initially write dummy header during encryption

diff --git a/deltatar/crypto.py b/deltatar/crypto.py
index b389ef6..f7ca6e3 100755 (executable)
--- a/deltatar/crypto.py
+++ b/deltatar/crypto.py
@@ -32,6 +32,7 @@ passed in advance: https://github.com/pyca/cryptography/pull/3421
 import binascii
 import ctypes
 import io
+from functools import reduce
 import os
 import struct
 import sys
@@ -75,12 +76,12 @@ ENCRYPTION_PARAMETERS = \
 
 I2N_HDR_MAGIC = b"PDTCRYPT"
 
-I2N_HDR_SIZE_MAGIC          = 8
-I2N_HDR_SIZE_VERSION        = 2
-I2N_HDR_SIZE_PARAMVERSION   = 2
-I2N_HDR_SIZE_NACL           = 16
-I2N_HDR_SIZE_IV             = 12
-I2N_HDR_SIZE_CTSIZE         = 8
+I2N_HDR_SIZE_MAGIC          = 8     #  8
+I2N_HDR_SIZE_VERSION        = 2     # 10
+I2N_HDR_SIZE_PARAMVERSION   = 2     # 12
+I2N_HDR_SIZE_NACL           = 16    # 28
+I2N_HDR_SIZE_IV             = 12    # 40
+I2N_HDR_SIZE_CTSIZE         = 8     # 48
 I2N_TLR_SIZE_TAG            = 16 # GCM auth tag, appended to data
 
 I2N_HDR_SIZE = I2N_HDR_SIZE_MAGIC        + I2N_HDR_SIZE_VERSION \
@@ -183,6 +184,16 @@ def hdr_from_params (version, paramversion, nacl, iv, ctsize=None):
     return True, bytes (buf)
 
 
+def hdr_make_dummy (s):
+    """
+    Create a header sized block of bytes initialized to a value derived from a
+    string. Used to verify we’ve jumped back correctly to the actual position
+    of the object header.
+    """
+    c = reduce (lambda a, c: a + ord(c), s, 0) % 0xFF
+    return bytearray (struct.pack ("B", c)) * I2N_HDR_SIZE
+
+
 def hdr_make (hdr):
     return hdr_from_params (version=hdr.get("version"),
                             paramversion=hdr.get("paramversion"),
@@ -315,11 +326,17 @@ class Crypto (object):
 
 
     def process (self, buf):
-        self.aes.update (buf)
+        if self.aes is not None:
+            return self.aes.update (buf)
+        # this branch is taken when .close() is called on the fileobj
+        return b""
 
 
 class Encrypt (Crypto):
 
+    curobj = None
+    hdrdum = None
+
     def __init__ (self, password, paramversion, nacl=None):
         if nacl is None:
             _, params = kdf_by_version (paramversion)
@@ -331,11 +348,9 @@ class Encrypt (Crypto):
         return struct.pack("<8sL", self.pfx, self.cnt % 0xffFFffFF)
 
 
-    def next (self, filename, version, paramversion, nacl, ctsize=None):
+    def next (self, filename, version, paramversion, nacl):
         iv = self.iv_make()
-        ok, hdr = hdr_from_params (version, paramversion, nacl, iv, ctsize)
-        if ok is False:
-            return None
+        self.curobj = (filename, version, paramversion, nacl)
         self.cnt += 1
         aad = "%s" % filename
         self.aes = Cipher \
@@ -343,13 +358,21 @@ class Encrypt (Crypto):
                         , modes.GCM (iv)
                         , backend = default_backend ()) \
                         .encryptor ()
-        return hdr
-
-        return self.aes.authenticate_additional_data (str.encode (aad))
+        self.aes.authenticate_additional_data (str.encode (aad))
+        self.hdrdum = hdr_make_dummy (filename)
+        return self.hdrdum
 
 
-    def done (self):
-        return self.aes.finalize ()
+    def done (self, cmpdata, ctsize):
+        if cmpdata != self.hdrdum:
+            raise "XXX bad sync for writing header" ## we need to converge on a sensible error handling strategy
+        data, tag = self.aes.finalize () ## XXX we could also put the tag in the header
+        ctsize += len (data)
+        (filename, version, paramversion, nacl) = self.curobj
+        ok, hdr = hdr_from_params (version, paramversion, nacl, iv, ctsize)
+        if ok is False:
+            raise "XXX error constructing header" ## we need to converge on a sensible error handling strategy
+        return data, tag, hdr
 
 
 class Decrypt (Crypto):