diff --git a/scripts/update_payload/__init__.py b/scripts/update_payload/__init__.py
index 8ee95e2..6e77678 100644
--- a/scripts/update_payload/__init__.py
+++ b/scripts/update_payload/__init__.py
@@ -17,6 +17,8 @@
 """Library for processing, verifying and applying Chrome OS update payloads."""
 
 # Just raise the interface classes to the root namespace.
+from __future__ import absolute_import
+
 from update_payload.checker import CHECKS_TO_DISABLE
 from update_payload.error import PayloadError
 from update_payload.payload import Payload
diff --git a/scripts/update_payload/applier.py b/scripts/update_payload/applier.py
index 511ed49..7830c00 100644
--- a/scripts/update_payload/applier.py
+++ b/scripts/update_payload/applier.py
@@ -24,6 +24,7 @@
 
 """
 
+from __future__ import absolute_import
 from __future__ import print_function
 
 import array
@@ -70,7 +71,7 @@
   """
   hasher = hashlib.sha256()
   block_length = 1024 * 1024
-  max_length = length if length >= 0 else sys.maxint
+  max_length = length if length >= 0 else sys.maxsize
 
   while max_length > 0:
     read_length = min(max_length, block_length)
@@ -108,7 +109,7 @@
   """
   data = array.array('c')
   if max_length < 0:
-    max_length = sys.maxint
+    max_length = sys.maxsize
   for ex in extents:
     if max_length == 0:
       break
@@ -176,7 +177,7 @@
   arg = ''
   pad_off = pad_len = 0
   if data_length < 0:
-    data_length = sys.maxint
+    data_length = sys.maxsize
   for ex, ex_name in common.ExtentIter(extents, base_name):
     if not data_length:
       raise PayloadError('%s: more extents than total data length' % ex_name)
@@ -416,7 +417,7 @@
                          "--dst_extents=%s" % out_extents_arg]
         subprocess.check_call(puffpatch_cmd)
       else:
-        raise PayloadError("Unknown operation %s", op.type)
+        raise PayloadError("Unknown operation %s" % op.type)
 
       # Pad with zeros past the total output length.
       if pad_len:
@@ -451,7 +452,7 @@
                          "--patch_file=%s" % patch_file_name]
         subprocess.check_call(puffpatch_cmd)
       else:
-        raise PayloadError("Unknown operation %s", op.type)
+        raise PayloadError("Unknown operation %s" % op.type)
 
       # Read output.
       with open(out_file_name, 'rb') as out_file:
diff --git a/scripts/update_payload/checker.py b/scripts/update_payload/checker.py
index 4558872..4c65516 100644
--- a/scripts/update_payload/checker.py
+++ b/scripts/update_payload/checker.py
@@ -24,6 +24,7 @@
   checker.Run(...)
 """
 
+from __future__ import absolute_import
 from __future__ import print_function
 
 import array
@@ -34,13 +35,14 @@
 import os
 import subprocess
 
+from six.moves import range
+
 from update_payload import common
 from update_payload import error
 from update_payload import format_utils
 from update_payload import histogram
 from update_payload import update_metadata_pb2
 
-
 #
 # Constants.
 #
@@ -71,6 +73,7 @@
     6: (_TYPE_DELTA,),
 }
 
+
 #
 # Helper functions.
 #
@@ -647,7 +650,7 @@
             'Apparent full payload contains old_{kernel,rootfs}_info.')
       self.payload_type = _TYPE_DELTA
 
-      for part, (msg, part_report) in self.old_part_info.iteritems():
+      for part, (msg, part_report) in self.old_part_info.items():
         # Check: {size, hash} present in old_{kernel,rootfs}_info.
         field = 'old_%s_info' % part
         self.old_fs_sizes[part] = self._CheckMandatoryField(msg, 'size',
@@ -668,7 +671,7 @@
       self.payload_type = _TYPE_FULL
 
     # Check: new_{kernel,rootfs}_info present; contains {size, hash}.
-    for part, (msg, part_report) in self.new_part_info.iteritems():
+    for part, (msg, part_report) in self.new_part_info.items():
       field = 'new_%s_info' % part
       self.new_fs_sizes[part] = self._CheckMandatoryField(msg, 'size',
                                                           part_report, field)
@@ -740,7 +743,7 @@
             (ex_name, common.FormatExtent(ex, self.block_size), usable_size))
 
