259 lines
9.6 KiB
Python
259 lines
9.6 KiB
Python
# SPDX-License-Identifier: MIT
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#
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# Copyright (c) 2017 PySecretHandshake contributors (see AUTHORS for more details)
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#
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# Permission is hereby granted, free of charge, to any person obtaining a copy
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# of this software and associated documentation files (the "Software"), to deal
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# in the Software without restriction, including without limitation the rights
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# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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# copies of the Software, and to permit persons to whom the Software is
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# furnished to do so, subject to the following conditions:
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#
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# The above copyright notice and this permission notice shall be included in all
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# copies or substantial portions of the Software.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# SOFTWARE.
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"""Cryptography functions"""
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from base64 import b64decode
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import hashlib
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import hmac
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from typing import Optional
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from nacl.bindings import crypto_box_afternm, crypto_box_open_afternm, crypto_scalarmult
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from nacl.exceptions import CryptoError
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from nacl.public import PrivateKey
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from nacl.signing import SigningKey, VerifyKey
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from .boxstream import BoxStreamKeys
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APPLICATION_KEY = b64decode("1KHLiKZvAvjbY1ziZEHMXawbCEIM6qwjCDm3VYRan/s=")
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class SHSError(Exception):
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"""A SHS exception."""
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class SHSCryptoBase: # pylint: disable=too-many-instance-attributes
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"""Base functions for SHS cryptography"""
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def __init__(
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self,
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local_key: SigningKey,
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ephemeral_key: Optional[PrivateKey] = None,
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application_key: Optional[bytes] = None,
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):
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self.application_key = application_key or APPLICATION_KEY
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self.b_alice: Optional[bytes] = None
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self.box_secret: Optional[bytes] = None
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self.hello: Optional[bytes] = None
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self.local_key = local_key
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self.remote_app_hmac: Optional[bytes] = None
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self.remote_ephemeral_key: Optional[bytes] = None
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self.remote_pub_key: Optional[VerifyKey] = None
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self.shared_hash: Optional[bytes] = None
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self.shared_secret: Optional[bytes] = None
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self._reset_keys(ephemeral_key or PrivateKey.generate())
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def _reset_keys(self, ephemeral_key: PrivateKey) -> None:
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self.local_ephemeral_key = ephemeral_key
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self.local_app_hmac = hmac.new(
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self.application_key, bytes(ephemeral_key.public_key), digestmod="sha512"
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).digest()[:32]
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def generate_challenge(self) -> bytes:
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"""Generate and return a challenge to be sent to the server."""
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return self.local_app_hmac + bytes(self.local_ephemeral_key.public_key)
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def verify_challenge(self, data: bytes) -> bool:
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"""Verify the correctness of challenge sent from the client."""
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assert len(data) == 64
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sent_hmac, remote_ephemeral_key = data[:32], data[32:]
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h = hmac.new(self.application_key, remote_ephemeral_key, digestmod="sha512")
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self.remote_app_hmac = h.digest()[:32]
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ok = self.remote_app_hmac == sent_hmac
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if ok:
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# this is (a * b)
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self.shared_secret = crypto_scalarmult(bytes(self.local_ephemeral_key), remote_ephemeral_key)
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self.remote_ephemeral_key = remote_ephemeral_key
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# this is hash(a * b)
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self.shared_hash = hashlib.sha256(self.shared_secret).digest()
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return ok
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def clean(self, new_ephemeral_key: Optional[PrivateKey] = None) -> None:
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"""Clean internal data"""
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self._reset_keys(new_ephemeral_key or PrivateKey.generate())
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self.shared_secret = None
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self.shared_hash = None
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self.remote_ephemeral_key = None
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def get_box_keys(self) -> BoxStreamKeys:
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"""Get the box stream’s keys"""
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assert self.box_secret
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assert self.remote_app_hmac
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assert self.remote_pub_key
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shared_secret = hashlib.sha256(self.box_secret).digest()
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return {
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"shared_secret": shared_secret,
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"encrypt_key": hashlib.sha256(shared_secret + bytes(self.remote_pub_key)).digest(),
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"decrypt_key": hashlib.sha256(shared_secret + bytes(self.local_key.verify_key)).digest(),
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"encrypt_nonce": self.remote_app_hmac[:24],
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"decrypt_nonce": self.local_app_hmac[:24],
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}
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class SHSServerCrypto(SHSCryptoBase):
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"""SHS server crypto algorithm"""
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def verify_client_auth(self, data: bytes) -> bool:
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"""Verify client authentication data"""
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assert self.remote_ephemeral_key
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assert self.shared_hash
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assert self.shared_secret
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assert len(data) == 112
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a_bob = crypto_scalarmult(bytes(self.local_key.to_curve25519_private_key()), self.remote_ephemeral_key)
