version code than all previous builds. The following table breaks this down:

Build Types

Type	Purpose	Stability	Audience	Identifier	Example Version
alpha	Sandbox. For developers to verify behavior and try features. Things seen here might never go to production. Most bugs here can be ignored.	Unstable: Expect bugs	Internal developers	0XX	1.2.3-alpha04 (10203004)
beta	Hand-off. For developers to present finished features. Bugs found here should be reported and immediately addressed, if they relate to recent changes.	Unstable: Report bugs	Internal stakeholders	2XX	1.2.3-beta04 (10203204)
release candidate	Hardening. Final testing for an app release that we believe is ready to go live. The focus here is regression testing to ensure that new changes have not introduced instability in areas that were previously working.	Stable: Hunt for bugs	External testers	4XX	1.2.3-rc04 (10203404)
production	Delivery. Deliver new features to end-users. Any bugs found here need to be prioritized. Some will require immediate attention but most can be worked into a future release.	Stable: Prioritize bugs	Public	8XX	1.2.3 (10203800)

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Examples

A primitive example to exercise the SDK exists in this repo, under Demo App.

There's also a more comprehensive Sample Wallet.

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Publishing

Publishing instructions for maintainers of this repository can be found in PUBLISHING.md

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Known Issues

1. ktlintFormat may fail on Java 16 or greater. As a workaround, the task is run under Java 11. This requires that JDK 11 be installed, even if a newer JDK is also installed. While this is configured to use the Java toolchain API, toolchain support for Java 11 does not work on Apple Silicon
2. During builds, a warning will be printed that says "Unable to detect AGP versions for included builds. All projects in the build should use the same AGP version." This can be safely ignored. The version under build-conventions is the same as the version used elsewhere in the application.
3. Android Studio will warn about the Gradle checksum. This is a known issue and can be safely ignored.