       # Record block usage.
-      for i in xrange(start_block, end_block):
+      for i in range(start_block, end_block):
         block_counters[i] += 1
 
       total_num_blocks += num_blocks
@@ -759,6 +762,11 @@
     Raises:
       error.PayloadError if any check fails.
     """
+    # Check: total_dst_blocks is not a floating point.
+    if isinstance(total_dst_blocks, float):
+      raise error.PayloadError('%s: contains invalid data type of '
+                               'total_dst_blocks.' % op_name)
+
     # Check: Does not contain src extents.
     if op.src_extents:
       raise error.PayloadError('%s: contains src_extents.' % op_name)
@@ -975,7 +983,7 @@
 
   def _SizeToNumBlocks(self, size):
     """Returns the number of blocks needed to contain a given byte size."""
-    return (size + self.block_size - 1) / self.block_size
+    return (size + self.block_size - 1) // self.block_size
 
   def _AllocBlockCounters(self, total_size):
     """Returns a freshly initialized array of block counters.
@@ -1054,7 +1062,7 @@
       op_num += 1
 
       # Check: Type is valid.
-      if op.type not in op_counts.keys():
+      if op.type not in op_counts:
         raise error.PayloadError('%s: invalid type (%d).' % (op_name, op.type))
       op_counts[op.type] += 1
 
@@ -1127,7 +1135,6 @@
         raise error.PayloadError('It seems like the last operation is the '
                                  'signature blob. This is an invalid payload.')
 
-
     # Compute the checksum of all data up to signature blob.
     # TODO(garnold) we're re-reading the whole data section into a string
     # just to compute the checksum; instead, we could do it incrementally as
diff --git a/scripts/update_payload/checker_unittest.py b/scripts/update_payload/checker_unittest.py
index 4881653..993b785 100755
--- a/scripts/update_payload/checker_unittest.py
+++ b/scripts/update_payload/checker_unittest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright (C) 2013 The Android Open Source Project
 #
@@ -17,30 +17,33 @@
 
 """Unit testing checker.py."""
 
-from __future__ import print_function
+# Disable check for function names to avoid errors based on old code
+# pylint: disable-msg=invalid-name
+
+from __future__ import absolute_import
 
 import array
 import collections
-import cStringIO
 import hashlib
+import io
 import itertools
 import os
 import unittest
 
-# pylint cannot find mox.
-# pylint: disable=F0401
-import mox
+from six.moves import zip
+
+import mock  # pylint: disable=import-error
 
 from update_payload import checker
 from update_payload import common
 from update_payload import test_utils
 from update_payload import update_metadata_pb2
 from update_payload.error import PayloadError
-from update_payload.payload import Payload # Avoid name conflicts later.
+from update_payload.payload import Payload  # Avoid name conflicts later.
 
 
 def _OpTypeByName(op_name):
-  """Returns the type of an operation from itsname."""
+  """Returns the type of an operation from its name."""
   op_name_to_type = {
       'REPLACE': common.OpType.REPLACE,
       'REPLACE_BZ': common.OpType.REPLACE_BZ,
@@ -63,7 +66,7 @@
   if checker_init_dargs is None:
     checker_init_dargs = {}
 
-  payload_file = cStringIO.StringIO()
+  payload_file = io.BytesIO()
   payload_gen_write_to_file_func(payload_file, **payload_gen_dargs)
   payload_file.seek(0)
   payload = Payload(payload_file)
@@ -73,7 +76,7 @@
 
 def _GetPayloadCheckerWithData(payload_gen):
   """Returns a payload checker from a given payload generator."""
-  payload_file = cStringIO.StringIO()
+  payload_file = io.BytesIO()
   payload_gen.WriteToFile(payload_file)
   payload_file.seek(0)
   payload = Payload(payload_file)
@@ -87,7 +90,7 @@
 # pylint: disable=W0212
 # Don't bark about missing members of classes you cannot import.
 # pylint: disable=E1101
-class PayloadCheckerTest(mox.MoxTestBase):
+class PayloadCheckerTest(unittest.TestCase):
   """Tests the PayloadChecker class.
 
   In addition to ordinary testFoo() methods, which are automatically invoked by
@@ -100,11 +103,42 @@
   all such tests is done in AddAllParametricTests().
   """
 
+  def setUp(self):
+    """setUp function for unittest testcase"""
+    self.mock_checks = []
+
+  def tearDown(self):
+    """tearDown function for unittest testcase"""
+    # Verify that all mock functions were called.
+    for check in self.mock_checks:
+      check.mock_fn.assert_called_once_with(*check.exp_args, **check.exp_kwargs)
+
+  class MockChecksAtTearDown(object):
+    """Mock data storage.
+
+    This class stores the mock functions and its arguments to be checked at a
+    later point.
+    """
+    def __init__(self, mock_fn, *args, **kwargs):
+      self.mock_fn = mock_fn
+      self.exp_args = args
+      self.exp_kwargs = kwargs
+
+  def addPostCheckForMockFunction(self, mock_fn, *args, **kwargs):
+    """Store a mock function and its arguments to self.mock_checks
+
+    Args:
+      mock_fn: mock function object
+      args: expected positional arguments for the mock_fn
+      kwargs: expected named arguments for the mock_fn
+    """
+    self.mock_checks.append(self.MockChecksAtTearDown(mock_fn, *args, **kwargs))
+
   def MockPayload(self):
     """Create a mock payload object, complete with a mock manifest."""
-    payload = self.mox.CreateMock(Payload)
+    payload = mock.create_autospec(Payload)
     payload.is_init = True
-    payload.manifest = self.mox.CreateMock(
+    payload.manifest = mock.create_autospec(
         update_metadata_pb2.DeltaArchiveManifest)
     return payload
 
