Plaintext modulus

FHE uses some funky math. At the lowest level, plaintexts are polynomials where the coefficient of each term is an integer modulo the plaintext modulus. The plaintext modulus parameter impacts correctness and performance — overflow occurs when the plaintext modulus is too small, but increasing it can negatively impact performance. Sunscreen balances these two considerations by setting a default plaintext modulus that prevents overflow in most applications while maintaining good performance.¹ However, you may at times wish to change it.

Why you might want to change the default plaintext modulus

A few reasons you would change the plain modulus include:

• If the default is too conservative, decreasing the plaintext modulus can improve performance.
• Very very rarely, the default can cause overflow in some FHE programs. Increasing the plaintext modulus solves this issue at the expense of performance.
• You wish to use batching, which requires very specific values.

When setting the plaintext modulus, you call compiler.plain_modulus_constraint() and pass a PlainModulusConstraint, which comes in two forms:

• Raw(x) sets the plaintext modulus to x.
• BatchingMinimum(x) chooses a value suitable for use with batching with at least x bits of precision. As noted in the name, this modulus should be used with batching.

How to change the plaintext modulus

You can manually set the PlainModulusConstraint when compiling your program like so:

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

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

fn main() {
    let app = Compiler::new()
        .fhe_program(my_program)
        .plain_modulus_constraint(PlainModulusConstraint::Raw(1_000_000))
        .compile()
        .unwrap();
}