circuitpython_hmac.py

# The MIT License (MIT)
#
# Copyright (c) 2020 Jim Bennett
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
"""
`circuitpython_hmac`
================================================================================

HMAC (Keyed-Hashing for Message Authentication) Python module.
Implements the HMAC algorithm as described by RFC 2104.


* Author(s): Jim Bennett

Implementation Notes
--------------------

**Software and Dependencies:**

* Adafruit CircuitPython firmware for the supported boards:
  https://github.com/adafruit/circuitpython/releases

"""

# imports

__version__ = "0.0.0-auto.0"
__repo__ = "https://github.com/jimbobbennett/CircuitPython_HMAC.git"

import adafruit_hashlib as _hashlib


def __translate(key, translation):
    return bytes(translation[x] for x in key)


TRANS_5C = bytes((x ^ 0x5C) for x in range(256))
TRANS_36 = bytes((x ^ 0x36) for x in range(256))


class HMAC:
    """RFC 2104 HMAC class.  Also complies with RFC 4231.

    This supports the API for Cryptographic Hash Functions (PEP 247).
    """

    blocksize = 64  # 512-bit HMAC; can be changed in subclasses.

    def __init__(self, key, msg=None, digestmod=None):
        """Create a new HMAC object.

        key:       key for the keyed hash object.
        msg:       Initial input for the hash, if provided.
        digestmod: A module supporting PEP 247.  *OR*
                   A hashlib constructor returning a new hash object. *OR*
                   A hash name suitable for hashlib.new().
                   Defaults to hashlib.md5.
                   Implicit default to hashlib.md5 is deprecated and will be
                   removed in Python 3.6.

        Note: key and msg must be a bytes or bytearray objects.
        """

        if not isinstance(key, (bytes, bytearray)):
            raise TypeError(
                "key: expected bytes or bytearray, but got %r" % type(key).__name__
            )

        if digestmod is None:
            digestmod = _hashlib.sha256

        if callable(digestmod):
            self.digest_cons = digestmod
        elif isinstance(digestmod, str):
            self.digest_cons = lambda d=b"": _hashlib.new(digestmod, d)
        else:
            self.digest_cons = lambda d=b"": digestmod.new(d)

        self.outer = self.digest_cons()
        self.inner = self.digest_cons()
        self.digest_size = self.inner.digest_size

        if hasattr(self.inner, "block_size"):
            blocksize = self.inner.block_size
            if blocksize < 16:
                blocksize = self.blocksize
        else:
            blocksize = self.blocksize

        # self.blocksize is the default blocksize. self.block_size is
        # effective block size as well as the public API attribute.
        self.block_size = blocksize

        if len(key) > blocksize:
            key = self.digest_cons(key).digest()

        key = key + bytes(blocksize - len(key))
        self.outer.update(__translate(key, TRANS_5C))
        self.inner.update(__translate(key, TRANS_36))
        if msg is not None:
            self.update(msg)

    @property
    def name(self):
        """Return the name of this object
        """
        return "hmac-" + self.inner.name

    def update(self, msg):
        """Update this hashing object with the string msg.
        """
        self.inner.update(msg)

    def copy(self):
        """Return a separate copy of this hashing object.

        An update to this copy won't affect the original object.
        """
        # Call __new__ directly to avoid the expensive __init__.
        other = self.__class__.__new__(self.__class__)
        other.digest_cons = self.digest_cons
        other.digest_size = self.digest_size
        other.inner = self.inner.copy()
        other.outer = self.outer.copy()
        return other

    def _current(self):
        """Return a hash object for the current state.

        To be used only internally with digest() and hexdigest().
        """
        hmac = self.outer.copy()
        hmac.update(self.inner.digest())
        return hmac

    def digest(self):
        """Return the hash value of this hashing object.

        This returns a string containing 8-bit data.  The object is
        not altered in any way by this function; you can continue
        updating the object after calling this function.
        """
        hmac = self._current()
        return hmac.digest()

    def hexdigest(self):
        """Like digest(), but returns a string of hexadecimal digits instead.
        """
        hmac = self._current()
        return hmac.hexdigest()


def new(key, msg=None, digestmod=None):
    """Create a new hashing object and return it.

    key: The starting key for the hash.
    msg: if available, will immediately be hashed into the object's starting
    state.

    You can now feed arbitrary strings into the object using its update()
    method, and can ask for the hash value at any time by calling its digest()
    method.
    """
    return HMAC(key, msg, digestmod)