@@ -173,19 +207,20 @@
     subreport = 'fake subreport'
 
     # Create a mock message.
-    msg = self.mox.CreateMock(update_metadata_pb2._message.Message)
-    msg.HasField(name).AndReturn(is_present)
+    msg = mock.create_autospec(update_metadata_pb2._message.Message)
+    self.addPostCheckForMockFunction(msg.HasField, name)
+    msg.HasField.return_value = is_present
     setattr(msg, name, val)
-
     # Create a mock report.
-    report = self.mox.CreateMock(checker._PayloadReport)
+    report = mock.create_autospec(checker._PayloadReport)
     if is_present:
       if is_submsg:
-        report.AddSubReport(name).AndReturn(subreport)
+        self.addPostCheckForMockFunction(report.AddSubReport, name)
+        report.AddSubReport.return_value = subreport
       else:
-        report.AddField(name, convert(val), linebreak=linebreak, indent=indent)
+        self.addPostCheckForMockFunction(report.AddField, name, convert(val),
+                                         linebreak=linebreak, indent=indent)
 
-    self.mox.ReplayAll()
     return (msg, report, subreport, name, val)
 
   def DoAddElemTest(self, is_present, is_mandatory, is_submsg, convert,
@@ -211,9 +246,9 @@
     else:
       ret_val, ret_subreport = checker.PayloadChecker._CheckElem(*args,
                                                                  **kwargs)
-      self.assertEquals(val if is_present else None, ret_val)
-      self.assertEquals(subreport if is_present and is_submsg else None,
-                        ret_subreport)
+      self.assertEqual(val if is_present else None, ret_val)
+      self.assertEqual(subreport if is_present and is_submsg else None,
+                       ret_subreport)
 
   def DoAddFieldTest(self, is_mandatory, is_present, convert, linebreak,
                      indent):
@@ -243,7 +278,7 @@
       self.assertRaises(PayloadError, tested_func, *args, **kwargs)
     else:
       ret_val = tested_func(*args, **kwargs)
-      self.assertEquals(val if is_present else None, ret_val)
+      self.assertEqual(val if is_present else None, ret_val)
 
   def DoAddSubMsgTest(self, is_mandatory, is_present):
     """Parametrized testing of _Check{Mandatory,Optional}SubMsg().
@@ -267,8 +302,8 @@
       self.assertRaises(PayloadError, tested_func, *args)
     else:
       ret_val, ret_subreport = tested_func(*args)
-      self.assertEquals(val if is_present else None, ret_val)
-      self.assertEquals(subreport if is_present else None, ret_subreport)
+      self.assertEqual(val if is_present else None, ret_val)
+      self.assertEqual(subreport if is_present else None, ret_subreport)
 
   def testCheckPresentIff(self):
     """Tests _CheckPresentIff()."""
@@ -294,15 +329,14 @@
       returned_signed_hash: The signed hash data retuned by openssl.
       expected_signed_hash: The signed hash data to compare against.
     """
-    try:
-      # Stub out the subprocess invocation.
-      self.mox.StubOutWithMock(checker.PayloadChecker, '_Run')
+    # Stub out the subprocess invocation.
+    with mock.patch.object(checker.PayloadChecker, '_Run') \
+         as mock_payload_checker:
       if expect_subprocess_call:
-        checker.PayloadChecker._Run(
-            mox.IsA(list), send_data=sig_data).AndReturn(
-                (sig_asn1_header + returned_signed_hash, None))
+        mock_payload_checker([], send_data=sig_data)
+        mock_payload_checker.return_value = (
+            sig_asn1_header + returned_signed_hash, None)
 
