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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro/intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../intro/prelim.html"><strong aria-hidden="true">1.1.</strong> Prerequisites</a></li><li class="chapter-item expanded "><a href="../intro/why.html"><strong aria-hidden="true">1.2.</strong> Why is a compiler needed for FHE?</a></li><li class="chapter-item expanded "><a href="../intro/compiler.html"><strong aria-hidden="true">1.3.</strong> Sunscreen's compiler</a></li></ol></li><li class="chapter-item expanded "><a href="../getting_started/getting_started.html"><strong aria-hidden="true">2.</strong> Getting started</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../getting_started/installation.html"><strong aria-hidden="true">2.1.</strong> Installation</a></li><li class="chapter-item expanded "><a href="../getting_started/example.html" class="active"><strong aria-hidden="true">2.2.</strong> My first FHE program</a></li></ol></li><li class="chapter-item expanded "><a href="../fhe_programs/fhe_programs.html"><strong aria-hidden="true">3.</strong> What's in an FHE program?</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/types/types.html"><strong aria-hidden="true">3.1.</strong> Types</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/types/signed.html"><strong aria-hidden="true">3.1.1.</strong> Signed</a></li><li class="chapter-item expanded "><a href="../fhe_programs/types/fractional.html"><strong aria-hidden="true">3.1.2.</strong> Fractional</a></li><li class="chapter-item expanded "><a href="../fhe_programs/types/rational.html"><strong aria-hidden="true">3.1.3.</strong> Rational</a></li></ol></li><li class="chapter-item expanded "><a href="../fhe_programs/writing_an_fhe_program/writing_an_fhe_program.html"><strong aria-hidden="true">3.2.</strong> How to write an FHE program</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/factoring_fhe_programs.html"><strong aria-hidden="true">3.2.1.</strong> Factoring FHE programs</a></li><li class="chapter-item expanded "><a href="../fhe_programs/writing_an_fhe_program/limitations.html"><strong aria-hidden="true">3.2.2.</strong> Limitations</a></li></ol></li><li class="chapter-item expanded "><a href="../troubleshooting.html"><strong aria-hidden="true">3.3.</strong> Troubleshooting</a></li></ol></li><li class="chapter-item expanded "><a href="../fhe_programs/runtime/runtime.html"><strong aria-hidden="true">4.</strong> Runtime</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/runtime/key_generation.html"><strong aria-hidden="true">4.1.</strong> Key generation</a></li><li class="chapter-item expanded "><a href="../fhe_programs/runtime/encryption.html"><strong aria-hidden="true">4.2.</strong> Encryption</a></li><li class="chapter-item expanded "><a href="../fhe_programs/runtime/running_fhe_programs.html"><strong aria-hidden="true">4.3.</strong> Running FHE programs</a></li><li class="chapter-item expanded "><a href="../fhe_programs/runtime/decryption.html"><strong aria-hidden="true">4.4.</strong> Decryption</a></li><li class="chapter-item expanded "><a href="../fhe_programs/runtime/serialization.html"><strong aria-hidden="true">4.5.</strong> Serialization</a></li></ol></li><li class="chapter-item expanded "><a href="../fhe_programs/apps.html"><strong aria-hidden="true">5.</strong> Applications</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/example.html"><strong aria-hidden="true">5.1.</strong> AMMs & private token swaps</a></li><li class="chapter-item expanded "><a href="../fhe_programs/pir_intro.html"><strong aria-hidden="true">5.2.</strong> Private information retrieval</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../fhe_programs/pir_simple.html"><strong aria-hidden="true">5.2.1.</strong> Take 1</a></li><li class="chapter-item expanded "><a href="../fhe_programs/pir_matrix.html"><strong aria-hidden="true">5.2.2.</strong> Take 2</a></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">5.3.</strong> Genomics & private tests</div></li></ol></li><li class="chapter-item expanded "><a href="../advanced/faq.html"><strong aria-hidden="true">6.</strong> FAQ</a></li><li class="chapter-item expanded "><a href="../advanced/advanced.html"><strong aria-hidden="true">7.</strong> Advanced topics</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../advanced/bfv.html"><strong aria-hidden="true">7.1.</strong> Why the BFV scheme?</a></li><li class="chapter-item expanded "><a href="../advanced/good_fhe_programs.html"><strong aria-hidden="true">7.2.</strong> Writing even better FHE programs</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../advanced/plain_modulus/plain_modulus.html"><strong aria-hidden="true">7.2.1.</strong> Plaintext modulus</a></li><li class="chapter-item expanded "><a href="../advanced/noise_margin.html"><strong aria-hidden="true">7.2.2.</strong> Noise</a></li><li class="chapter-item expanded "><a href="../advanced/pruning_keys.html"><strong aria-hidden="true">7.2.3.</strong> Pruning public keys</a></li></ol></li><li class="chapter-item expanded "><a href="../advanced/wasm.html"><strong aria-hidden="true">7.3.</strong> WASM support</a></li><li class="chapter-item expanded "><a href="../advanced/carryless_arithmetic.html"><strong aria-hidden="true">7.4.</strong> Funky math: carryless arithmetic</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">7.5.</strong> Batching</div></li></ol></li></ol>
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<h1 class="menu-title">Sunscreen Documentation</h1>
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<main>
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<h1 id="my-first-fhe-program"><a class="header" href="#my-first-fhe-program">My first FHE program</a></h1>