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box_secret = hashlib.sha256(self.application_key + self.shared_secret + a_bob).digest()
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self.hello = crypto_box_open_afternm(data, b"\x00" * 24, box_secret)
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signature, public_key = self.hello[:64], self.hello[64:]
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signed = self.application_key + bytes(self.local_key.verify_key) + self.shared_hash
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pkey = VerifyKey(public_key)
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# will raise an exception if verification fails
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pkey.verify(signed, signature)
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self.remote_pub_key = pkey
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b_alice = crypto_scalarmult(
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bytes(self.local_ephemeral_key), bytes(self.remote_pub_key.to_curve25519_public_key())
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)
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self.box_secret = hashlib.sha256(self.application_key + self.shared_secret + a_bob + b_alice).digest()[:32]
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return True
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def generate_accept(self) -> bytes:
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"""Generate an accept message"""
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assert self.box_secret
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assert self.hello
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assert self.shared_hash
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okay = self.local_key.sign(self.application_key + self.hello + self.shared_hash).signature
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return crypto_box_afternm(okay, b"\x00" * 24, self.box_secret)
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def clean(self, new_ephemeral_key: Optional[PrivateKey] = None) -> None:
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super().clean(new_ephemeral_key=new_ephemeral_key)
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self.hello = None
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self.b_alice = None
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class SHSClientCrypto(SHSCryptoBase):
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"""An object that encapsulates all the SHS client-side crypto.
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:param local_key: the keypair used by the client
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:param server_pub_key: the server's public key
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:param ephemeral_key: a fresh local private key
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:param application_key: the unique application key, defaults to SSB's
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"""
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def __init__(
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self,
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local_key: SigningKey,
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server_pub_key: bytes,
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ephemeral_key: PrivateKey,
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application_key: Optional[bytes] = None,
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):
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super().__init__(local_key, ephemeral_key, application_key)
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self.a_bob: Optional[bytes] = None
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self.remote_pub_key = VerifyKey(server_pub_key)
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def verify_server_challenge(self, data: bytes) -> bool:
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"""Verify the correctness of challenge sent from the server."""
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assert self.remote_pub_key
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assert super().verify_challenge(data)
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assert self.shared_hash
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assert self.shared_secret
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curve_pkey = self.remote_pub_key.to_curve25519_public_key()
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# a_bob is (a * B)
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a_bob = crypto_scalarmult(bytes(self.local_ephemeral_key), bytes(curve_pkey))
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self.a_bob = a_bob
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# this shall be hash(K | a * b | a * B)
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self.box_secret = hashlib.sha256(self.application_key + self.shared_secret + a_bob).digest()
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# and message_to_box will correspond to H = sign(A)[K | Bp | hash(a * b)] | Ap
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signed_message = self.local_key.sign(self.application_key + bytes(self.remote_pub_key) + self.shared_hash)
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message_to_box = signed_message.signature + bytes(self.local_key.verify_key)
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self.hello = message_to_box
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return True
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def generate_client_auth(self) -> bytes:
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"""Generate box[K|a*b|a*B](H)"""
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assert self.box_secret
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assert self.hello
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nonce = b"\x00" * 24
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# return box(K | a * b | a * B)[H]
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return crypto_box_afternm(self.hello, nonce, self.box_secret)
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def verify_server_accept(self, data: bytes) -> bool:
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"""Verify that the server's accept message is sane"""
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assert self.a_bob
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assert self.hello
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assert self.remote_ephemeral_key
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assert self.remote_pub_key
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assert self.shared_hash
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assert self.shared_secret
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curve_lkey = self.local_key.to_curve25519_private_key()
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# b_alice is (A * b)
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b_alice = crypto_scalarmult(bytes(curve_lkey), self.remote_ephemeral_key)
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self.b_alice = b_alice
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# this is hash(K | a * b | a * B | A * b)
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self.box_secret = hashlib.sha256(self.application_key + self.shared_secret + self.a_bob + b_alice).digest()
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nonce = b"\x00" * 24
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try:
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# let's use the box secret to unbox our encrypted message
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signature = crypto_box_open_afternm(data, nonce, self.box_secret)
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except CryptoError as exc:
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raise SHSError("Error decrypting server acceptance message") from exc
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# we should have received sign(B)[K | H | hash(a * b)]
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# let's see if that signature can verify the reconstructed data on our side
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self.remote_pub_key.verify(self.application_key + self.hello + self.shared_hash, signature)
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return True
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def clean(self, new_ephemeral_key: Optional[PrivateKey] = None) -> None:
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super().clean(new_ephemeral_key=new_ephemeral_key)
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self.a_bob = None
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self.b_alice = None
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