Add challenge tests.

Signed-off-by: Pol Henarejos <pol.henarejos@cttc.es>

tests/pico-hsm/test_003_challenge.py

@@ -0,0 +1,57 @@
+"""
+/*
+ * This file is part of the Pico HSM distribution (https://github.com/polhenarejos/pico-hsm).
+ * Copyright (c) 2022 Pol Henarejos.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+"""
+
+import pytest
+import math
+from collections import Counter
+
+def mean(x):
+    sum = 0
+    for i in x:
+        sum += i
+    return sum/len(x)
+
+def var(x):
+    sum = 0
+    m = mean(x)
+    for i in x:
+        sum += (i-m)**2
+    return sum/len(x)
+
+@pytest.mark.parametrize(
+    "length", [1, 256, 1024]
+)
+def test_challenge(device, length):
+    data = device.get_challenge(length)
+    assert(len(data) == length)
+
+def test_randomness(device):
+    data = []
+    N = 1000
+    for k2 in range(N):
+        data += device.get_challenge(1024)
+
+    _, values = zip(*Counter(data).items())
+
+    nm = mean(values)/(N*1024/256)
+    sm = math.sqrt(var(values))/mean(values)
+
+    assert(0.99 <= nm <= 1.01)
+    assert(sm <= 0.02)
+