#!/usr/bin/env python
# Copyright 2016 Google LLC
#
# 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.

"""Pure-Python RSA cryptography implementation.

Uses the ``rsa``, ``pyasn1`` and ``pyasn1_modules`` packages
to parse PEM files storing PKCS#1 or PKCS#8 keys as well as
certificates. There is no support for p12 files.
"""

from __future__ import absolute_import

from pyasn1.codec.der import decoder  # type: ignore
from pyasn1_modules import pem  # type: ignore
from pyasn1_modules.rfc2459 import Certificate  # type: ignore
from pyasn1_modules.rfc5208 import PrivateKeyInfo  # type: ignore
import rsa  # type: ignore
import six

from google.auth import _helpers
from google.auth import exceptions
from google.auth.crypt import base

_POW2 = (128, 64, 32, 16, 8, 4, 2, 1)
_CERTIFICATE_MARKER = b"-----BEGIN CERTIFICATE-----"
_PKCS1_MARKER = ("-----BEGIN RSA PRIVATE KEY-----", "-----END RSA PRIVATE KEY-----")
_PKCS8_MARKER = ("-----BEGIN PRIVATE KEY-----", "-----END PRIVATE KEY-----")
_PKCS8_SPEC = PrivateKeyInfo()


def _bit_list_to_bytes(bit_list):
    """Converts an iterable of 1s and 0s to bytes.

    Combines the list 8 at a time, treating each group of 8 bits
    as a single byte.

    Args:
        bit_list (Sequence): Sequence of 1s and 0s.

    Returns:
        bytes: The decoded bytes.
    """
    num_bits = len(bit_list)
    byte_vals = bytearray()
    for start in six.moves.xrange(0, num_bits, 8):
        curr_bits = bit_list[start : start + 8]
        char_val = sum(val * digit for val, digit in six.moves.zip(_POW2, curr_bits))
        byte_vals.append(char_val)
    return bytes(byte_vals)


class RSAVerifier(base.Verifier):
    """Verifies RSA cryptographic signatures using public keys.

    Args:
        public_key (rsa.key.PublicKey): The public key used to verify
            signatures.
    """

    def __init__(self, public_key):
        self._pubkey = public_key

    @_helpers.copy_docstring(base.Verifier)
    def verify(self, message, signature):
        message = _helpers.to_bytes(message)
        try:
            return rsa.pkcs1.verify(message, signature, self._pubkey)
        except (ValueError, rsa.pkcs1.VerificationError):
            return False

    @classmethod
    def from_string(cls, public_key):
        """Construct an Verifier instance from a public key or public
        certificate string.

        Args:
            public_key (Union[str, bytes]): The public key in PEM format or the
                x509 public key certificate.

        Returns:
            google.auth.crypt._python_rsa.RSAVerifier: The constructed verifier.

        Raises:
            ValueError: If the public_key can't be parsed.
        """
        public_key = _helpers.to_bytes(public_key)
        is_x509_cert = _CERTIFICATE_MARKER in public_key

        # If this is a certificate, extract the public key info.
        if is_x509_cert:
            der = rsa.pem.load_pem(public_key, "CERTIFICATE")
            asn1_cert, remaining = decoder.decode(der, asn1Spec=Certificate())
            if remaining != b"":
                raise exceptions.InvalidValue("Unused bytes", remaining)

            cert_info = asn1_cert["tbsCertificate"]["subjectPublicKeyInfo"]
            key_bytes = _bit_list_to_bytes(cert_info["subjectPublicKey"])
            pubkey = rsa.PublicKey.load_pkcs1(key_bytes, "DER")
        else:
            pubkey = rsa.PublicKey.load_pkcs1(public_key, "PEM")
        return cls(pubkey)


class RSASigner(base.Signer, base.FromServiceAccountMixin):
    """Signs messages with an RSA private key.

    Args:
        private_key (rsa.key.PrivateKey): The private key to sign with.
        key_id (str): Optional key ID used to identify this private key. This
            can be useful to associate the private key with its associated
            public key or certificate.
    """

    def __init__(self, private_key, key_id=None):
        self._key = private_key
        self._key_id = key_id

    @property  # type: ignore
    @_helpers.copy_docstring(base.Signer)
    def key_id(self):
        return self._key_id

    @_helpers.copy_docstring(base.Signer)
    def sign(self, message):
        message = _helpers.to_bytes(message)
        return rsa.pkcs1.sign(message, self._key, "SHA-256")

    @classmethod
    def from_string(cls, key, key_id=None):
        """Construct an Signer instance from a private key in PEM format.

        Args:
            key (str): Private key in PEM format.
            key_id (str): An optional key id used to identify the private key.

        Returns:
            google.auth.crypt.Signer: The constructed signer.

        Raises:
            ValueError: If the key cannot be parsed as PKCS#1 or PKCS#8 in
                PEM format.
        """
        key = _helpers.from_bytes(key)  # PEM expects str in Python 3
        marker_id, key_bytes = pem.readPemBlocksFromFile(
            six.StringIO(key), _PKCS1_MARKER, _PKCS8_MARKER
        )

        # Key is in pkcs1 format.
        if marker_id == 0:
            private_key = rsa.key.PrivateKey.load_pkcs1(key_bytes, format="DER")
        # Key is in pkcs8.
        elif marker_id == 1:
            key_info, remaining = decoder.decode(key_bytes, asn1Spec=_PKCS8_SPEC)
            if remaining != b"":
                raise exceptions.InvalidValue("Unused bytes", remaining)
            private_key_info = key_info.getComponentByName("privateKey")
            private_key = rsa.key.PrivateKey.load_pkcs1(
                private_key_info.asOctets(), format="DER"
            )
        else:
            raise exceptions.MalformedError("No key could be detected.")

        return cls(private_key, key_id=key_id)
