moviewyrm/bookwyrm/signatures.py

""" signs activitypub activities """
import hashlib
from urllib.parse import urlparse
import datetime
from base64 import b64encode, b64decode

from Crypto import Random
from Crypto.PublicKey import RSA
from Crypto.Signature import pkcs1_15  # pylint: disable=no-name-in-module
from Crypto.Hash import SHA256

MAX_SIGNATURE_AGE = 300


def create_key_pair():
    """ a new public/private key pair, used for creating new users """
    random_generator = Random.new().read
    key = RSA.generate(1024, random_generator)
    private_key = key.export_key().decode("utf8")
    public_key = key.publickey().export_key().decode("utf8")

    return private_key, public_key


def make_signature(sender, destination, date, digest):
    """ uses a private key to sign an outgoing message """
    inbox_parts = urlparse(destination)
    signature_headers = [
        "(request-target): post %s" % inbox_parts.path,
        "host: %s" % inbox_parts.netloc,
        "date: %s" % date,
        "digest: %s" % digest,
    ]
    message_to_sign = "\n".join(signature_headers)
    signer = pkcs1_15.new(RSA.import_key(sender.key_pair.private_key))
    signed_message = signer.sign(SHA256.new(message_to_sign.encode("utf8")))
    signature = {
        "keyId": "%s#main-key" % sender.remote_id,
        "algorithm": "rsa-sha256",
        "headers": "(request-target) host date digest",
        "signature": b64encode(signed_message).decode("utf8"),
    }
    return ",".join('%s="%s"' % (k, v) for (k, v) in signature.items())


def make_digest(data):
    """ creates a message digest for signing """
    return "SHA-256=" + b64encode(hashlib.sha256(data.encode("utf-8")).digest()).decode(
        "utf-8"
    )


def verify_digest(request):
    """ checks if a digest is syntactically valid and matches the message """
    algorithm, digest = request.headers["digest"].split("=", 1)
    if algorithm == "SHA-256":
        hash_function = hashlib.sha256
    elif algorithm == "SHA-512":
        hash_function = hashlib.sha512
    else:
        raise ValueError("Unsupported hash function: {}".format(algorithm))

    expected = hash_function(request.body).digest()
    if b64decode(digest) != expected:
        raise ValueError("Invalid HTTP Digest header")


class Signature:
    """ read and validate incoming signatures """

    def __init__(self, key_id, headers, signature):
        self.key_id = key_id
        self.headers = headers
        self.signature = signature

    @classmethod
    def parse(cls, request):
        """ extract and parse a signature from an http request """
        signature_dict = {}
        for pair in request.headers["Signature"].split(","):
            k, v = pair.split("=", 1)
            v = v.replace('"', "")
            signature_dict[k] = v

        try:
            key_id = signature_dict["keyId"]
            headers = signature_dict["headers"]
            signature = b64decode(signature_dict["signature"])
        except KeyError:
            raise ValueError("Invalid auth header")

        return cls(key_id, headers, signature)

    def verify(self, public_key, request):
        """ verify rsa signature """
        if http_date_age(request.headers["date"]) > MAX_SIGNATURE_AGE:
            raise ValueError("Request too old: %s" % (request.headers["date"],))
        public_key = RSA.import_key(public_key)

        comparison_string = []
        for signed_header_name in self.headers.split(" "):
            if signed_header_name == "(request-target)":
                comparison_string.append("(request-target): post %s" % request.path)
            else:
                if signed_header_name == "digest":
                    verify_digest(request)
                comparison_string.append(
                    "%s: %s" % (signed_header_name, request.headers[signed_header_name])
                )
        comparison_string = "\n".join(comparison_string)

        signer = pkcs1_15.new(public_key)
        digest = SHA256.new()
        digest.update(comparison_string.encode())

        # raises a ValueError if it fails
        signer.verify(digest, self.signature)


def http_date_age(datestr):
    """ age of a signature in seconds """
    parsed = datetime.datetime.strptime(datestr, "%a, %d %b %Y %H:%M:%S GMT")
    delta = datetime.datetime.utcnow() - parsed
    return delta.total_seconds()