diff --git a/host/blockdiff.py b/host/blockdiff.py
new file mode 100755
index 0000000..d29dcbb
--- /dev/null
+++ b/host/blockdiff.py
@@ -0,0 +1,113 @@
+#!/usr/bin/python
+#
+# Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Block diff utility."""
+
+import sys
+import optparse
+
+
+class BlockDiffError(Exception):
+  pass
+
+
+def BlockDiff(block_size, file1, file2, name1, name2, max_length=-1):
+  """Performs a binary diff of two files by blocks.
+
+  Args:
+    block_size: the size of a block to diff by
+    file1: first file object
+    file2: second file object
+    name1: name of first file (for error reporting)
+    name2: name of second file (for error reporting)
+    max_length: the maximum length to read/diff in bytes (optional)
+  Returns:
+    A list of (start, length) pairs representing block extents that differ
+    between the two files.
+  Raises:
+    BlockDiffError if there were errors while diffing.
+
+  """
+  if max_length < 0:
+    max_length = sys.maxint
+  diff_list = []
+  num_blocks = extent_start = extent_length = 0
+  while max_length or extent_length:
+    read_length = min(max_length, block_size)
+    data1 = file1.read(read_length)
+    data2 = file2.read(read_length)
+    if len(data1) != len(data2):
+      raise BlockDiffError('read %d bytes from %s but %d bytes from %s' %
+                          (len(data1), name1, len(data2), name2))
+
+    if data1 != data2:
+      # Data is different, mark it down.
+      if extent_length:
+        # Stretch the current diff extent.
+        extent_length += 1
+      else:
+        # Start a new diff extent.
+        extent_start = num_blocks
+        extent_length = 1
+    elif extent_length:
+      # Record the previous extent.
+      diff_list.append((extent_start, extent_length))
+      extent_length = 0
+
+    # Are we done reading?
+    if not data1:
+      break
+
+    max_length -= len(data1)
+    num_blocks += 1
+
+  return diff_list
+
+
+def main(argv):
+  # Parse command-line arguments.
+  parser = optparse.OptionParser(
+      usage='Usage: %prog FILE1 FILE2',
+      description='Compare FILE1 and FILE2 by blocks.')
+
+  parser.add_option('-b', '--block-size', metavar='NUM', type=int, default=4096,
+                    help='the block size to use (default: %default)')
+  parser.add_option('-m', '--max-length', metavar='NUM', type=int, default=-1,
+                    help='maximum number of bytes to compared')
+
+  opts, args = parser.parse_args(argv[1:])
+
+  try:
+    name1, name2 = args
+  except ValueError:
+    parser.error('unexpected number of arguments')
+
+  # Perform the block diff.
+  try:
+    with open(name1) as file1:
+      with open(name2) as file2:
+        diff_list = BlockDiff(opts.block_size, file1, file2, name1, name2,
+                              opts.max_length)
+  except BlockDiffError as e:
+    print >> sys.stderr, 'Error:', e
+    return 2
+
+  # Print the diff, if such was found.
+  if diff_list:
+    total_diff_blocks = 0
+    for extent_start, extent_length in diff_list:
+      total_diff_blocks += extent_length
+      print('%d->%d (%d)' %
+            (extent_start, extent_start + extent_length, extent_length))
+
+    print('total diff: %d blocks' % total_diff_blocks)
+    return 1
+
+  return 0
+
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))