Merge pull request #888 from tlsfuzzer/layperson-explanation-fixups

handle the 95% CI for 45% trimmed mean equal 0 for small pieces of code

tlsfuzzer/analysis.py

@@ -40,7 +40,7 @@
 mpl.use('Agg')
 
 
-VERSION = 5
+VERSION = 6
 
 
 _diffs = None
@@ -1323,10 +1323,22 @@ def _write_summary(self, difference, p_vals, sign_p_vals, worst_pair,
                     "Results suggesting side-channel found, "
                     "collecting more data necessary for confirmation")
             else:
-                small_ci = min((diff_conf_int[key][2]-diff_conf_int[key][0])/2
-                               for key in
-                               ["mean", "trim_mean_05", "trim_mean_25",
-                                "trim_mean_45"])
+                small_cis = list(
+                    (diff_conf_int[key][2]-diff_conf_int[key][0])/2
+                    for key in
+                    ["mean", "median", "trim_mean_05", "trim_mean_25",
+                     "trim_mean_45"])
+                if max(small_cis) == 0:
+                    print("WARNING: all 95% CIs are equal 0. Too small sammple"
+                          " or too low clock resolution for the measurement.")
+                # when measuring values below clock frequency
+                # or very small pieces of code with high resolution clock
+                # it may cause the 95% CI to equal 0.0; that's not a realistic
+                # value so ignore it
+                # (for median it would be nice to actually check if we're not
+                # in the vicinity of the clock resolution, and ignore median
+                # then, but that's much more complex so don't do it for now)
+                small_ci = min(i for i in small_cis if i != 0)
                 if small_ci < 1e-10:
                     explanation = (
                         "Implementation verified as not "
@@ -1339,7 +1351,7 @@ def _write_summary(self, difference, p_vals, sign_p_vals, worst_pair,
                     explanation = (
                         "Large confidence intervals detected, "
                         "collecting more data necessary. Side channel "
-                        "leakege smaller than {0:.3e}s is possible".format(
+                        "leakage smaller than {0:.3e}s is possible".format(
                             small_ci))
                 else:
                     explanation = (