operations.py

import numpy as np

from utils import polyadd, polymul


def add_plain(ct, pt, q, t, poly_mod):
    """Add a ciphertext and a plaintext.
    Args:
        ct: ciphertext.
        pt: integer to add.
        q: ciphertext modulus.
        t: plaintext modulus.
        poly_mod: polynomial modulus.
    Returns:
        Tuple representing a ciphertext.
    """
    size = len(poly_mod) - 1
    # encode the integer into a plaintext polynomial
    m = np.array([pt] + [0] * (size - 1), dtype=np.int64) % t
    delta = q // t
    scaled_m = delta * m  % q
    new_ct0 = polyadd(ct[0], scaled_m, q, poly_mod)
    return (new_ct0, ct[1])


def mul_plain(ct, pt, q, t, poly_mod):
    """Multiply a ciphertext and a plaintext.
    Args:
        ct: ciphertext.
        pt: integer to multiply.
        q: ciphertext modulus.
        t: plaintext modulus.
        poly_mod: polynomial modulus.
    Returns:
        Tuple representing a ciphertext.
    """
    size = len(poly_mod) - 1
    # encode the integer into a plaintext polynomial
    m = np.array([pt] + [0] * (size - 1), dtype=np.int64) % t
    new_c0 = polymul(ct[0], m, q, poly_mod)
    new_c1 = polymul(ct[1], m, q, poly_mod)
    return (new_c0, new_c1)