-      self.mox.ReplayAll()
       if expect_pass:
         self.assertIsNone(checker.PayloadChecker._CheckSha256Signature(
             sig_data, 'foo', expected_signed_hash, 'bar'))
@@ -310,13 +344,11 @@
         self.assertRaises(PayloadError,
                           checker.PayloadChecker._CheckSha256Signature,
                           sig_data, 'foo', expected_signed_hash, 'bar')
-    finally:
-      self.mox.UnsetStubs()
 
   def testCheckSha256Signature_Pass(self):
     """Tests _CheckSha256Signature(); pass case."""
     sig_data = 'fake-signature'.ljust(256)
-    signed_hash = hashlib.sha256('fake-data').digest()
+    signed_hash = hashlib.sha256(b'fake-data').digest()
     self.DoCheckSha256SignatureTest(True, True, sig_data,
                                     common.SIG_ASN1_HEADER, signed_hash,
                                     signed_hash)
@@ -324,7 +356,7 @@
   def testCheckSha256Signature_FailBadSignature(self):
     """Tests _CheckSha256Signature(); fails due to malformed signature."""
     sig_data = 'fake-signature'  # Malformed (not 256 bytes in length).
-    signed_hash = hashlib.sha256('fake-data').digest()
+    signed_hash = hashlib.sha256(b'fake-data').digest()
     self.DoCheckSha256SignatureTest(False, False, sig_data,
                                     common.SIG_ASN1_HEADER, signed_hash,
                                     signed_hash)
@@ -332,7 +364,7 @@
   def testCheckSha256Signature_FailBadOutputLength(self):
     """Tests _CheckSha256Signature(); fails due to unexpected output length."""
     sig_data = 'fake-signature'.ljust(256)
-    signed_hash = 'fake-hash'  # Malformed (not 32 bytes in length).
+    signed_hash = b'fake-hash'  # Malformed (not 32 bytes in length).
     self.DoCheckSha256SignatureTest(False, True, sig_data,
                                     common.SIG_ASN1_HEADER, signed_hash,
                                     signed_hash)
@@ -340,16 +372,16 @@
   def testCheckSha256Signature_FailBadAsnHeader(self):
     """Tests _CheckSha256Signature(); fails due to bad ASN1 header."""
     sig_data = 'fake-signature'.ljust(256)
-    signed_hash = hashlib.sha256('fake-data').digest()
-    bad_asn1_header = 'bad-asn-header'.ljust(len(common.SIG_ASN1_HEADER))
+    signed_hash = hashlib.sha256(b'fake-data').digest()
+    bad_asn1_header = b'bad-asn-header'.ljust(len(common.SIG_ASN1_HEADER))
     self.DoCheckSha256SignatureTest(False, True, sig_data, bad_asn1_header,
                                     signed_hash, signed_hash)
 
   def testCheckSha256Signature_FailBadHash(self):
     """Tests _CheckSha256Signature(); fails due to bad hash returned."""
     sig_data = 'fake-signature'.ljust(256)
-    expected_signed_hash = hashlib.sha256('fake-data').digest()
-    returned_signed_hash = hashlib.sha256('bad-fake-data').digest()
+    expected_signed_hash = hashlib.sha256(b'fake-data').digest()
+    returned_signed_hash = hashlib.sha256(b'bad-fake-data').digest()
     self.DoCheckSha256SignatureTest(False, True, sig_data,
                                     common.SIG_ASN1_HEADER,
                                     expected_signed_hash, returned_signed_hash)
@@ -455,23 +487,23 @@
     # Add old kernel/rootfs partition info, as required.
     if fail_mismatched_oki_ori or fail_old_kernel_fs_size or fail_bad_oki:
       oki_hash = (None if fail_bad_oki
-                  else hashlib.sha256('fake-oki-content').digest())
+                  else hashlib.sha256(b'fake-oki-content').digest())
       payload_gen.SetPartInfo(common.KERNEL, False, old_kernel_fs_size,
                               oki_hash)
     if not fail_mismatched_oki_ori and (fail_old_rootfs_fs_size or
                                         fail_bad_ori):
       ori_hash = (None if fail_bad_ori
-                  else hashlib.sha256('fake-ori-content').digest())
+                  else hashlib.sha256(b'fake-ori-content').digest())
       payload_gen.SetPartInfo(common.ROOTFS, False, old_rootfs_fs_size,
                               ori_hash)
 
     # Add new kernel/rootfs partition info.
     payload_gen.SetPartInfo(
         common.KERNEL, True, new_kernel_fs_size,
-        None if fail_bad_nki else hashlib.sha256('fake-nki-content').digest())
+        None if fail_bad_nki else hashlib.sha256(b'fake-nki-content').digest())
     payload_gen.SetPartInfo(
         common.ROOTFS, True, new_rootfs_fs_size,
-        None if fail_bad_nri else hashlib.sha256('fake-nri-content').digest())
+        None if fail_bad_nri else hashlib.sha256(b'fake-nri-content').digest())
 
     # Set the minor version.
     payload_gen.SetMinorVersion(0)
@@ -518,7 +550,7 @@
 
     # Passes w/ all real extents.
     extents = self.NewExtentList((0, 4), (8, 3), (1024, 16))
-    self.assertEquals(
+    self.assertEqual(
         23,
         payload_checker._CheckExtents(extents, (1024 + 16) * block_size,
                                       collections.defaultdict(int), 'foo'))
@@ -553,34 +585,34 @@
     block_size = payload_checker.block_size
     data_length = 10000
 
