core.py

#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
#
#  Licensed under the Apache License, Version 2.0 (the "License");
#  you may not use this file except in compliance with the License.
#  You may obtain a copy of the License at
#
#      https://www.apache.org/licenses/LICENSE-2.0
#
#  Unless required by applicable law or agreed to in writing, software
#  distributed under the License is distributed on an "AS IS" BASIS,
#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#  See the License for the specific language governing permissions and
#  limitations under the License.

"""Core mathematical operations.

This is the actual core RSA implementation, which is only defined
mathematically on integers.
"""


def assert_int(var: int, name: str) -> None:
    if isinstance(var, int):
        return

    raise TypeError('%s should be an integer, not %s' % (name, var.__class__))


def encrypt_int(message: int, ekey: int, n: int) -> int:
    """Encrypts a message using encryption key 'ekey', working modulo n"""

    assert_int(message, 'message')
    assert_int(ekey, 'ekey')
    assert_int(n, 'n')

    if message < 0:
        raise ValueError('Only non-negative numbers are supported')

    if message > n:
        raise OverflowError("The message %i is too long for n=%i" % (message, n))

    return pow(message, ekey, n)


def decrypt_int(cyphertext: int, dkey: int, n: int) -> int:
    """Decrypts a cypher text using the decryption key 'dkey', working modulo n"""

    assert_int(cyphertext, 'cyphertext')
    assert_int(dkey, 'dkey')
    assert_int(n, 'n')

    message = pow(cyphertext, dkey, n)
    return message