Add conversion filters for serial numbers (#713)

* Refactoring.

* Add parse_filter and to_filter plugins.

* Mention filters when serial numbers are accepted or returned.

plugins/module_utils/crypto/math.py

@@ -54,17 +54,93 @@ def quick_is_not_prime(n):
 python_version = (sys.version_info[0], sys.version_info[1])
 if python_version >= (2, 7) or python_version >= (3, 1):
     # Ansible still supports Python 2.6 on remote nodes
+
+    def count_bytes(no):
+        """
+        Given an integer, compute the number of bytes necessary to store its absolute value.
+        """
+        no = abs(no)
+        if no == 0:
+            return 0
+        return (no.bit_length() + 7) // 8
+
     def count_bits(no):
+        """
+        Given an integer, compute the number of bits necessary to store its absolute value.
+        """
         no = abs(no)
         if no == 0:
             return 0
         return no.bit_length()
 else:
     # Slow, but works
+    def count_bytes(no):
+        """
+        Given an integer, compute the number of bytes necessary to store its absolute value.
+        """
+        no = abs(no)
+        count = 0
+        while no > 0:
+            no >>= 8
+            count += 1
+        return count
+
     def count_bits(no):
+        """
+        Given an integer, compute the number of bits necessary to store its absolute value.
+        """
         no = abs(no)
         count = 0
         while no > 0:
             no >>= 1
             count += 1
         return count
+
+if sys.version_info[0] >= 3:
+    # Python 3 (and newer)
+    def _convert_int_to_bytes(count, no):
+        return no.to_bytes(count, byteorder='big')
+
+    def _to_hex(no):
+        return hex(no)[2:]
+else:
+    # Python 2
+    def _convert_int_to_bytes(count, n):
+        h = '%x' % n
+        if len(h) > 2 * count:
+            raise Exception('Number {1} needs more than {0} bytes!'.format(count, n))
+        return ('0' * (2 * count - len(h)) + h).decode('hex')
+
+    def _to_hex(no):
+        return '%x' % no
+
+
+def convert_int_to_bytes(no, count=None):
+    """
+    Convert the absolute value of an integer to a byte string in network byte order.
+
+    If ``count`` is provided, it must be sufficiently large so that the integer's
+    absolute value can be represented with these number of bytes. The resulting byte
+    string will have length exactly ``count``.
+
+    The value zero will be converted to an empty byte string if ``count`` is provided.
+    """
+    no = abs(no)
+    if count is None:
+        count = count_bytes(no)
+    return _convert_int_to_bytes(count, no)
+
+
+def convert_int_to_hex(no, digits=None):
+    """
+    Convert the absolute value of an integer to a string of hexadecimal digits.
+
+    If ``digits`` is provided, the string will be padded on the left with ``0``s so
+    that the returned value has length ``digits``. If ``digits`` is not sufficient,
+    the string will be longer.
+    """
+    no = abs(no)
+    value = _to_hex(no)
+    if digits is not None and len(value) < digits:
+        value = '0' * (digits - len(value)) + value
+    return value