def _sub_cipher_cipher(self, ct1, ct2):
"""Subtract two ciphertexts.
Args:
ct1 (Ciphertext): First polynomial argument (Minuend).
ct2 (Ciphertext): Second polynomial argument (Subtrahend).
Returns:
A Ciphertext object with value equivalent to result of subtraction of two provided
arguments.
"""
ct1, ct2 = copy.deepcopy(ct1.data), copy.deepcopy(ct2.data)
result = ct2 if len(ct2) > len(ct1) else ct1
min_size, max_size = min(len(ct1), len(ct2)), max(len(ct1), len(ct2))
for i in range(min_size):
for j in range(len(self.coeff_modulus)):
result[i][j] = poly_sub_mod(ct1[i][j], ct2[i][j],
self.coeff_modulus[j],
self.poly_modulus)
for i in range(min_size + 1, max_size):
for j in range(len(self.coeff_modulus)):
result[i][j] = poly_negate_mod(result[i][j],
self.coeff_modulus[j])
return CipherText(result)
def encrypt_symmetric(context, secret_key):
"""Create encryption of zero values with a secret key which can be used in subsequent
processes to add a message into it.
Args:
context (Context): A valid context required for extracting the encryption parameters.
secret_key (SecretKey): A secret key generated with same encryption parameters.
Returns:
A ciphertext object containing encryption of zeroes by symmetric encryption procedure.
"""
coeff_modulus = context.param.coeff_modulus
coeff_mod_size = len(coeff_modulus)
# Sample uniformly at random
c1 = sample_poly_uniform(context.param)
# Sample e <-- chi
e = sample_poly_normal(context.param)
# calculate -(a*s + e) (mod q) and store in c0
c0 = [0] * coeff_mod_size
for i in range(coeff_mod_size):
c0[i] = poly_negate_mod(
poly_add_mod(
poly_mul_mod(c1[i], secret_key[i], coeff_modulus[i]), e[i], coeff_modulus[i]
),
coeff_modulus[i],
)
return CipherText([c0, c1])
def test_poly_negate_mod(op1, mod, result):
assert poly_negate_mod(op1, mod) == result