[refactor] move preamble module to common

This move allows server to use it on #8868 as described in #8890

-- Relates: #8890

1 files changed, 88 insertions, 0 deletions

diff --git a/src/leap/soledad/common/preamble.py b/src/leap/soledad/common/preamble.py
new file mode 100644
index 00000000..724923c5
--- /dev/null
+++ b/src/leap/soledad/common/preamble.py
@@ -0,0 +1,88 @@
+# -*- coding: utf-8 -*-
+# preamble.py
+# Copyright (C) 2017 LEAP Encryption Access Project
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+"""
+Preamble is a metadata payload present on encrypted documents. It holds data
+about encryption scheme, iv, document id and sync related data.
+   BLOB_SIGNATURE_MAGIC, -> used to differentiate from other data formats
+   ENC_SCHEME, -> cryptographic scheme (symmetric or asymmetric)
+   ENC_METHOD, -> cipher used, such as AES-GCM or AES-CTR or GPG
+   current_time, -> time.time()
+   self.iv, -> initialization vector if any, or 0 when not applicable
+   str(self.doc_id), -> document id
+   str(self.rev), -> current revision
+   self._content_size) -> size, rounded to ceiling
+"""
+import warnings
+import struct
+import time
+from collections import namedtuple
+PACMAN = struct.Struct('2sbbQ16s255p255pQ')
+LEGACY_PACMAN = struct.Struct('2sbbQ16s255p255p')  # DEPRECATED
+BLOB_SIGNATURE_MAGIC = '\x13\x37'
+ENC_SCHEME = namedtuple('SCHEME', 'symkey')(1)
+ENC_METHOD = namedtuple('METHOD', 'aes_256_ctr aes_256_gcm')(1, 2)
+
+
+class InvalidPreambleException(Exception):
+    pass
+
+
+class Preamble:
+
+    def __init__(self, doc_id, rev, scheme, method,
+                 timestamp=0, iv='', magic=None, content_size=0):
+        self.doc_id = doc_id
+        self.rev = rev
+        self.scheme = scheme
+        self.method = method
+        self.iv = iv
+        self.timestamp = int(timestamp) or int(time.time())
+        self.magic = magic or BLOB_SIGNATURE_MAGIC
+        self.content_size = int(content_size)
+
+    def encode(self):
+        preamble = PACMAN.pack(
+            self.magic,
+            self.scheme,
+            self.method,
+            self.timestamp,
+            self.iv,
+            str(self.doc_id),
+            str(self.rev),
+            self.content_size)
+        return preamble
+
+
+def decode_preamble(encoded_preamble):
+    preamble_size = len(encoded_preamble)
+    try:
+        if preamble_size == LEGACY_PACMAN.size:
+            unpacked_data = LEGACY_PACMAN.unpack(encoded_preamble)
+            magic, sch, meth, ts, iv, doc_id, rev = unpacked_data
+            warnings.warn("Decoding a legacy preamble without size. " +
+                          "This will be deprecated in 0.12. Doc was: " +
+                          "doc_id: %s rev: %s" % (doc_id, rev), Warning)
+            return Preamble(doc_id, rev, sch, meth, ts, iv, magic)
+        elif preamble_size == PACMAN.size:
+            unpacked_data = PACMAN.unpack(encoded_preamble)
+            magic, sch, meth, ts, iv, doc_id, rev, size = unpacked_data
+            return Preamble(doc_id, rev, sch, meth, ts, iv, magic, int(size))
+        else:
+            raise InvalidPreambleException("Unexpected preamble size %d",
+                                           preamble_size)
+    except struct.error as e:
+        raise InvalidPreambleException(e)