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<p>Now that we have installed the Sunscreen crate as a dependency, let's get started writing our first private app using FHE! Writing our program will be a gradual process and we'll add more code as we progress through this section. </p>
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<p>In this example, we'll just multiply two encrypted integers.</p>
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<pre><pre class="playground"><code class="language-rust no_run edition2021">use sunscreen::{
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fhe_program,
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types::{bfv::Signed, Cipher},
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Compiler, Error, Runtime,
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};
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#[fhe_program(scheme = "bfv")]
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fn simple_multiply(a: Cipher<Signed>, b: Cipher<Signed>) -> Cipher<Signed> {
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a * b
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}
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fn main() {
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}
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</code></pre></pre>
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<p>Notice that the <code>simple_multiply</code> function is like any other function in Rust, except for the <code>#[fhe_program(...)]</code> attribute. This is where the magic happens— it declares your function as an FHE program that can be compiled. The <code>scheme</code> argument should always be <code>"bfv"</code>, though we may support additional FHE schemes in the future.</p>
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<p><code>simple_multiply</code>'s signature specifies that it takes in two <code>Cipher<Signed></code> values and returns one. <code>Cipher<T></code> indicates the contained type <code>T</code> is encrypted (i.e. a ciphertext) and <code>Signed</code> is Sunscreen's signed integer type; thus, <code>Cipher<Signed></code> indicates that we have an encrypted signed integer. The body of <code>simple_multiply</code> multiplies the ciphertexts <code>a</code> and <code>b</code> together. As with any function in Rust, omitting a <code>;</code> returns an expression's value from the current scope.</p>
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<p>Having specified our program, let's compile it.</p>
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<pre><pre class="playground"><code class="language-rust no_run edition2021">use sunscreen::{
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fhe_program,
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types::{bfv::Signed, Cipher},
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Compiler, Error, Runtime,
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};
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#[fhe_program(scheme = "bfv")]
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fn simple_multiply(a: Cipher<Signed>, b: Cipher<Signed>) -> Cipher<Signed> {
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a * b
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}
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fn main() -> Result<(), Error> {
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let fhe_program = Compiler::with_fhe_program(simple_multiply)
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.compile()?;
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Ok(())
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}
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</code></pre></pre>
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<p>The <code>Compiler::with_fhe_program(simple_multiply)</code> line creates a compiler and <code>.compile()</code>, tells it to compile our FHE program. Compilation performs optimization and fills in implementation details, including figuring out FHE scheme parameters and inserting special operations. </p>
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<p>What's the <code>?</code> after at the end of <code>.compile()</code>? For the uninitiated, the <a href="https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html"><code>?</code></a> operator propagates errors. Fallible expressions in Rust emit <a href="https://doc.rust-lang.org/std/result/enum.Result.html"><code>Results</code></a>, which can contain either a value or an error. Using <code>?</code> unwraps the value in a successful result or immediately returns the error from a failed one, letting the caller of the current function deal with it. We should see the former after compilation, as our program is well-formed.</p>
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<p>Next, we need a public and private key pair. In order to generate keys, we'll first construct a <code>Runtime</code> with the parameters we got from compilation. This allows us to encrypt/decrypt data and run FHE programs.</p>
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<pre><pre class="playground"><code class="language-rust no_run edition2021">use sunscreen::{
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fhe_program,
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types::{bfv::Signed, Cipher},
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Compiler, Error, Runtime,
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};
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#[fhe_program(scheme = "bfv")]
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fn simple_multiply(a: Cipher<Signed>, b: Cipher<Signed>) -> Cipher<Signed> {
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a * b
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}
