92 lines
2.7 KiB
Python
92 lines
2.7 KiB
Python
import base64
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import secrets
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import math
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import Crypto.Util
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# This commutative cipher is based on the SRA cryptographical system, which is just a modification of RSA where the
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# modulus n is known, but both the encryption and decryption exponents are kept secret. As long as both keys use the
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# same modulus, this cryptography system is commutative, i.e. Ea(Eb(x)) = Eb(Ea(x)) if encryption with key a is denoted
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# as Ea() and encryption with key b is denoted as Eb.
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class CommutativeCipher:
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def __init__(self, p, q):
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self.n = p*q
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carmichael_function = (p-1) * (q-1)
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# Make the exponent have almost as many bits as the modulus
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number_of_bits = int(math.ceil(math.log(self.n) / math.log(2)))
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self.e = Crypto.Util.number.getPrime(number_of_bits-10, randfunc=secrets.token_bytes)
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self.d = pow(self.e, -1, carmichael_function)
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def encode(self, message):
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message_was_base64 = False
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message_was_bytes = False
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if isinstance(message, str):
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message_bytes = base64.b64decode(message)
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message = message_bytes
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message_was_base64 = True
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try:
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message_int = int.from_bytes(message)
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message = message_int
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message_was_bytes = True
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except TypeError:
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# Assume message is already an integer
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pass
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if not isinstance(message, int):
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raise Exception(
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'The message to encrypt was not of the correct type (base64 string, bytes-like object, or integer'
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)
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if message >= self.n:
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raise Exception(
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'The message is equal to or larger than the modulus'
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)
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encrypted = pow(message, self.e, self.n)
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if message_was_bytes:
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# Find number of bits
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number_of_bits = int(math.ceil(math.log(encrypted) / math.log(2)))
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number_of_bytes = int(math.ceil(number_of_bits / 8))
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encrypted = encrypted.to_bytes(number_of_bytes)
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if message_was_base64:
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encrypted = base64.b64encode(encrypted)
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return encrypted
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def decode(self, cipher):
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cipher_was_base64 = False
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cipher_was_bytes = False
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if isinstance(cipher, str):
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cipher_was_base64 = True
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cipher = base64.b64decode(cipher)
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try:
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cipher_int = int.from_bytes(cipher)
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cipher = cipher_int
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cipher_was_bytes = True
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except TypeError:
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pass
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if not isinstance(cipher, int):
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raise Exception('The passed cipher was not a valid type (base64 string, bytes object or integer)')
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if cipher >= self.n:
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raise Exception('The passed cipher is equal to or larger than the modulus')
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decrypted = pow(cipher, self.d, self.n)
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if cipher_was_bytes:
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number_of_bits = int(math.ceil(math.log(decrypted)/math.log(2)))
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number_of_bytes = int(math.ceil(number_of_bits / 8))
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decrypted = decrypted.to_bytes(number_of_bytes)
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if cipher_was_base64:
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decrypted = base64.b64encode(decrypted)
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return decrypted |