An Introduction to Leo: Crafting Privacy-Preserving Blockchain Applications

Table of Contents

1. Privacy vs. Transparency in Blockchain
2. Core Concepts of Leo Programming
3. Advanced Features and Techniques
4. Real-World Applications and Use Cases
5. Deploying Your First Leo Program
6. Next Steps

Privacy vs. Transparency in Blockchain

Traditional blockchains offer transparency but sacrifice privacy, exposing sensitive data and limiting adoption for many real-world applications leaving developers to face a significant challenge: how to build applications that maintain the integrity and transparency of blockchain while preserving user privacy.

The Leo Programming Language

Enter Leo. Leo is a statically-typed programming language specifically designed for writing zero-knowledge applications; it combines the expressiveness needed for complex computations with the ability to generate zero-knowledge proofs, making it a powerful tool for developers looking to build privacy-focused blockchain applications giving devs the ability to build applications that maintain the integrity and transparency of blockchain while preserving user privacy.

Zero-knowledge proofs allow one party (the prover) to prove to another party (the verifier) that a statement is true, without revealing any information beyond the validity of the statement itself. Think of a "ZK Proof" like proving you know the password to an account without actually saying the password. Imagine proving you have enough money in your bank account to buy a car, without revealing your exact balance or any transaction history. You demonstrate you meet the financial requirement without exposing any private financial details.

In the context of blockchain applications, this enables:

• Private transactions and computations
• Scalable off-chain computations with on-chain verification
• Enhanced privacy for users while maintaining the integrity of the blockchain

Leo, as a programming language, abstracts much of the complexity involved in creating zero-knowledge proofs, allowing developers to focus on building innovative applications.

How Leo Works: Bridging High-Level Code and Zero-Knowledge Circuits

• Intuitive Syntax: Leo's syntax is familiar to developers coming from languages like Rust or TypeScript, lowering the barrier to entry for zero-knowledge programming.

• Zero-Knowledge Abstraction: Leo abstracts the complexities of zero-knowledge proof systems, allowing developers to focus on application logic rather than cryptographic intricacies.

• Efficient Compilation: The Leo compiler transforms high-level code into optimized arithmetic circuits suitable for generating zero-knowledge proofs.

• Privacy by Design: Leo's type system and built-in primitives encourage developers to think in terms of privacy from the ground up.

2. Core Concepts of Leo Programming

Program Structure

A Leo program typically consists of the following components:

program token.aleo {
    // Record definitions
    // Mapping definitions
    // Transition functions
    // Helper functions
}

Records: Private Data Structures

Records in Leo represent private data that only the owner can decrypt and use:

record Token {
    owner: address,
    amount: u64,
}

Transitions: Entry Points for Interaction

Transitions are public functions that serve as entry points for interacting with your program:

transition transfer(sender: Token, receiver: address, amount: u64) -> (Token, Token) {
    assert(sender.amount >= amount);

    let remaining: Token = Token {
        owner: sender.owner,
        amount: sender.amount - amount,
    };

    let transferred: Token = Token {
        owner: receiver,
        amount: amount,
    };

    return (remaining, transferred);
}

Mappings: Public State Management

• Mappings in Leo are used to store public state:

mapping balances: address => u64;

Visibility and Privacy

Leo provides fine-grained control over data visibility:

• private: The default, visible only to the owner
• public: Visible on-chain
• constant: Immutable and publicly visible

3. Advanced Features and Techniques

• Leo provides built-in functions for common cryptographic operations:

• Efficient Leo programs minimize the number of constraints generated:

• For sensitive computations, use constant-time operations to prevent timing attacks:

4. Real-World Applications and Use Cases

Having ease of developing using zero knowledge proofs like this opens up a new paradigm to building new types of applications that you could not build before. A few interesting possible implementations of what a few different sectors affected by this tech could look like:

Anonymous Credentials

• A system for proving credential ownership without revealing identity:

// Define a record to represent a credential

record Credential {
    owner: address,        // The address of the credential owner
    credential_type: u8,   // The type of credential (e.g., 1 for driver's license, 2 for passport)
    expiration: u32,       // The expiration date of the credential
}

