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First partially working implementation

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This commit is contained in:
Pedro Ferreira
2017-05-25 12:47:01 +02:00
commit fe95688f9d
16 changed files with 672 additions and 0 deletions
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4
ssb/shs/__init__.py Normal file
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from .crypto import SHSClientCrypto, SHSServerCrypto
from .socket import SHSClient, SHSServer
__all__ = ('SHSClient', 'SHSClientCrypto', 'SHSServer', 'SHSServerCrypto')

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ssb/shs/crypto.py Normal file
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import hashlib
import hmac
from base64 import b64decode
from nacl.bindings import crypto_scalarmult, crypto_box_afternm, crypto_box_open_afternm
from nacl.exceptions import CryptoError
from nacl.public import PrivateKey
from nacl.signing import VerifyKey
APPLICATION_KEY = b64decode('1KHLiKZvAvjbY1ziZEHMXawbCEIM6qwjCDm3VYRan/s=')
class SHSError(Exception):
"""A SHS exception."""
pass
class SHSCryptoBase(object):
def __init__(self, local_key, ephemeral_key=None, application_key=None):
self.local_key = local_key
self.application_key = application_key or APPLICATION_KEY
self._reset_keys(ephemeral_key or PrivateKey.generate())
def _reset_keys(self, ephemeral_key):
self.local_ephemeral_key = ephemeral_key
self.local_app_hmac = (hmac.new(self.application_key, bytes(ephemeral_key.public_key), digestmod='sha512')
.digest()[:32])
def generate_challenge(self):
"""Generate and return a challenge to be sent to the server."""
return self.local_app_hmac + bytes(self.local_ephemeral_key.public_key)
def verify_challenge(self, data):
"""Verify the correctness of challenge sent from the client."""
assert len(data) == 64
sent_hmac, remote_ephemeral_key = data[:32], data[32:]
h = hmac.new(self.application_key, remote_ephemeral_key, digestmod='sha512')
self.remote_app_hmac = h.digest()[:32]
ok = self.remote_app_hmac == sent_hmac
if ok:
# this is (a * b)
self.shared_secret = crypto_scalarmult(bytes(self.local_ephemeral_key), remote_ephemeral_key)
self.remote_ephemeral_key = remote_ephemeral_key
# this is hash(a * b)
self.shared_hash = hashlib.sha256(self.shared_secret).digest()
return ok
def clean(self, new_ephemeral_key=None):
self._reset_keys(new_ephemeral_key or PrivateKey.generate())
self.shared_secret = None
self.shared_hash = None
self.remote_ephemeral_key = None
def get_box_keys(self):
shared_secret = hashlib.sha256(self.box_secret).digest()
return {
'shared_secret': shared_secret,
'encrypt_key': hashlib.sha256(shared_secret + bytes(self.remote_pub_key)).digest(),
'decrypt_key': hashlib.sha256(shared_secret + bytes(self.local_key.verify_key)).digest(),
'encrypt_nonce': self.remote_app_hmac[:24],
'decrypt_nonce': self.local_app_hmac[:24]
}
class SHSServerCrypto(SHSCryptoBase):
def verify_client_auth(self, data):
assert len(data) == 112
a_bob = crypto_scalarmult(bytes(self.local_key.to_curve25519_private_key()), self.remote_ephemeral_key)
box_secret = hashlib.sha256(self.application_key + self.shared_secret + a_bob).digest()
self.hello = crypto_box_open_afternm(data, b'\x00' * 24, box_secret)
signature, public_key = self.hello[:64], self.hello[64:]
signed = self.application_key + bytes(self.local_key.verify_key) + self.shared_hash
pkey = VerifyKey(public_key)
# will raise an exception if verification fails
pkey.verify(signed, signature)
self.remote_pub_key = pkey
b_alice = crypto_scalarmult(bytes(self.local_ephemeral_key),
bytes(self.remote_pub_key.to_curve25519_public_key()))
self.box_secret = hashlib.sha256(self.application_key + self.shared_secret + a_bob + b_alice).digest()[:32]
return True
def generate_accept(self):
okay = self.local_key.sign(self.application_key + self.hello + self.shared_hash).signature
d = crypto_box_afternm(okay, b'\x00' * 24, self.box_secret)
return d
def clean(self, new_ephemeral_key=None):
super(SHSServerCrypto, self).clean(new_ephemeral_key=new_ephemeral_key)
self.hello = None
self.local_lterm_shared = None
class SHSClientCrypto(SHSCryptoBase):
"""An object that encapsulates all the SHS client-side crypto.
:param local_key: the :class:`ssb.keys.KeyPair` used by the client
:param local_ephemeral_key: a fresh local :class:`nacl.public.PrivateKey`
:param server_pub_key: the server's public key (``byte`` string)
:param application_key: the unique application key (``byte`` string), defaults to SSB's
"""
def __init__(self, local_key, server_pub_key, ephemeral_key, application_key=None):
super(SHSClientCrypto, self).__init__(local_key, ephemeral_key, application_key)
self.remote_pub_key = VerifyKey(server_pub_key)
def verify_server_challenge(self, data):
"""Verify the correctness of challenge sent from the server."""
# TODO: use super.verify_challenge and add extra logic
return super(SHSClientCrypto, self).verify_challenge(data)
def generate_client_auth(self):
"""Generate box[K|a*b|a*B](H)"""
curve_pkey = self.remote_pub_key.to_curve25519_public_key()
# remote_lterm_shared is (a * B)
remote_lterm_shared = crypto_scalarmult(bytes(self.local_ephemeral_key), bytes(curve_pkey))
self.remote_lterm_shared = remote_lterm_shared
# this shall be hash(K | a * b | a * B)
box_secret = hashlib.sha256(self.application_key + self.shared_secret + remote_lterm_shared).digest()
# and message_to_box will correspond to H = sign(A)[K | Bp | hash(a * b)] | Ap
signed_message = self.local_key.sign(self.application_key + bytes(self.remote_pub_key) + self.shared_hash)
message_to_box = signed_message.signature + bytes(self.local_key.verify_key)
self.client_auth = message_to_box
nonce = b"\x00" * 24
# return box(K | a * b | a * B)[H]
return crypto_box_afternm(message_to_box, nonce, box_secret)
def verify_server_accept(self, data):
"""Verify that the server's accept message is sane"""
curve_lkey = self.local_key.to_curve25519_private_key()
# local_lterm_shared is (A * b)
local_lterm_shared = crypto_scalarmult(bytes(curve_lkey), self.remote_ephemeral_key)
self.local_lterm_shared = local_lterm_shared
# this is hash(K | a * b | a * B | A * b)
self.box_secret = hashlib.sha256(self.application_key + self.shared_secret + self.remote_lterm_shared +
local_lterm_shared).digest()
nonce = b"\x00" * 24
try:
# let's use the box secret to unbox our encrypted message
signature = crypto_box_open_afternm(data, nonce, self.box_secret)
except CryptoError:
raise SHSError('Error decrypting server acceptance message')
# we should have received sign(B)[K | H | hash(a * b)]
# let's see if that signature can verify the reconstructed data on our side
self.remote_pub_key.verify(self.application_key + self.client_auth + self.shared_hash, signature)
return True
def clean(self, new_ephemeral_key=None):
super(SHSClientCrypto, self).clean(new_ephemeral_key=new_ephemeral_key)
self.remote_lterm_shared = None
self.local_lterm_shared = None

