python-fido2/fido2/utils.py

# Copyright (c) 2013 Yubico AB
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"""Various utility functions.

This module contains various functions used throughout the rest of the project.
"""

from base64 import urlsafe_b64decode, urlsafe_b64encode
from threading import Timer, Event
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hmac, hashes
from binascii import b2a_hex
from numbers import Number
import six

__all__ = [
    'Timeout',
    'websafe_encode',
    'websafe_decode',
    'sha256',
    'hmac_sha256',
    'bytes2int',
    'int2bytes'
]


def sha256(data):
    """Produces a SHA256 hash of the input.

    :param data: The input data to hash.
    :return: The resulting hash.
    """
    h = hashes.Hash(hashes.SHA256(), default_backend())
    h.update(data)
    return h.finalize()


def hmac_sha256(key, data):
    """Performs an HMAC-SHA256 operation on the given data, using the given key.

    :param key: The key to use.
    :param data: The input data to hash.
    :return: The resulting hash.
    """
    h = hmac.HMAC(key, hashes.SHA256(), default_backend())
    h.update(data)
    return h.finalize()


def bytes2int(value):
    """Parses an arbitrarily sized integer from a byte string.

    :param value: A byte string encoding a big endian unsigned integer.
    :return: The parsed int.
    """
    return int(b2a_hex(value), 16)


def int2bytes(value, minlen=-1):
    """Encodes an int as a byte string.

    :param value: The integer value to encode.
    :param minlen: An optional minimum length for the resulting byte string.
    :return: The value encoded as a big endian byte string.
    """
    ba = []
    while value > 0xff:
        ba.append(0xff & value)
        value >>= 8
    ba.append(value)
    ba.extend([0] * (minlen - len(ba)))
    return bytes(bytearray(reversed(ba)))


def websafe_decode(data):
    """Decodes a websafe-base64 encoded string (bytes or str).
    See: "Base 64 Encoding with URL and Filename Safe Alphabet" from Section 5
    in RFC4648 without padding.

    :param data: The input to decode.
    :return: The decoded bytes.
    """
    if isinstance(data, six.text_type):
        data = data.encode('ascii')
    data += b'=' * (-len(data) % 4)
    return urlsafe_b64decode(data)


def websafe_encode(data):
    """Encodes a byte string into websafe-base64 encoding.

    :param data: The input to encode.
    :return: The encoded string.
    """
    return urlsafe_b64encode(data).replace(b'=', b'').decode('ascii')


class Timeout(object):
    """Utility class for adding a timeout to an event.

    :param time_or_event: A number, in seconds, or a threading.Event object.
    :ivar event: The Event associated with the Timeout.
    :ivar timer: The Timer associated with the Timeout, if any.
    """

    def __init__(self, time_or_event):

        if isinstance(time_or_event, Number):
            self.event = Event()
            self.timer = Timer(time_or_event,
                               self.event.set)
        else:
            self.event = time_or_event
            self.timer = None

    def __enter__(self):
        if self.timer:
            self.timer.start()
        return self.event

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.timer:
            self.timer.cancel()
            self.timer.join()