// This transition proves ownership of a valid credential without revealing the owner's identity
transition prove_credential(
    credential: Credential,  // The credential to be proven
    challenge: field         // A challenge provided by the verifier
) -> bool {
    // Ensure the credential hasn't expired
    assert(credential.expiration > block.height);

    // Create a proof by hashing the owner's address with the challenge
    // This proves ownership without revealing the address
    let proof: field = hash(credential.owner, challenge);

    // Return the proof
    return proof;
}

Privacy-Preserving Supply Chain

• Tracking products through a supply chain while maintaining privacy:

// Define a record to represent the status of a product in the supply chain
record ProductStatus {
    product_id: field,     // Unique identifier for the product
    current_owner: address, // Current owner of the product
    status: u8,            // Current status of the product (e.g., 1 for manufactured, 2 for shipped, 3 for delivered)
}

// This transition updates the status of a product in the supply chain
transition update_status(
    product: ProductStatus,  // The current status of the product
    new_status: u8,          // The new status to update to
    new_owner: address       // The new owner of the product
) -> ProductStatus {
    // Return a new ProductStatus record with updated information
    return ProductStatus {
        product_id: product.product_id,  // Keep the same product ID
        current_owner: new_owner,        // Update the owner
        status: new_status,              // Update the status
    };
}

5. Deploying Your First Leo Program

Project Setup

Prerequisites

Ensure you have Rust 1.76.0 or above installed before running Leo programs.

Installing Leo

1. Clone the Leo repository:

   git clone https://github.com/ProvableHQ/leo.git

1. Navigate to the Leo directory and build from source:

   cd leo
   cargo install --path .

1. Verify the installation by running the leo command. You should see output similar to:

Take some time to explore the various CLI commands and familiarize yourself with the available options.

Creating Your First Leo Project

1. Create a new Leo project:

   leo new hello
   cd hello

1. This command creates a new directory with the following structure:

   hello/
   ├── settings.json
   ├── .gitignore
   ├── .env
   ├── program.json
   └── src/
       └── main.leo

Understanding the Project Files

• .gitignore: Default Git ignore file for Leo projects
• .env: Contains environment variables (e.g., NETWORK, PRIVATE_KEY)
• program.json: The manifest file for the Leo project
• build/: Directory for compiled code
• src/main.leo: The main Leo source code file

Examining the Code

Open src/main.leo. You'll see the following code:

// The 'hello' program.
program hello.aleo {
    transition main(public a: u32, b: u32) -> u32 {
        let c: u32 = a + b;
        return c;
    }
}

This simple program:

• Defines a program named hello.aleo
• Creates a transition function named main
• Takes two u32 (32-bit unsigned integer) inputs, a and b
• Adds these inputs and returns the result

Running the Program

1. Execute the program:

   leo run main 1u32 2u32

1. You should see output similar to:

   Leo Compiled 'main.leo' into Aleo instructions
   ⛓  Constraints
    •  'hello.aleo/main' - 33 constraints (called 1 time)
   ➡️  Output
    • 3u32

   Leo ✅ Finished 'hello.aleo/main' (in "/hello/build")

Here's what it looks like in a VS Code terminal:

Congratulations! You've just run your first zero-knowledge program in the Leo language.

Additional Leo CLI Commands

• Clean the build directory:

  leo clean

• Update Leo to the latest version:

  leo update

• Execute the program and output a transaction object:

  leo execute main 1u32 2u32

1. Next Steps

Now that you've created and run your first Leo program, consider:

• Exploring building out more complex apps - there's some really unique stuff that can be built here
• Modifying the main function to perform different operations that are unique and out of the box
• Adding new functions to your program that have some application level logic

Remember, Leo is designed for creating privacy-preserving applications on the blockchain, so as you become more comfortable with the basics, start exploring how to leverage Leo's features for building secure and private decentralized applications.

Heres some additional resources as you continue to go down the rabbit hole:

1. The Aleo Workshop Repository
2. The Awesome Aleo Repository
3. The Leo Developer ToolKit
4. Create a Full Stack Aleo App with Leo, React, and Javascript
5. Aleo Developer Workshop Video Some great resources to dive deeper down the rabbit hole: