Add PGPPacket._serialize_multiprecision_integer

This is the inverse of _parse_multiprecision_integer.  See the
_parse_multiprecision_integer commit for references to RFC 4880.

diff --git a/gpg-migrate.py b/gpg-migrate.py
index 5a3c8f0ef16a96ca0c4ac3ea8790ed2383485fb4..70c5250dce1b57330ec92ba285cf6702cd8f9359 100755 (executable)
--- a/gpg-migrate.py
+++ b/gpg-migrate.py
@@ -1,6 +1,7 @@
 #!/usr/bin/python
 
 import hashlib as _hashlib
+import math as _math
 import re as _re
 import subprocess as _subprocess
 import struct as _struct
@@ -684,6 +685,24 @@ class PGPPacket (dict):
             length_data,
             ])
 
+    @staticmethod
+    def _serialize_multiprecision_integer(integer):
+        r"""Serialize RFC 4880's multipricision integers
+
+        >>> PGPPacket._serialize_multiprecision_integer(1)
+        b'\x00\x01\x01'
+        >>> PGPPacket._serialize_multiprecision_integer(511)
+        b'\x00\t\x01\xff'
+        """
+        bit_length = int(_math.log(integer, 2)) + 1
+        chunks = [
+            _struct.pack('>H', bit_length),
+            ]
+        while integer > 0:
+            chunks.insert(1, bytes([integer & 0xff]))
+            integer = integer >> 8
+        return b''.join(chunks)
+
 
 def packets_from_bytes(data):
     offset = 0