Add an authenticating serializer

Although Celery already includes an authenticating serializer, it is not very
difficult to implement a serializer. The serializer included with Celery uses
X.509 certificates and OpenSSL, which is flexible, but complex.

This serializer uses a lightweight PKI scheme based on ed25519 and the
well-regarded libsodium library. All that needs to be done, is to generate a
secure random 32 byte sequence. The serializer has a single signing key and a
list of trusted verification keys.

Addresses #68

debian/control

@@ -27,6 +27,7 @@ Build-Depends:
  python3-gi,
  python3-jinja2,
  python3-kombu,
+ python3-nacl,
  python3-netaddr,
  python3-psycopg2,
  python3-pydbus,
@@ -66,6 +67,7 @@ Depends:
  python3-gi,
  python3-jinja2,
  python3-kombu,
+ python3-nacl,
  python3-netaddr,
  python3-psycopg2,
  python3-pydbus,

docker/Dockerfile

@@ -69,6 +69,7 @@ RUN export DEBIAN_FRONTEND="noninteractive" && \
     python3-pip \
     python3-psycopg2 \
     python3-pydbus \
+    python3-pynacl \
     python3-pyrad \
     python3-pyroute2 \
     python3-pysnmp4 \

setup.py.in

@@ -25,6 +25,7 @@ setup(name='@PACKAGE_NAME@',
           "psycopg2",
           "pydbus",
           'pygobject',
+          'pynacl',
           "pyrad",
           "pyroute2",
           "pysnmp",

src/hades/agent/signature.py

@@ -0,0 +1,132 @@
+# -*- coding: utf-8 -*-
+"""
+A serializer that prepends a JSON header with an ed25519 signature before the
+data.
+
+The header is a completely separate JSON document, that is separated from the
+body by optional whitespace. This allows parsing the header without parsing the
+payload. Also it is not required to encode the payload. Raw data can also be
+inspected by humans easily. The only problem with this scheme is, that the
+payload may not start with space, because this white space would be striped
+and signature verification would fail.
+
+The JSON payload ``{"Foobar": 1}`` would be encoded as follows:
+
+.. code-block :: json
+
+    {
+      "signature": "u9rPWbQh3TNpW8wrimL5SHtelkgm32cTPfzaUgp+djMDGz/Vjf/mb6BtQcXpJ1noJl2xILTWxhrpqtv9ykf2Bw==",
+      "signer": "w7ADgLSZTlXIDY/qbcfxUCeXht8VcpGoJYOj0lQu1Qw=",
+      "content_type": "application/json",
+      "content_encoding": "utf-8"
+    }
+    {
+      "Foobar": 1
+    }
+
+"""
+import io
+import json
+from typing import Iterable, Union
+
+import nacl.encoding
+import nacl.signing
+from kombu.serialization import dumps, loads, register as kombu_register
+from kombu.utils.encoding import bytes_to_str
+
+__all__ = ['ED25519Serializer', 'register']
+
+
+class ED25519Serializer(object):
+    key_codec = nacl.encoding.Base64Encoder()
+    json_decoder = json.JSONDecoder()
+    whitespace = b' \t\n\r'
+
+    def __init__(self, signing_key: nacl.signing.SigningKey,
+                 verify_keys: Iterable[nacl.signing.VerifyKey],
+                 serializer='json', content_encoding='utf-8'):
+        self._signing_key = signing_key
+        self._verify_keys = {
+            self.key_codec.encode(bytes(key)).decode('ascii'): key
+            for key in verify_keys
+        }
+        self._serializer = serializer
+        self._signer = self.key_codec.encode(
+            bytes(self._signing_key.verify_key)).decode('ascii')
+        self._content_encoding = content_encoding
+
+    def _ensure_bytes(self, data: Union[bytes, str]):
+        if isinstance(data, bytes):
+            return data
+        return data.encode(self._content_encoding)
+
+    def serialize(self, data):
+        content_type, content_encoding, body = dumps(
+            data, serializer=self._serializer)
+        if content_encoding != self._content_encoding:
+            raise ValueError("Content encoding of inner serializer {!r} must "
+                             "match ({!r} != {!r})"
+                             .format(self._serializer, content_encoding,
+                                     self._content_encoding))
+        body = self._ensure_bytes(body)
+        if len(body) > 0 and body[0] in self.whitespace:
+            raise ValueError("Inner data may not begin with the following "
+                             "characters {!r}"
+                             .format(str(self.whitespace)))
+        message = self._signing_key.sign(body)
+        signature = self.key_codec.encode(message.signature).decode('ascii')
+        header = {
+            'signature':        signature,
+            'signer':           self._signer,
+            'content_type':     content_type,
+            'content_encoding': content_encoding,
+        }
+        buffer = io.BytesIO()
+        wrapper = io.TextIOWrapper(buffer, self._content_encoding,
+                                   write_through=True)
+        with wrapper:
+            json.dump(header, wrapper)
+            buffer.write(b"\n")
+            buffer.write(message.message)
+            return buffer.getvalue()
+
+    def parse_header(self, data):
+        return self.json_decoder.raw_decode(data.decode(self._content_encoding))
+
+    def deserialize(self, data):
+        data = self._ensure_bytes(data)
+        header, end = self.parse_header(data)
+        # Skip whitespace
+        length = len(data)
+        while end < length and data[end] in self.whitespace:
+            end += 1
+        header, body = header, data[end:]
+
+        signer, signature, content_type, content_encoding = (
+            header['signer'], header['signature'],
+            header['content_type'], header['content_encoding']
+        )
+        signature = self.key_codec.decode(signature)
+        if content_encoding != self._content_encoding:
+            raise ValueError("Invalid inner content encoding ({!r} != {!r})"
+                             .format(content_encoding, self._content_encoding))
+
+        try:
+            verify_key = self._verify_keys[signer]
+        except KeyError:
+            raise ValueError("Unknown signer {!r}".format(signer)) from None
+        verify_key.verify(body, signature)
+        return loads(bytes_to_str(body), content_type, content_encoding,
+                     force=True)
+
+
+def register(signing_key: nacl.signing.SigningKey,
+             verify_keys: Iterable[nacl.signing.VerifyKey],
+             name: str = 'ed25519', serializer='json',
+             content_type: str = 'application/x-data-ed25519',
+             content_encoding: str = 'utf-8'):
+    """Register serializer with :mod:`kombu`"""
+    s = ED25519Serializer(signing_key, verify_keys, serializer,
+                          content_encoding)
+    kombu_register(name, s.serialize, s.deserialize, content_type,
+                   content_encoding)