Noise

Every FHE operation introduces noise into ciphertexts.¹ If too much noise accrues, then the message becomes garbled and cannot be decrypted. The total amount of allowed noise before this problem occurs is referred to as the noise budget.

Fortunately, our compiler handles this noise issue for you. It uses a value known as the noise_margin that influences how conservative it needs to be in parameter selection. So long as noise_budget >= noise_margin, we're good to go. Sunscreen's default noise margin prevents garbled ciphertexts coming from a single FHE program.

Why you might want to change the default noise margin

There are a few reasons you may wish to change the noise margin:

• If you wish to use an output of one FHE program as an input to another FHE program, you need to allow for additional noise growth in subsequent programs. See our calculator for a complete example of this scenario.
• Decreasing the noise margin can result in improved performance if your application can tolerate a higher rate of faults.

How to change the noise margin

To change the noise margin, simply call .additional_noise_budget() when compiling your program. For example:

use sunscreen::{
    fhe_program,
    types::{bfv::Signed, Cipher},
    Compiler, Runtime, PublicKey
};

#[fhe_program(scheme = "bfv")]
fn my_program() {
}

fn main() {
    let app = Compiler::new()
        .fhe_program(my_program)
        .additional_noise_budget(60)
        .compile()
        .unwrap();
}