-    op = self.mox.CreateMock(
-        update_metadata_pb2.InstallOperation)
+    op = mock.create_autospec(update_metadata_pb2.InstallOperation)
     op.type = common.OpType.REPLACE
 
     # Pass.
     op.src_extents = []
     self.assertIsNone(
         payload_checker._CheckReplaceOperation(
-            op, data_length, (data_length + block_size - 1) / block_size,
+            op, data_length, (data_length + block_size - 1) // block_size,
             'foo'))
 
     # Fail, src extents founds.
     op.src_extents = ['bar']
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, data_length, (data_length + block_size - 1) / block_size, 'foo')
+        op, data_length, (data_length + block_size - 1) // block_size, 'foo')
 
     # Fail, missing data.
     op.src_extents = []
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, None, (data_length + block_size - 1) / block_size, 'foo')
+        op, None, (data_length + block_size - 1) // block_size, 'foo')
 
     # Fail, length / block number mismatch.
     op.src_extents = ['bar']
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, data_length, (data_length + block_size - 1) / block_size + 1, 'foo')
+        op, data_length, (data_length + block_size - 1) // block_size + 1,
+        'foo')
 
   def testCheckReplaceBzOperation(self):
     """Tests _CheckReplaceOperation() where op.type == REPLACE_BZ."""
@@ -588,7 +620,7 @@
     block_size = payload_checker.block_size
     data_length = block_size * 3
 
-    op = self.mox.CreateMock(
+    op = mock.create_autospec(
         update_metadata_pb2.InstallOperation)
     op.type = common.OpType.REPLACE_BZ
 
@@ -596,25 +628,32 @@
     op.src_extents = []
     self.assertIsNone(
         payload_checker._CheckReplaceOperation(
-            op, data_length, (data_length + block_size - 1) / block_size + 5,
+            op, data_length, (data_length + block_size - 1) // block_size + 5,
             'foo'))
 
     # Fail, src extents founds.
     op.src_extents = ['bar']
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, data_length, (data_length + block_size - 1) / block_size + 5, 'foo')
+        op, data_length, (data_length + block_size - 1) // block_size + 5,
+        'foo')
 
     # Fail, missing data.
     op.src_extents = []
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, None, (data_length + block_size - 1) / block_size, 'foo')
+        op, None, (data_length + block_size - 1) // block_size, 'foo')
 
     # Fail, too few blocks to justify BZ.
     op.src_extents = []
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
+        op, data_length, (data_length + block_size - 1) // block_size, 'foo')
+
+    # Fail, total_dst_blocks is a floating point value.
+    op.src_extents = []
+    self.assertRaises(
+        PayloadError, payload_checker._CheckReplaceOperation,
         op, data_length, (data_length + block_size - 1) / block_size, 'foo')
 
   def testCheckReplaceXzOperation(self):
@@ -623,7 +662,7 @@
     block_size = payload_checker.block_size
     data_length = block_size * 3
 
-    op = self.mox.CreateMock(
+    op = mock.create_autospec(
         update_metadata_pb2.InstallOperation)
     op.type = common.OpType.REPLACE_XZ
 
@@ -631,25 +670,32 @@
     op.src_extents = []
     self.assertIsNone(
         payload_checker._CheckReplaceOperation(
-            op, data_length, (data_length + block_size - 1) / block_size + 5,
+            op, data_length, (data_length + block_size - 1) // block_size + 5,
             'foo'))
 
     # Fail, src extents founds.
     op.src_extents = ['bar']
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, data_length, (data_length + block_size - 1) / block_size + 5, 'foo')
+        op, data_length, (data_length + block_size - 1) // block_size + 5,
+        'foo')
 
     # Fail, missing data.
     op.src_extents = []
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
-        op, None, (data_length + block_size - 1) / block_size, 'foo')
+        op, None, (data_length + block_size - 1) // block_size, 'foo')
 
     # Fail, too few blocks to justify XZ.
     op.src_extents = []
     self.assertRaises(
         PayloadError, payload_checker._CheckReplaceOperation,
+        op, data_length, (data_length + block_size - 1) // block_size, 'foo')
+
+    # Fail, total_dst_blocks is a floating point value.
+    op.src_extents = []
+    self.assertRaises(
+        PayloadError, payload_checker._CheckReplaceOperation,
         op, data_length, (data_length + block_size - 1) / block_size, 'foo')
 
   def testCheckAnyDiff(self):
@@ -724,9 +770,9 @@
     old_part_size = test_utils.MiB(4)
     new_part_size = test_utils.MiB(8)
     old_block_counters = array.array(
-        'B', [0] * ((old_part_size + block_size - 1) / block_size))
+        'B', [0] * ((old_part_size + block_size - 1) // block_size))
     new_block_counters = array.array(
-        'B', [0] * ((new_part_size + block_size - 1) / block_size))
+        'B', [0] * ((new_part_size + block_size - 1) // block_size))
     prev_data_offset = 1876
     blob_hash_counts = collections.defaultdict(int)
 
