# Copyright 2019 Cypress Semiconductor Corporation
# SPDX-License-Identifier: Apache-2.0
#
# 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
#
#     http://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.

from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives.kdf.hkdf import HKDF

from cryptography.hazmat.primitives import serialization

import codecs
import os

class ECDH_KDF(object):

    def __init__(self, private_key, public_key, key_length, salt, info): 
        
        salt_length = 16

        self.backend = default_backend()
        self.host_key_pair = serialization.load_pem_private_key(self.read_key_bytes(private_key), password=None, backend=self.backend)
        
        #Deserialize public key: extract from private or directly from pem file 
        try:
            self.device_public_key = serialization.load_pem_private_key(self.read_key_bytes(public_key), password=None, backend=self.backend).public_key()
        except:
            self.device_public_key = serialization.load_pem_public_key(self.read_key_bytes(public_key), backend=self.backend)

        self.key_length = key_length
        self.iv_length = 16

        if salt is not None:
            self.salt = ECDH_KDF.get_bytes(salt)
        else:
            self.salt = os.urandom(salt_length)
        self.info = ECDH_KDF.get_bytes(info)
        
        self.derived_key = self.derive_key()
        self.aes_key = self.derived_key[:self.key_length]
        self.iv = self.derived_key[self.key_length:]

    @staticmethod
    def get_bytes(inputdata):
        if type(inputdata) is str:
            return str.encode(inputdata,'utf-8')
        elif type(inputdata) is bytes:
            return inputdata
        else:
            raise Exception("Unknown input data type...")
        
    def read_key_bytes(self, key_file):
        with open(key_file, "rb") as key_file:
            return key_file.read()

    def derive_key(self):  

        shared_key = self.host_key_pair.exchange(ec.ECDH(), self.device_public_key)
        derived_key = HKDF(algorithm=hashes.SHA256(),                   # Perform key derivation.
                   length=self.key_length + self.iv_length,
                   salt=self.salt,
                   info=self.info,
                   backend=self.backend).derive(shared_key)

        return derived_key

def main():
    pass

if __name__ == "__main__":
    main()