Struct DiscreteLogTable

pub struct DiscreteLogTable<G: Group> { /* private fields */ }

Lookup table for discrete logarithms.

For Ciphertexts to be partially homomorphic, the encrypted values must be group scalars linearly mapped to group elements: x -> [x]G, where G is the group generator. After decryption it is necessary to map the decrypted group element back to a scalar (i.e., get its discrete logarithm with base G). Because of discrete logarithm assumption, this task is computationally infeasible in the general case; however, if the possible range of encrypted values is small, it is possible to “cheat” by precomputing mapping [x]G -> x for all allowed x ahead of time. This is exactly what DiscreteLogTable does.

Examples

let mut rng = thread_rng();
let receiver = Keypair::<Ristretto>::generate(&mut rng);
let ciphertexts = (0_u64..16)
    .map(|i| receiver.public().encrypt(i, &mut rng));
// Assume that we know that the plaintext is in range 0..16,
// e.g., via a zero-knowledge proof.
let lookup_table = DiscreteLogTable::new(0..16);
// Then, we can use the lookup table to decrypt values.
// A single table may be shared for multiple decryption operations
// (i.e., it may be constructed ahead of time).
for (i, enc) in ciphertexts.enumerate() {
    assert_eq!(
        receiver.secret().decrypt(enc, &lookup_table),
        Some(i as u64)
    );
}

Implementations

impl<G: Group> DiscreteLogTable<G>

pub fn new(values: impl IntoIterator<Item = u64>) -> Self

Creates a lookup table for the specified values.

pub fn get(&self, decrypted_element: &G::Element) -> Option<u64>

Gets the discrete log of decrypted_element, or None if it is not present among values stored in this table.

Trait Implementations

impl<G: Clone + Group> Clone for DiscreteLogTable<G>

fn clone(&self) -> DiscreteLogTable<G>

Returns a copy of the value.

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

Performs copy-assignment from source.

impl<G: Debug + Group> Debug for DiscreteLogTable<G>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.