@@ -769,16 +815,14 @@
       fake_data = 'fake-data'.ljust(op.data_length)
       if not allow_unhashed and not fail_data_hash:
         # Create a valid data blob hash.
-        op.data_sha256_hash = hashlib.sha256(fake_data).digest()
-        payload.ReadDataBlob(op.data_offset, op.data_length).AndReturn(
-            fake_data)
+        op.data_sha256_hash = hashlib.sha256(fake_data.encode('utf-8')).digest()
+        payload.ReadDataBlob.return_value = fake_data.encode('utf-8')
 
       elif fail_data_hash:
         # Create an invalid data blob hash.
         op.data_sha256_hash = hashlib.sha256(
-            fake_data.replace(' ', '-')).digest()
-        payload.ReadDataBlob(op.data_offset, op.data_length).AndReturn(
-            fake_data)
+            fake_data.replace(' ', '-').encode('utf-8')).digest()
+        payload.ReadDataBlob.return_value = fake_data.encode('utf-8')
 
     total_dst_blocks = 0
     if not fail_missing_dst_extents:
@@ -807,7 +851,6 @@
             payload_checker.minor_version <= 3):
         op.dst_length = total_dst_blocks * block_size
 
-    self.mox.ReplayAll()
     should_fail = (fail_src_extents or fail_dst_extents or
                    fail_mismatched_data_offset_length or
                    fail_missing_dst_extents or fail_src_length or
@@ -857,7 +900,8 @@
       rootfs_data_length -= block_size
 
     payload_gen.AddOperation(common.ROOTFS, rootfs_op_type,
-                             dst_extents=[(0, rootfs_data_length / block_size)],
+                             dst_extents=
+                             [(0, rootfs_data_length // block_size)],
                              data_offset=0,
                              data_length=rootfs_data_length)
 
@@ -889,13 +933,13 @@
     rootfs_part_size = test_utils.MiB(2)
     kernel_part_size = test_utils.KiB(16)
     payload_gen.SetPartInfo(common.ROOTFS, True, rootfs_part_size,
-                            hashlib.sha256('fake-new-rootfs-content').digest())
+                            hashlib.sha256(b'fake-new-rootfs-content').digest())
     payload_gen.SetPartInfo(common.KERNEL, True, kernel_part_size,
-                            hashlib.sha256('fake-new-kernel-content').digest())
+                            hashlib.sha256(b'fake-new-kernel-content').digest())
     payload_gen.SetMinorVersion(0)
     payload_gen.AddOperationWithData(
         common.ROOTFS, common.OpType.REPLACE,
-        dst_extents=[(0, rootfs_part_size / block_size)],
+        dst_extents=[(0, rootfs_part_size // block_size)],
         data_blob=os.urandom(rootfs_part_size))
 
     do_forge_sigs_data = (fail_empty_sigs_blob or fail_sig_missing_fields or
@@ -908,7 +952,7 @@
         if fail_sig_missing_fields:
           sig_data = None
         else:
-          sig_data = test_utils.SignSha256('fake-payload-content',
+          sig_data = test_utils.SignSha256(b'fake-payload-content',
                                            test_utils._PRIVKEY_FILE_NAME)
         sigs_gen.AddSig(5 if fail_unknown_sig_version else 1, sig_data)
 
@@ -984,9 +1028,9 @@
     kernel_filesystem_size = test_utils.KiB(16)
     rootfs_filesystem_size = test_utils.MiB(2)
     payload_gen.SetPartInfo(common.ROOTFS, True, rootfs_filesystem_size,
-                            hashlib.sha256('fake-new-rootfs-content').digest())
+                            hashlib.sha256(b'fake-new-rootfs-content').digest())
     payload_gen.SetPartInfo(common.KERNEL, True, kernel_filesystem_size,
-                            hashlib.sha256('fake-new-kernel-content').digest())
+                            hashlib.sha256(b'fake-new-kernel-content').digest())
     payload_gen.SetMinorVersion(0)
 
     rootfs_part_size = 0
@@ -997,7 +1041,7 @@
       rootfs_op_size += block_size
     payload_gen.AddOperationWithData(
         common.ROOTFS, common.OpType.REPLACE,
-        dst_extents=[(0, rootfs_op_size / block_size)],
+        dst_extents=[(0, rootfs_op_size // block_size)],
         data_blob=os.urandom(rootfs_op_size))
 
     kernel_part_size = 0
@@ -1008,7 +1052,7 @@
       kernel_op_size += block_size
     payload_gen.AddOperationWithData(
         common.KERNEL, common.OpType.REPLACE,
-        dst_extents=[(0, kernel_op_size / block_size)],
+        dst_extents=[(0, kernel_op_size // block_size)],
         data_blob=os.urandom(kernel_op_size))
 
