Merge pull request #176 from cthulahoops/digests

Verify http digest is correct if specified in signature.

fedireads/broadcast.py

@@ -5,7 +5,7 @@ import requests
 
 from fedireads import models
 from fedireads.tasks import app
-from fedireads.signatures import make_signature
+from fedireads.signatures import make_signature, make_digest
 
 
 def get_public_recipients(user, software=None):
@@ -67,12 +67,16 @@ def sign_and_send(sender, activity, destination):
         # this shouldn't happen. it would be bad if it happened.
         raise ValueError('No private key found for sender')
 
+    data = json.dumps(activity).encode('utf-8')
+    digest = make_digest(data)
+
     response = requests.post(
         destination,
-        data=json.dumps(activity),
+        data=data,
         headers={
             'Date': now,
-            'Signature': make_signature(sender, destination, now),
+            'Digest': digest,
+            'Signature': make_signature(sender, destination, now, digest),
             'Content-Type': 'application/activity+json; charset=utf-8',
         },
     )

fedireads/remote_user.py

@@ -114,6 +114,10 @@ def get_or_create_remote_server(domain):
         'https://%s/.well-known/nodeinfo' % domain,
         headers={'Accept': 'application/activity+json'}
     )
+
+    if response.status_code != 200:
+        return None
+
     data = response.json()
     try:
         nodeinfo_url = data.get('links')[0].get('href')

fedireads/signatures.py

@@ -1,4 +1,6 @@
+import hashlib
 from urllib.parse import urlparse
+import datetime
 from base64 import b64encode, b64decode
 
 from Crypto import Random
@@ -6,6 +8,7 @@ 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():
     random_generator = Random.new().read
@@ -16,12 +19,13 @@ def create_key_pair():
     return private_key, public_key
 
 
-def make_signature(sender, destination, date):
+def make_signature(sender, destination, date, digest):
     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.private_key))
@@ -29,11 +33,26 @@ def make_signature(sender, destination, date):
     signature = {
         'keyId': '%s#main-key' % sender.remote_id,
         'algorithm': 'rsa-sha256',
-        'headers': '(request-target) host date',
+        '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):
+    return 'SHA-256=' + b64encode(hashlib.sha512(data).digest()).decode('utf-8')
+
+def verify_digest(request):
+    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:
+        return ValueError("Invalid HTTP Digest header")
 
 class Signature:
     def __init__(self, key_id, headers, signature):
@@ -60,6 +79,9 @@ class 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 = []
@@ -68,6 +90,8 @@ class Signature:
                 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]
@@ -80,3 +104,8 @@ class Signature:
 
         # raises a ValueError if it fails
         signer.verify(digest, self.signature)
+
+def http_date_age(datestr):
+    parsed = datetime.datetime.strptime(datestr, '%a, %d %b %Y %H:%M:%S GMT')
+    delta = datetime.datetime.utcnow() - parsed
+    return delta.total_seconds()

fedireads/tests/test_signing.py

@@ -1,6 +1,8 @@
+import time
 from collections import namedtuple
 from urllib.parse import urlsplit
 
+import json
 import responses
 
 from django.test import TestCase, Client
@@ -9,7 +11,10 @@ from django.utils.http import http_date
 from fedireads.models import User
 from fedireads.activitypub import get_follow_request
 from fedireads.settings import DOMAIN
-from fedireads.signatures import create_key_pair, make_signature
+from fedireads.signatures import create_key_pair, make_signature, make_digest
+
+def get_follow_data(follower, followee):
+    return json.dumps(get_follow_request(follower, followee)).encode('utf-8')
 
 Sender = namedtuple('Sender', ('remote_id', 'private_key', 'public_key'))
 
@@ -27,35 +32,44 @@ class Signature(TestCase):
             public_key,
         )
 
-    def send_follow(self, sender, signature, now):
+    def send(self, signature, now, data):
         c = Client()
         return c.post(
             urlsplit(self.rat.inbox).path,
-            data=get_follow_request(
-                sender,
-                self.rat,
-            ),
+            data=data,
             content_type='application/json',
             **{
                 'HTTP_DATE': now,
                 'HTTP_SIGNATURE': signature,
+                'HTTP_DIGEST': make_digest(data),
                 'HTTP_CONTENT_TYPE': 'application/activity+json; charset=utf-8',
                 'HTTP_HOST': DOMAIN,
             }
         )
 
+    def send_test_request(
+            self,
+            sender,
+            signer=None,
+            send_data=None,
+            digest=None,
+            date=None):
+        now = date or http_date()
+        data = get_follow_data(sender, self.rat)
+        signature = make_signature(
+            signer or sender, self.rat.inbox, now, digest or make_digest(data))
+        return self.send(signature, now, send_data or data)
+
     def test_correct_signature(self):
-        now = http_date()
-        signature = make_signature(self.mouse, self.rat.inbox, now)
-        return self.send_follow(self.mouse, signature, now).status_code == 200
+        response = self.send_test_request(sender=self.mouse)
+        self.assertEqual(response.status_code, 200)
 
     def test_wrong_signature(self):
         ''' Messages must be signed by the right actor.
             (cat cannot sign messages on behalf of mouse)
         '''
-        now = http_date()
-        signature = make_signature(self.cat, self.rat.inbox, now)
-        assert self.send_follow(self.mouse, signature, now).status_code == 401
+        response = self.send_test_request(sender=self.mouse, signer=self.cat)
+        self.assertEqual(response.status_code, 401)
 
     @responses.activate
     def test_remote_signer(self):
@@ -67,10 +81,7 @@ class Signature(TestCase):
             }},
             status=200)
 
-        now = http_date()
-        sender = self.fake_remote
-        signature = make_signature(sender, self.rat.inbox, now)
-        response = self.send_follow(sender, signature, now)
+        response = self.send_test_request(sender=self.fake_remote)
         self.assertEqual(response.status_code, 200)
 
     @responses.activate
@@ -81,8 +92,26 @@ class Signature(TestCase):
             json={'error': 'not found'},
             status=404)
 
-        now = http_date()
-        sender = self.fake_remote
-        signature = make_signature(sender, self.rat.inbox, now)
-        response = self.send_follow(sender, signature, now)
+        response = self.send_test_request(sender=self.fake_remote)
+        self.assertEqual(response.status_code, 401)
+
+    def test_changed_data(self):
+        '''Message data must match the digest header.'''
+        response = self.send_test_request(
+            self.mouse,
+            send_data=get_follow_data(self.mouse, self.cat))
+        self.assertEqual(response.status_code, 401)
+
+    def test_invalid_digest(self):
+        response = self.send_test_request(
+            self.mouse,
+            digest='SHA-256=AAAAAAAAAAAAAAAAAA')
+        self.assertEqual(response.status_code, 401)
+
+    def test_old_message(self):
+        '''Old messages should be rejected to prevent replay attacks.'''
+        response = self.send_test_request(
+            self.mouse,
+            date=http_date(time.time() - 301)
+        )
         self.assertEqual(response.status_code, 401)