import base64
import secrets
import math
import Crypto.Util


# This commutative cipher is based on the SRA cryptographical system, which is just a modification of RSA where the
# modulus n is known, but both the encryption and decryption exponents are kept secret. As long as both keys use the
# same modulus, this cryptography system is commutative, i.e. Ea(Eb(x)) = Eb(Ea(x)) if encryption with key a is denoted
# as Ea() and encryption with key b is denoted as Eb.
class CommutativeCipher:
	def __init__(self, p, q):
		self.n = p*q
		carmichael_function = (p-1) * (q-1)

		# Make the exponent have almost as many bits as the modulus
		number_of_bits = int(math.ceil(math.log(self.n) / math.log(2)))
		self.e = Crypto.Util.number.getPrime(number_of_bits-10, randfunc=secrets.token_bytes)

		self.d = pow(self.e, -1, carmichael_function)

	def encode(self, message):
		message_was_base64 = False
		message_was_bytes = False

		if isinstance(message, str):
			message_bytes = base64.b64decode(message)
			message = message_bytes
			message_was_base64 = True

		try:
			message_int = int.from_bytes(message)
			message = message_int
			message_was_bytes = True
		except TypeError:
			# Assume message is already an integer
			pass

		if not isinstance(message, int):
			raise Exception(
				'The message to encrypt was not of the correct type (base64 string, bytes-like object, or integer'
			)

		if message >= self.n:
			raise Exception(
				'The message is equal to or larger than the modulus'
			)

		encrypted = pow(message, self.e, self.n)

		if message_was_bytes:
			# Find number of bits
			number_of_bits = int(math.ceil(math.log(encrypted) / math.log(2)))
			number_of_bytes = int(math.ceil(number_of_bits / 8))
			encrypted = encrypted.to_bytes(number_of_bytes)

		if message_was_base64:
			encrypted = base64.b64encode(encrypted)

		return encrypted

	def decode(self, cipher):
		cipher_was_base64 = False
		cipher_was_bytes = False

		if isinstance(cipher, str):
			cipher_was_base64 = True
			cipher = base64.b64decode(cipher)

		try:
			cipher_int = int.from_bytes(cipher)
			cipher = cipher_int
			cipher_was_bytes = True
		except TypeError:
			pass

		if not isinstance(cipher, int):
			raise Exception('The passed cipher was not a valid type (base64 string, bytes object or integer)')

		if cipher >= self.n:
			raise Exception('The passed cipher is equal to or larger than the modulus')

		decrypted = pow(cipher, self.d, self.n)
		if cipher_was_bytes:
			number_of_bits = int(math.ceil(math.log(decrypted)/math.log(2)))
			number_of_bytes = int(math.ceil(number_of_bits / 8))
			decrypted = decrypted.to_bytes(number_of_bytes)

		if cipher_was_base64:
			decrypted = base64.b64encode(decrypted)

		return decrypted