     # Generate payload (complete w/ signature) and create the test object.
@@ -1054,6 +1098,7 @@
       else:
         self.assertIsNone(payload_checker.Run(**kwargs2))
 
+
 # This implements a generic API, hence the occasional unused args.
 # pylint: disable=W0613
 def ValidateCheckOperationTest(op_type_name, allow_unhashed,
@@ -1104,13 +1149,13 @@
                (values) associated with them.
     validate_func: A function used for validating test argument combinations.
   """
-  for value_tuple in itertools.product(*arg_space.itervalues()):
-    run_dargs = dict(zip(arg_space.iterkeys(), value_tuple))
+  for value_tuple in itertools.product(*iter(arg_space.values())):
+    run_dargs = dict(zip(iter(arg_space.keys()), value_tuple))
     if validate_func and not validate_func(**run_dargs):
       continue
     run_method_name = 'Do%sTest' % tested_method_name
     test_method_name = 'test%s' % tested_method_name
-    for arg_key, arg_val in run_dargs.iteritems():
+    for arg_key, arg_val in run_dargs.items():
       if arg_val or isinstance(arg_val, int):
         test_method_name += '__%s=%s' % (arg_key, arg_val)
     setattr(PayloadCheckerTest, test_method_name,
diff --git a/scripts/update_payload/common.py b/scripts/update_payload/common.py
index dfb8181..b934cf8 100644
--- a/scripts/update_payload/common.py
+++ b/scripts/update_payload/common.py
@@ -16,8 +16,11 @@
 
 """Utilities for update payload processing."""
 
+from __future__ import absolute_import
 from __future__ import print_function
 
+import base64
+
 from update_payload import update_metadata_pb2
 from update_payload.error import PayloadError
 
@@ -26,9 +29,9 @@
 # Constants.
 #
 SIG_ASN1_HEADER = (
-    '\x30\x31\x30\x0d\x06\x09\x60\x86'
-    '\x48\x01\x65\x03\x04\x02\x01\x05'
-    '\x00\x04\x20'
+    b'\x30\x31\x30\x0d\x06\x09\x60\x86'
+    b'\x48\x01\x65\x03\x04\x02\x01\x05'
+    b'\x00\x04\x20'
 )
 
 BRILLO_MAJOR_PAYLOAD_VERSION = 2
@@ -43,6 +46,7 @@
 # Tuple of (name in system, name in protobuf).
 CROS_PARTITIONS = ((KERNEL, KERNEL), (ROOTFS, 'rootfs'))
 
+
 #
 # Payload operation types.
 #
@@ -138,7 +142,7 @@
 
   try:
     data = file_obj.read(length)
-  except IOError, e:
+  except IOError as e:
     raise PayloadError('error reading from file (%s): %s' % (file_obj.name, e))
 
   if len(data) != length:
@@ -164,7 +168,7 @@
 
 def FormatSha256(digest):
   """Returns a canonical string representation of a SHA256 digest."""
-  return digest.encode('base64').strip()
+  return base64.b64encode(digest).decode('utf-8')
 
 
 #
diff --git a/scripts/update_payload/format_utils.py b/scripts/update_payload/format_utils.py
index 6248ba9..e73badf 100644
--- a/scripts/update_payload/format_utils.py
+++ b/scripts/update_payload/format_utils.py
@@ -16,6 +16,8 @@
 
 """Various formatting functions."""
 
+from __future__ import division
+
 
 def NumToPercent(num, total, min_precision=1, max_precision=5):
   """Returns the percentage (string) of |num| out of |total|.
@@ -50,7 +52,7 @@
   precision = min(min_precision, max_precision)
   factor = 10 ** precision
   while precision <= max_precision:
-    percent = num * 100 * factor / total
+    percent = num * 100 * factor // total
     if percent:
       break
     factor *= 10
@@ -102,8 +104,8 @@
     magnitude = next_magnitude
 
   if exp != 0:
-    whole = size / magnitude
-    frac = (size % magnitude) * (10 ** precision) / magnitude
+    whole = size // magnitude
+    frac = (size % magnitude) * (10 ** precision) // magnitude
     while frac and not frac % 10:
       frac /= 10
     return '%d%s %s' % (whole, '.%d' % frac if frac else '', suffixes[exp - 1])
diff --git a/scripts/update_payload/format_utils_unittest.py b/scripts/update_payload/format_utils_unittest.py
index 42ea621..4dcd652 100755
--- a/scripts/update_payload/format_utils_unittest.py
+++ b/scripts/update_payload/format_utils_unittest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright (C) 2013 The Android Open Source Project
 #
@@ -17,6 +17,11 @@
 
 """Unit tests for format_utils.py."""
 
