Key-switching

The key-switching allows to convert a ciphertext encrypted with a secret key, to a ciphertext encrypted with another secret key. This requires a special Key-switching key ($ksk$). Key-switching is used primarily to change the dimension security parameter of a ciphertext, and as part of the bootstrapping procedure.

Operation	$E_{k_1}[m] \rightarrow E_{k_2} [m]$
Type	Leveled
Side effects	Modifies encryption key Potentially modifies security parameters

Generating a key-switching key

A Key-switching key $ksk$ is an encryption of the bits of the original secret key (secret_key_before) under the destination secret key (secret_key_after). To generate a key-switching key, you can use the structure LWEKSK with the following parameters:

• secret_key_before: the key under which the ciphertext is encrypted

• secret_key_after: the key under which we want the ciphertext to be encrypted after the key-switch

• base_log: number of bits of the decomposition base

• level: precision of the decomposition

Both the base_log and the level parameters are used to managed the noise of the ciphertext and the message precision on the plaintext. They also impact directly the computation cost and the key-switching key size. The level is usually chosen small to obtain a good tradeoff between all parameters.

Here is a code example:

/// file: main.rs
use concrete::*;

fn main() {
    // generate two secret keys
    let secret_key_before = LWESecretKey::new(&LWE128_1024);
    let secret_key_after = LWESecretKey::new(&LWE128_630);

    // generate the key switching key
    let ksk = LWEKSK::new(&secret_key_before, &secret_key_after, 8, 3);

    println!("Well done :-)");
}

Performing a key-switch

Performing a key-switch is done by calling the method keyswitch , which takes a key-switching key as input, and outputs a ciphertext under the new key. Note that in this example, the ciphertext also changes the dimension security parameter, from 1024 to 630.

/// file: main.rs
use concrete::*;

fn main() -> Result<(), CryptoAPIError> {
    // encoder
    let encoder = Encoder::new(100., 110., 5, 0)?;

    // generate two secret keys
    let secret_key_before = LWESecretKey::new(&LWE128_1024);
    let secret_key_after = LWESecretKey::new(&LWE128_630);

    // generate the key switching key
    let ksk = crypto_api::LWEKSK::new(&secret_key_before, &secret_key_after, 2, 6);

    // a list of messages that we encrypt
    let messages: Vec<f64> = vec![106.276, 104.3, 100.12, 101.1, 107.78];
    let ciphertext_before = VectorLWE::encode_encrypt(&secret_key_before, &messages, &encoder)?;

    // key switch
    let ciphertext_after = ciphertext_before.keyswitch(&ksk);

    // decryption
    let decryptions: Vec<f64> = ciphertext_before.decrypt_decode(&secret_key_before)?;

    Ok(())
}