# Copyright 2015 Google Inc. All Rights Reserved.
#
# 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.
"""Utilities for encryption functions on Windows."""
# TODO(user) Create unittests for this module (which only run on Windows).
import base64
import ctypes
from ctypes import windll
from ctypes import wintypes

advapi32 = windll.advapi32

RSA_KEY_LENGTH = 2048

# Windows Constants -- Most of these values are from wincrypt.h
# Copies of wincrypt.h can be found online (Google Search) although
# Microsoft doesn't seem to publish it online anywhere. This is a stable
# API, however, so these shouldn't need to ever change (unless we change
# the implementation).
PROV_RSA_FULL = 1
MS_ENHANCED_PROV_W = u'Microsoft Enhanced Cryptographic Provider v1.0'
MS_ENHANCED_PROV = MS_ENHANCED_PROV_W
CRYPT_VERIFYCONTEXT = 0xF0000000
ALG_CLASS_KEY_EXCHANGE = 40960
ALG_TYPE_RSA = 1024
ALG_SID_RSA_ANY = 0
CALG_RSA_KEYX = (ALG_CLASS_KEY_EXCHANGE|ALG_TYPE_RSA|ALG_SID_RSA_ANY)
CRYPT_OAEP = 0x00000040
PUBLICKEYBLOB = 6

# GetLastError
kernel32 = windll.kernel32
kernel32.GetLastError.restype = wintypes.DWORD
kernel32.GetLastError.argtypes = []
get_last_error = kernel32.GetLastError


class WindowsException(Exception):

  def __init__(self, extra_data=None):
    windows_error_code = get_last_error()
    message = 'Windows Error: 0x%0x' % windows_error_code
    if extra_data:
      message += '\nExtra Info: %s' % extra_data
    super(WindowsException, self).__init__(message)


class WinCrypt(object):
  """Base Class for Windows encryption functions."""

  def __init__(self):
    self.crypt_acquire_context = advapi32.CryptAcquireContextW
    self.crypt_acquire_context.restype = wintypes.BOOL
    self.crypt_acquire_context.argtypes = [wintypes.HANDLE,
                                           wintypes.LPCWSTR,
                                           wintypes.LPCWSTR,
                                           wintypes.DWORD,
                                           wintypes.DWORD]

    self.crypt_release_context = advapi32.CryptReleaseContext
    self.crypt_release_context.restype = wintypes.BOOL
    self.crypt_release_context.argtypes = [wintypes.HANDLE,
                                           wintypes.DWORD]

    self.crypt_gen_key = advapi32.CryptGenKey
    self.crypt_gen_key.restype = wintypes.BOOL
    self.crypt_gen_key.argtypes = [wintypes.HANDLE,
                                   wintypes.DWORD,
                                   wintypes.DWORD,
                                   ctypes.POINTER(wintypes.HANDLE)]

    self.crypt_destroy_key = advapi32.CryptDestroyKey
    self.crypt_destroy_key.restype = wintypes.BOOL
    self.crypt_destroy_key.argtypes = [wintypes.HANDLE]

    self.crypt_decrypt = advapi32.CryptDecrypt
    self.crypt_decrypt.restype = wintypes.BOOL
    self.crypt_decrypt.argtypes = [wintypes.HANDLE,
                                   wintypes.HANDLE,
                                   wintypes.BOOL,
                                   wintypes.DWORD,
                                   ctypes.POINTER(wintypes.BYTE),
                                   ctypes.POINTER(wintypes.DWORD)]

    self.crypt_export_key = advapi32.CryptExportKey
    self.crypt_export_key.restype = wintypes.BOOL
    self.crypt_export_key.argtypes = [wintypes.HANDLE,
                                      wintypes.HANDLE,
                                      wintypes.DWORD,
                                      wintypes.DWORD,
                                      ctypes.POINTER(wintypes.BYTE),
                                      ctypes.POINTER(wintypes.DWORD)]

  def GetKeyPair(self):
    """Returns a handle for an RSA key pair."""
    # Key and Provider handles
    crypt_provider_handle = wintypes.HANDLE()
    key_container_name = None
    provider = MS_ENHANCED_PROV
    provider_type = PROV_RSA_FULL
    acquire_context_flags = CRYPT_VERIFYCONTEXT
    algorithm_id = CALG_RSA_KEYX
    key_handle = wintypes.HANDLE()

    # Get Crypt Context
    if not self.crypt_acquire_context(ctypes.byref(crypt_provider_handle),
                                      key_container_name,
                                      provider,
                                      provider_type,
                                      acquire_context_flags):
      raise WindowsException

    # Generate RSA key pair
    # The bit length is passed in the upper 16 bits of the flag argument.
    gen_key_flags = RSA_KEY_LENGTH << 16
    if not self.crypt_gen_key(crypt_provider_handle,
                              algorithm_id,
                              gen_key_flags,
                              key_handle):
      raise WindowsException()

    return key_handle

  def GetPublicKey(self, key):
    """Returns the public key for the referenced private key handle."""
    user_crypto_key = None  # Not used for public key
    key_type = PUBLICKEYBLOB
    export_key_flags = 0

    # Get length of public key
    key_data = None
    key_len = ctypes.c_ulong()
    self.crypt_export_key(key,
                          user_crypto_key,
                          key_type,
                          export_key_flags,
                          key_data,
                          ctypes.byref(key_len))

    # Now, export public key
    byte_array_type = ctypes.c_byte * key_len.value
    key_data = byte_array_type()
    if not self.crypt_export_key(key,
                                 user_crypto_key,
                                 key_type,
                                 export_key_flags,
                                 key_data,
                                 ctypes.byref(key_len)):
      raise WindowsException()

    public_key = (ctypes.c_char * key_len.value).from_buffer(key_data)
    return public_key

  def DecryptMessage(self, key, enc_message, destroy_key=True):
    """Returns a decrypted message from the given encrypted message and key.

    Can optionally destroy the key (used only on Windows).

    Args:
      key: An openssl key pair (private key) or a Windows key handle.
      enc_message: A base64 encoded encrypted message.
      destroy_key: If True, the key pointed to by the key handle is destroyed.

    Returns:
      Decrypted version of enc_message

    Raises:
      WindowsException: If message fails to decrypt
    """
    decoded_message = base64.b64decode(enc_message)
    little_endian_message = decoded_message[::-1]
    data_len = ctypes.c_ulong(len(little_endian_message))
    data_buf = (ctypes.c_byte * data_len.value).from_buffer_copy(
        little_endian_message)

    hash_object = None
    final = True
    decrypt_flags = CRYPT_OAEP

    if not self.crypt_decrypt(key,
                              hash_object,
                              final,
                              decrypt_flags,
                              data_buf,
                              ctypes.byref(data_len)):
      raise WindowsException(data_len)

    message = (ctypes.c_char * data_len.value).from_buffer(data_buf)

    if destroy_key:
      self.crypt_destroy_key(key)

    return message.value

  def GetModulusExponentFromPublicKey(self, public_key):
    """Returns a base64 encoded modulus and exponent from the public key."""
    # Windows Public key format is:
    #   12 bytes of header info
    #   4  bytes saying how many bits the key is
    #   4  bytes of exponent
    #   256 bytes of modulus (for a 2048 bit key, can vary)
    # All values are in little-endian format, so need to be reversed.
    modulus = public_key[20:][::-1]
    exponent = public_key[16:20][::-1]
    b64_mod = base64.b64encode(modulus)
    b64_exp = base64.b64encode(exponent)

    return (b64_mod, b64_exp)

