Struct KeySwitchKeyParams

pub struct KeySwitchKeyParams {
    pub special_modulus_factor_count: usize,
    pub digits_without_special: Box<RNSGadgetVectorDigitIndices>,
}

Parameters for a key-switching key.

More concretely, (hybrid) key-switching is parameterized by two parameters:

• the set of digits used for the gadget decomposition
• the special modulus

For an explanation, see the doc of the corresponding members.

Example

A standard choice is to use a small value for special_modulus_factor_count (e.g. 1 or 2), and then set digit_count = rns_base_len / special_modulus_factor_count.

let rns_base_len = 10; // length of the RNS base of the ciphertext ring
let special_modulus_factor_count = 2;
let digit_count = rns_base_len / special_modulus_factor_count;
let params = KeySwitchKeyParams::default(digit_count, special_modulus_factor_count, rns_base_len);

Fields

special_modulus_factor_count: usize

the special modulus is the product of the last special_modulus_factor_count rns factors of the ciphertext ring. Key-switching can only be performed on ciphertexts over an rns base that does not include the special modulus (i.e. must possibly be modulus-switched down before performing key-switching, possibly introducing noise). On the other hand, a high special modulus reduces the (additive) noise growth caused by key-switching. Note that the special modulus can be 1 (if special_modulus_factor_count = 0), in which case only digit-based key-switching will be performed.

digits_without_special: Box<RNSGadgetVectorDigitIndices>

The groups of RNS factors that are used as digits during the gadget decomposition (see also RNSGadgetVectorDigitIndices). The (additive) noise growth during key-switching depends on the largest digit (i.e. the maximal size of the product of rns factors belonging to a single digit), however a larger number of digits will make key-switching keys larger, and key-switching more expensive. Note that noise growth becomes minimal if the largest digit is smaller or equal the special modulus.

The special modulus RNS factors should not be included in this list.

Implementations

impl KeySwitchKeyParams

pub fn default( digit_count: usize, special_modulus_factor_count: usize, rns_base_len: usize, ) -> Self

Creates new KeySwitchKeyParams, making a balanced selection of digits. This should be a reasonable choice in basically all situations.

pub fn expected_rns_base_len(&self) -> usize

Returns the length of the RNS base that key-switching keys with these parameters are defined over.

Trait Implementations

impl Clone for KeySwitchKeyParams

fn clone(&self) -> KeySwitchKeyParams

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.