Pruning public keys

For convenience, the generate_keys function creates several keys in the returned PublicKey object.

Why you might want to prune PublicKey

Some of these keys can be fairly large, the size of which is determined by scheme parameters. However, they may or may not be needed in your application:

• Galois keys (galois_key) are needed to run FHE programs that perform batching rotations or row swapping.
• Relinearization keys (relin_key) are needed to run FHE programs that multiply ciphertexts.

How to prune PublicKey

Each compiled FHE program contains a list the keys it needs at runtime in fhe_program.metadata.required_keys. You can then remove unneeded keys. For example:

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

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

fn main() {
   let app = Compiler::new()
       .fhe_program(noop)
       .compile()
       .unwrap();

   let runtime = FheRuntime::new(app.params()).unwrap();
let (public_key, private_key) = runtime.generate_keys().unwrap();

// Shadow and overwrite the public_key, removing the galois_key and relin_key
let public_key = PublicKey {
    galois_key: None, // only do this if not using batching
    relin_key: None, // only do this if your FHE program never multiplies ciphertexts
    ..public_key
};
}