def decode(self, plain):
"""Decodes a plaintext polynomial and returns the integer.
Mathematically this amounts to evaluating the input polynomial at x=2.
Args:
plain: The plaintext to be decoded.
Returns:
An integer value.
"""
result = 0
bit_index = get_significant_count(plain.data)
while bit_index > 0:
bit_index -= 1
coeff = plain.data[bit_index]
# Left shift result.
next_result = result << 1
if (next_result < 0) != (result < 0):
# Check for overflow.
raise OverflowError("output out of range")
if coeff >= self.plain_modulus:
# Coefficient is bigger than plaintext modulus
raise ValueError("plain does not represent a valid plaintext polynomial")
coeff_is_negative = coeff >= self.coeff_neg_threshold
pos_value = coeff
if coeff_is_negative:
pos_value = self.plain_modulus - pos_value
coeff_value = pos_value
if coeff_is_negative:
coeff_value = -coeff_value
next_result_was_negative = next_result < 0
next_result += coeff_value
next_result_is_negative = next_result < 0
if (
next_result_was_negative == coeff_is_negative
and next_result_was_negative != next_result_is_negative
):
# Accumulation and coefficient had same signs, but accumulator changed signs
# after addition, so must be overflow.
raise OverflowError("output out of range")
result = next_result
return result
def decrypt(self, encrypted):
"""Decrypts the encrypted ciphertext objects.
Args:
encrypted: A ciphertext object which has to be decrypted.
Returns:
A PlainText object containing the decrypted result.
"""
# Calculate [c0 + c1 * sk + c2 * sk^2 ...]_q
temp_product_modq = self._mul_ct_sk(copy.deepcopy(encrypted.data))
# Divide scaling variant using BEHZ FullRNS techniques
result = self._context.rns_tool.decrypt_scale_and_round(temp_product_modq)
# removing leading zeroes in plaintext representation.
plain_coeff_count = get_significant_count(result)
return PlainText(result[:plain_coeff_count])
def test_get_significant_count(ptr, result):
assert result == get_significant_count(ptr)