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fn main() -> Result<(), Error> {
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let fhe_program = Compiler::with_fhe_program(simple_multiply)
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.compile()?;
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let runtime = Runtime::new(&fhe_program.metadata.params)?;
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let (public_key, private_key) = runtime.generate_keys()?;
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let a = runtime.encrypt(Signed::from(15), &public_key)?;
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let b = runtime.encrypt(Signed::from(5), &public_key)?;
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Ok(())
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}
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</code></pre></pre>
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<p>After constructing our runtime, we generate a public and private key pair by calling <code>runtime.generate_keys()</code>.</p>
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<p>Next, we call <code>Signed::from(15)</code> to make an unencrypted <code>Signed</code> integer equal to <code>15</code>. Rust doesn't allow implicit type conversion as some languages do, so we use the <code>From</code> trait to explicitly convert a Rust <code>i64</code> into a Sunscreen <code>Signed</code>.</p>
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<p>Once we have our plaintext value 15, we encrypt it by calling <code>runtime.encrypt(...)</code>, passing in the value and our public key. We repeat this process for <code>b</code> with the value <code>5</code>. Now that we have the two ciphertexts <code>a</code> and <code>b</code> to give to <code>simple_multiply</code>, we're ready to run our FHE program!</p>
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<pre><pre class="playground"><code class="language-rust no_run edition2021">use sunscreen::{
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fhe_program,
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types::{bfv::Signed, Cipher},
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Compiler, Error, Runtime,
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};
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#[fhe_program(scheme = "bfv")]
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fn simple_multiply(a: Cipher<Signed>, b: Cipher<Signed>) -> Cipher<Signed> {
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a * b
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}
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fn main() -> Result<(), Error> {
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let fhe_program = Compiler::with_fhe_program(simple_multiply)
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.compile()?;
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let runtime = Runtime::new(&fhe_program.metadata.params)?;
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let (public_key, private_key) = runtime.generate_keys()?;
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let a = runtime.encrypt(Signed::from(15), &public_key)?;
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let b = runtime.encrypt(Signed::from(5), &public_key)?;
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let results = runtime.run(&fhe_program, vec![a, b], &public_key)?;
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let c: Signed = runtime.decrypt(&results[0], &private_key)?;
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assert_eq!(c, 75.into());
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Ok(())
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}
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</code></pre></pre>
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<p>We call <code>runtime.run(...)</code> to execute our FHE program. For the first argument, we pass in our previously compiled program <code>fhe_program</code>. The second argument is always a <a href="https://doc.rust-lang.org/std/vec/struct.Vec.html">Vec</a> containing the arguments to the FHE program. In this case, we pass in the encrypted <code>a</code> and <code>b</code> values. You'll need to pass in the <code>public_key</code> as well.</p>
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<p>What would happen if we forgot to encrypt one of our values or gave an encrypted <code>Fractional</code> value where the program wanted an encrypted <code>Signed</code> value? Fortunately, the <code>run</code> method first performs some sanity checks to ensure the arguments match the call signature. If the types of the values we pass in don't match the signature, the <code>run</code> method returns an error <code>Result</code>. The <code>?</code> propagates this error, but our program exits because this is the <code>main()</code> method!</p>
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<p>Next, we call <code>runtime.decrypt(...)</code> with the first result and our private key. Programs <em>can</em> return more than one value; hence, <code>results</code> is a <code>Vec</code>. Since our FHE program only returns one value, we decrypt the value at index <code>0</code>. The left hand side of the assignment denotes the decrypted data is a <code>Signed</code> whereas <code>runtime.decrypt(...)</code> ensures this type matches the ciphertext's encrypted value before decryption. If we had assigned a different type to <code>c</code>, say <code>Fractional</code>, then decrypt would return an error.</p>
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<p>Finally, we verify the result equals 75 (i.e. 15 * 5) as expected. </p>
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