Add PGPPacket._string_to_key for calculating decryption keys

So far we only implement the 'simple' string-to-key type.  From RFC
4880 [1]:

  Simple S2K hashes the passphrase to produce the session key.  The
  manner in which this is done depends on the size of the session key
  (which will depend on the cipher used) and the size of the hash
  algorithm's output.  If the hash size is greater than the session
  key size, the high-order (leftmost) octets of the hash are used as
  the key.

  If the hash size is less than the key size, multiple instances of
  the hash context are created -- enough to produce the required key
  data.  These instances are preloaded with 0, 1, 2, ... octets of
  zeros (that is to say, the first instance has no preloading, the
  second gets preloaded with 1 octet of zero, the third is preloaded
  with two octets of zeros, and so forth).

  As the data is hashed, it is given independently to each hash
  context.  Since the contexts have been initialized differently, they
  will each produce different hash output.  Once the passphrase is
  hashed, the output data from the multiple hashes is concatenated,
  first hash leftmost, to produce the key data, with any excess octets
  on the right discarded.

[1]: http://tools.ietf.org/search/rfc4880#section-3.7.1.1

diff --git a/gpg-migrate.py b/gpg-migrate.py
index 393d4603cb8d72f141339b45172b50bbba44431d..0bf1133aca14eb87564fad122b5dccde65cac768 100755 (executable)
--- a/gpg-migrate.py
+++ b/gpg-migrate.py
@@ -848,6 +848,32 @@ class PGPPacket (dict):
                     self['public-key-algorithm']))
         return b''.join(chunks)
 
+    def _string_to_key(self, string, key_size):
+        if key_size % 8:
+            raise ValueError(
+                '{}-bit key is not an integer number of bytes'.format(
+                    key_size))
+        key_size_bytes = key_size // 8
+        hash_name = self._hashlib_name[
+            self['string-to-key-hash-algorithm']]
+        string_hash = _hashlib.new(hash_name)
+        hashes = _math.ceil(key_size_bytes / string_hash.digest_size)
+        key = b''
+        if self['string-to-key-type'] == 'simple':
+            update_bytes = string
+        else:
+            raise NotImplementedError(
+                'key calculation for string-to-key type {}'.format(
+                    self['string-to-key-type']))
+        for padding in range(hashes):
+            string_hash = _hashlib.new(hash_name)
+            string_hash.update(padding * b'\x00')
+            if self['string-to-key-type'] == 'simple':
+                string_hash.update(update_bytes)
+            key += string_hash.digest()
+        key = key[:key_size_bytes]
+        return key
+
     def decrypt_symmetric_encryption(self, data):
         """Decrypt OpenPGP's Cipher Feedback mode"""
         algorithm = self['symmetric-encryption-algorithm']
@@ -865,8 +891,9 @@ class PGPPacket (dict):
         passphrase = _getpass.getpass(
             'passphrase for {}: '.format(self['fingerprint'][-8:]))
         passphrase = passphrase.encode('ascii')
+        key = self._string_to_key(string=passphrase, key_size=key_size)
         cipher = module.new(
-            key=passphrase,
+            key=key,
             mode=module.MODE_CFB,
             IV=self['initial-vector'],
             segment_size=segment_size_bits)