crypto.math module utils: add some tests, fix quick_is_not_prime() for small primes (#733)

* Fix quick_is_not_prime() for small primes. Add some tests.

* Fix return value of convert_int_to_bytes(0, 0) on Python 2.

* Add some more test cases.

* Simplify the changelog and point out that these errors only happen for cases not happening in regular use.

plugins/module_utils/crypto/math.py

@@ -42,9 +42,18 @@ def quick_is_not_prime(n):
     that we could not detect quickly whether it is not prime.
     '''
     if n <= 2:
-        return True
+        return n < 2
     # The constant in the next line is the product of all primes < 200
-    if simple_gcd(n, 7799922041683461553249199106329813876687996789903550945093032474868511536164700810) > 1:
+    prime_product = 7799922041683461553249199106329813876687996789903550945093032474868511536164700810
+    gcd = simple_gcd(n, prime_product)
+    if gcd > 1:
+        if n < 200 and gcd == n:
+            # Explicitly check for all primes < 200
+            return n not in (
+                2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83,
+                89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179,
+                181, 191, 193, 197, 199,
+            )
         return True
     # TODO: maybe do some iterations of Miller-Rabin to increase confidence
     # (https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test)
@@ -106,6 +115,8 @@ if sys.version_info[0] >= 3:
 else:
     # Python 2
     def _convert_int_to_bytes(count, n):
+        if n == 0 and count == 0:
+            return ''
         h = '%x' % n
         if len(h) > 2 * count:
             raise Exception('Number {1} needs more than {0} bytes!'.format(count, n))