Use the strongest available random source when shuffling

We need to provide cryptographically secure random shuffling.

Looking at Python's implementation of random.shuffle()[1], we see that
(a) it is a straightforward implementation of Durstenfeld's shuffle
algorithm[2] (which is as unbiased as its random number source), and
(b) it allows for a custom `random` function to be passed in.

Hence, AFAICS, we should get a cryptographically secure shuffle as long
as we pass in a `random` function that itself is cryptographically
secure.

Quoting the documentation of the secrets module:

  The secrets module provides access to the most secure source of
  randomness that your operating system provides.

  class secrets.SystemRandom

    A class for generating random numbers using the highest-quality
    sources provided by the operating system.

Therefore, the best we can do is AFAICS to simply use the `random`
function available from secrets.SystemRandom class.

[1]: https://github.com/python/cpython/blob/v3.6.10rc1/Lib/random.py#L263
[2]: https://en.wikipedia.org/wiki/Fisher–Yates_shuffle#The_modern_algorithm
[3]: https://docs.python.org/3.6/library/secrets.html#random-numbers

Author: jherland

dp3t/protocols/lowcost.py

@@ -77,6 +77,18 @@ def batch_start_from_time(time):
     return (int(time.timestamp()) // SECONDS_PER_BATCH) * SECONDS_PER_BATCH
 
 
+def secure_shuffle(items):
+    """Perform a cryptographically secure shuffling of the given items
+
+    Args:
+        items (obj:list): A list of items to be shuffled
+
+    Returns:
+        Nothing. Items are shuffled in place
+    """
+    random.shuffle(items, secrets.SystemRandom().random)
+
+
 #########################################
 ### BASIC CRYPTOGRAPHIC FUNCTIONALITY ###
 #########################################
@@ -130,8 +142,7 @@ def generate_ephids_for_day(current_day_key, shuffle=True):
 
     # Shuffle the resulting ephids
     if shuffle:
-        # WARNING: replace with a secure shuffle
-        random.shuffle(ephids)
+        secure_shuffle(ephids)
 
     return ephids
 
@@ -310,7 +321,7 @@ def add_observation(self, ephid, time):
         self.observations[batch_start].append(ephid)
 
         # Shuffle observations to hide receive order
-        random.shuffle(self.observations[batch_start])
+        secure_shuffle(self.observations[batch_start])
 
     def get_tracing_information(
         self,
@@ -451,4 +462,4 @@ def housekeeping_after_batch(self, batch):
             self.observations[day_time].extend(observations)
 
             # Reshuffle to make sure we do not store ordering data
-            random.shuffle(self.observations[day_time])
+            secure_shuffle(self.observations[day_time])