+# Disable check for function names to avoid errors based on old code
+# pylint: disable-msg=invalid-name
+
+from __future__ import absolute_import
+
 import unittest
 
 from update_payload import format_utils
diff --git a/scripts/update_payload/histogram.py b/scripts/update_payload/histogram.py
index 1ac2ab5..bad2dc3 100644
--- a/scripts/update_payload/histogram.py
+++ b/scripts/update_payload/histogram.py
@@ -16,6 +16,9 @@
 
 """Histogram generation tools."""
 
+from __future__ import absolute_import
+from __future__ import division
+
 from collections import defaultdict
 
 from update_payload import format_utils
@@ -110,7 +113,7 @@
     hist_bar = '|'
     for key, count in self.data:
       if self.total:
-        bar_len = count * self.scale / self.total
+        bar_len = count * self.scale // self.total
         hist_bar = '|%s|' % ('#' * bar_len).ljust(self.scale)
 
       line = '%s %s %s' % (
diff --git a/scripts/update_payload/histogram_unittest.py b/scripts/update_payload/histogram_unittest.py
index e757dd0..ccde2bb 100755
--- a/scripts/update_payload/histogram_unittest.py
+++ b/scripts/update_payload/histogram_unittest.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright (C) 2013 The Android Open Source Project
 #
@@ -17,6 +17,11 @@
 
 """Unit tests for histogram.py."""
 
+# Disable check for function names to avoid errors based on old code
+# pylint: disable-msg=invalid-name
+
+from __future__ import absolute_import
+
 import unittest
 
 from update_payload import format_utils
diff --git a/scripts/update_payload/payload.py b/scripts/update_payload/payload.py
index 1ed5f99..ea5ed30 100644
--- a/scripts/update_payload/payload.py
+++ b/scripts/update_payload/payload.py
@@ -16,6 +16,7 @@
 
 """Tools for reading, verifying and applying Chrome OS update payloads."""
 
+from __future__ import absolute_import
 from __future__ import print_function
 
 import hashlib
@@ -64,7 +65,7 @@
     """Update payload header struct."""
 
     # Header constants; sizes are in bytes.
-    _MAGIC = 'CrAU'
+    _MAGIC = b'CrAU'
     _VERSION_SIZE = 8
     _MANIFEST_LEN_SIZE = 8
     _METADATA_SIGNATURE_LEN_SIZE = 4
@@ -111,7 +112,6 @@
             payload_file, self._METADATA_SIGNATURE_LEN_SIZE, True,
             hasher=hasher)
 
-
   def __init__(self, payload_file, payload_file_offset=0):
     """Initialize the payload object.
 
diff --git a/scripts/update_payload/test_utils.py b/scripts/update_payload/test_utils.py
index 4f5fed0..e153669 100644
--- a/scripts/update_payload/test_utils.py
+++ b/scripts/update_payload/test_utils.py
@@ -16,9 +16,10 @@
 
 """Utilities for unit testing."""
 
+from __future__ import absolute_import
 from __future__ import print_function
 
-import cStringIO
+import io
 import hashlib
 import os
 import struct
@@ -70,7 +71,7 @@
   """
   try:
     file_obj.write(struct.pack(common.IntPackingFmtStr(size, is_unsigned), val))
-  except IOError, e:
+  except IOError as e:
     raise payload.PayloadError('error writing to file (%s): %s' %
                                (file_obj.name, e))
 
@@ -335,7 +336,7 @@
 
     if do_generate_sigs_data:
       # First, sign some arbitrary data to obtain the size of a signature blob.
-      fake_sig = SignSha256('fake-payload-data', privkey_file_name)
+      fake_sig = SignSha256(b'fake-payload-data', privkey_file_name)
       fake_sigs_gen = SignaturesGenerator()
       fake_sigs_gen.AddSig(1, fake_sig)
       sigs_len = len(fake_sigs_gen.ToBinary())
@@ -345,7 +346,7 @@
 
     if do_generate_sigs_data:
       # Once all payload fields are updated, dump and sign it.
-      temp_payload_file = cStringIO.StringIO()
+      temp_payload_file = io.BytesIO()
       self.WriteToFile(temp_payload_file, data_blobs=self.data_blobs)
       sig = SignSha256(temp_payload_file.getvalue(), privkey_file_name)
       sigs_gen = SignaturesGenerator()