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ssb/shs/socket.py Normal file
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from asyncio import open_connection, start_server
from ..boxstream import get_stream_pair
from .crypto import SHSClientCrypto, SHSServerCrypto
class SHSClientException(Exception):
pass
class SHSSocket(object):
async def read(self):
async for msg in self.read_stream.process():
yield msg
async def write(self, data):
await self.write_stream.write(data)
async def disconnect(self):
self.writer.close()
class SHSServer(SHSSocket):
def __init__(self, host, port, server_kp, application_key=None):
self.host = host
self.port = port
self.crypto = SHSServerCrypto(server_kp.private_key, application_key=application_key)
self._on_connect = None
async def _handshake(self, reader, writer):
data = await reader.read(64)
if not self.crypto.verify_challenge(data):
raise SHSClientException('Client challenge is not valid')
writer.write(self.crypto.generate_challenge())
data = await reader.read(112)
if not self.crypto.verify_client_auth(data):
raise SHSClientException('Client auth is not valid')
writer.write(self.crypto.generate_accept())
async def handle_connection(self, reader, writer):
self.crypto.clean()
await self._handshake(reader, writer)
keys = self.crypto.get_box_keys()
self.crypto.clean()
self.read_stream, self.write_stream = get_stream_pair(reader, writer, **keys)
self.writer = writer
if self._on_connect:
await self._on_connect()
async def listen(self, loop=None):
await start_server(self.handle_connection, self.host, self.port, loop=loop)
class SHSClient(SHSSocket):
def __init__(self, host, port, client_kp, server_pub_key, ephemeral_key=None, application_key=None):
self.host = host
self.port = port
self.crypto = SHSClientCrypto(client_kp.private_key, server_pub_key, ephemeral_key=ephemeral_key,
application_key=application_key)
async def _handshake(self, reader, writer):
writer.write(self.crypto.generate_challenge())
data = await reader.read(64)
if not self.crypto.verify_server_challenge(data):
raise SHSClientException('Server challenge is not valid')
writer.write(self.crypto.generate_client_auth())
data = await reader.read(80)
if not self.crypto.verify_server_accept(data):
raise SHSClientException('Server accept is not valid')
async def connect(self, loop=None):
reader, writer = await open_connection(self.host, self.port, loop=loop)
await self._handshake(reader, writer)
keys = self.crypto.get_box_keys()
self.crypto.clean()
self.read_stream, self.write_stream = get_stream_pair(reader, writer, **keys)
self.writer = writer