[tor-bugs] #31823 [Core Tor/Stem]: HSv3 descriptor support in stem [encoding]

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#31823: HSv3 descriptor support in stem [encoding]
-------------------------------------------------+-------------------------
 Reporter:  asn                                  |          Owner:  atagar
     Type:  defect                               |         Status:
                                                 |  needs_revision
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Stem                        |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-hs scaling onionbalance          |  Actual Points:  2
  network-team-roadmap-september tor-spec        |
Parent ID:  #26768                               |         Points:  5
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by atagar):

 > Hm. I understand that _descriptor_content() arguments are default
 values, but what defines a good default value here?

 Hi George. Yup, I'm delighted to improve our defaults and this is probably
 simply a matter of us misunderstanding each other's code. Here's what you
 had...

 {{{
 def _get_fake_clients_bytes():
   """
   Generate fake client authorization data for the middle layer
   """

   final_bytes = b''
   num_fake_clients = 16  # default for when client auth is disabled

   for _ in range(num_fake_clients):
     client_id = base64.b64encode(os.urandom(8)).rstrip(b'=')
     client_iv = base64.b64encode(os.urandom(16)).rstrip(b'=')
     descriptor_cookie = base64.b64encode(os.urandom(16)).rstrip(b'=')

     final_bytes += b'%s %s %s %s\n' % (b'auth-client', client_id,
 client_iv, descriptor_cookie)

   return final_bytes


 def _get_middle_descriptor_layer_body(encrypted):
   """
   Get the middle descriptor layer as bytes
   (It's just fake client auth data since client auth is disabled)
   """

   from cryptography.hazmat.primitives.asymmetric.x25519 import
 X25519PrivateKey
   from cryptography.hazmat.primitives import serialization

   fake_pub_key = X25519PrivateKey.generate().public_key()
   fake_pub_key_bytes = fake_pub_key.public_bytes(encoding =
 serialization.Encoding.Raw, format = serialization.PublicFormat.Raw)
   fake_pub_key_bytes_b64 = base64.b64encode(fake_pub_key_bytes)
   fake_clients = _get_fake_clients_bytes()

   return b'desc-auth-type x25519\n' \
     b'desc-auth-ephemeral-key %s\n' \
     b'%s' \
     b'%s' % (fake_pub_key_bytes_b64, fake_clients, encrypted)
 }}}

 This didn't explain why values were fabricated this way so I figured it
 was simply overly complicated and inflexible (callers should have a
 mechanism to override all descriptor values).

 Since you're calling out **desc-auth-ephemeral-key** in particular here's
 your version of just that...

 {{{
 fake_pub_key = X25519PrivateKey.generate().public_key()
 fake_pub_key_bytes = fake_pub_key.public_bytes(encoding =
 serialization.Encoding.Raw, format = serialization.PublicFormat.Raw)

 b'desc-auth-ephemeral-key %s' % base64.b64encode(fake_pub_key_bytes)
 }}}

 As such, to do the same as your code I could simply change...

 {{{
 return _descriptor_content(attr, exclude, (
   ('desc-auth-type', 'x25519'),
   ('desc-auth-ephemeral-key', base64.b64encode(os.urandom(32))),
 ), (
   ('encrypted', b'\n' + inner_layer._encrypt(revision_counter,
 subcredential, blinded_key)),
 ))
 }}}

 ... to...

 {{{

 return _descriptor_content(attr, exclude, (
   ('desc-auth-type', 'x25519'),
   ('desc-auth-ephemeral-key',
 base64.b64encode(stem.util._pubkey_bytes(X25519PrivateKey.generate()))),
 ), (
   ('encrypted', b'\n' + inner_layer._encrypt(revision_counter,
 subcredential, blinded_key)),
 ))
 }}}

 Is that what you would like? The base64 encoding of a random key? If so
 that's a very simple tweak to make. Or are you requesting something else?

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31823#comment:14>
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