# "Hello, Multiply!": Write Your First Program for the RISC Zero zkVM

In this guide, we'll be writing a program that demonstrates a number is composite and we know its factors. If you'd like to start with an under-the-hood explanation of what this program will do, we've written one here. To jump ahead and see the finished product, look at the factors example in our examples repository. If you're ready to code, let's dive in!

## Step 1: Clone the template repository​

To get started, clone the Rust starter template. (An explanation of the template code is also available.) If you intend to publish your project, you should instead follow the directions for creating a repository from a template on GitHub.

## Step 2: Modify file names (and their references)​

### Name the new project​

Let's give our "Hello, Multiply!" project a name. Rename the folder host to factors.

Update the corresponding workspace member entry in the top-level Cargo.toml file:

[workspace]members = [    "methods",    "factors"]

### Give the guest program a name​

This project will call a program that executes on the guest zkVM. It's currently named methods/guest/src/bin/method_name.rs. We want to name it something that represents what the guest program does -- let's call it multiply.rs.

In order to access this guest code from the host driver program, the host program factors/src/main.rs includes two guest methods:

use methods::{METHOD_NAME_ELF, METHOD_NAME_ID};

Both of these must be changed to reflect the new guest program name:

use methods::{MULTIPLY_ELF, MULTIPLY_ID};

(As an aside, if you add more than one callable guest program to your next RISC Zero zkVM project, you'll need to include these ELF and ID references once for each guest file.)

While we're at it, let's change the rest of the references in factors/src/main.rs. Don't worry about why these lines are included yet; for now, we're just being diligent not to leave dead references behind. Here are what the other two lines with METHOD_NAME_ELF and METHOD_NAME_ID should look like after updating:

    let mut prover = Prover::new(MULTIPLY_ELF, MULTIPLY_ID)        .expect("Prover should be constructed from valid method source code and corresponding image ID");...    receipt.verify(MULTIPLY_ID)        .expect("Code you have proven should successfully verify; did you specify the correct image ID?");

### Intermission: Build and run the project!​

In the main project folder, build and run the project using cargo run --release. Nothing exciting will happen yet, but it should give you an error if you made one of the above changes incorrectly. Use this command any time you'd like to check your progress.

## Step 3 (Host): Share two values with the guest​

In this step, we'll be continuing to modify factors/src/main.rs. Let's start by picking some aesthetically pleasing primes:

fn main() {    let a: u64 = 17;    let b: u64 = 23;

Currently, our host driver program creates and runs a prover. The prover.run() command will cause our guest program to execute:

    let mut prover = Prover::new(MULTIPLY_ELF, MULTIPLY_ID)        .expect("Prover should be constructed from valid method source code and corresponding image ID");    let receipt = prover.run()        .expect("Code should be provable unless it had an error or overflowed the maximum cycle count");

We'd like the host to make the values of a and b available to the guest prior to execution. Because the prover is responsible for managing guest-readable memory, we need to share them after the prover is created. To accomplish this, let's send our two values to the guest between the lines listed above:

   let mut prover = Prover::new(MULTIPLY_ELF, MULTIPLY_ID)       .expect("Prover should be constructed from valid method source code and corresponding image ID");   prover.add_input_u32_slice(&to_vec(&a).unwrap());   prover.add_input_u32_slice(&to_vec(&b).unwrap());   let receipt = prover.run()       .expect("Code should be provable unless it had an error or overflowed the maximum cycle count");

Make sure to uncomment the use risc0_zkvm::serde::{from_slice, to_vec}; line at the top of the file so that we can access the helper functions in the lines we just added.

## Step 4 (Guest): Multiply two values and commit their result​

Now it's time to start writing guest code. Open the main guest program file methods/guest/src/bin/multiply.rs. In its final form, we'll tell the guest to read the values of a and b from the host and multiply them together. We'll then publicly commit their product to the receipt portion of the journal.

Here is the complete guest program. We'll break this down step by step below:

pub fn main() {    // Load the first number from the host    let a: u64 = env::read();    // Load the second number from the host    let b: u64 = env::read();    // Verify that neither of them are 1 (i.e. nontrivial factors)    if a == 1 || b == 1 {        panic!("Trivial factors")    }    // Compute the product while being careful with integer overflow    let product = a.checked_mul(b).expect("Integer overflow");    env::commit(&product);}

### Load values from the host​

First, add use risc0_zkvm::guest::env; at the top of the file (outside of the function body) to bring env into scope.

Then, we use env::read() to load both numbers:

    let a: u64 = env::read();    let b: u64 = env::read();

### Confirm that factors are non-trivial​

Next, we'll add a line that panics if either chosen value is 1. This will leave us with a guest program that only completes if the product of a and b is genuinely composite.

    // Verify that neither of them are 1 (i.e. nontrivial factors)    if a == 1 || b == 1 {        panic!("Trivial factors")    }

Now we can compute their product and commit it. Once committed to the journal, anyone with the receipt can read this value. As a final step, we'll have the host read and print the receipt's journal contents. In a real-world scenario, we'd want to hand the receipt to someone else, but reading it ourselves is a nice way to check our "Hello, Multiply!" project.

## Step 5 (Host): Generate a receipt and read its journal contents​

For this step, we return to the main file for the host driver program at factors/src/main.rs, which currently ends with receipt generation after the prover runs:

    let receipt = prover.run().unwrap()        .expect("Code should be provable unless it had an error or overflowed the maximum cycle count");

Now that we have a value to read from the receipt, let's extract the journal's contents. Below the line above, add the following lines.

    // Extract journal of receipt (i.e. output c, where c = a * b)    let c: u64 = from_slice(&receipt.journal).unwrap();    // Print an assertion    println!("Hello, world! I know the factors of {}, and I can prove it!", c);

If your program printed the "Hello, world!" assertion and receipt verification was a success, congratulations! If not, we hope that troubleshooting will get you familiar with the system, and we'd love to chat with you on Discord. If you're ready to start building more complex projects, we recommend taking a look at the other examples in our Getting Started resources for more project ideas that use zero-knowledge proofs.