################################################################################
# Name : Some fixups
# Author : Tyson Smith & Jesse Schwartzentruber
#
# Copyright 2014 BlackBerry Limited
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
################################################################################
import struct
import zlib
_KNOWN_EXTS = {}
def _coerce_ascii(c):
"""
coerce an ASCII value into the alphabetical range
"""
while True:
if c > 127: c -= 128
if c < 64: c += 64
if (c >= 65 and c <= 90) or (c >= 97 and c <= 122):
return c
c += 7 # just fudge it until it falls into the ascii range
def fix_png(data):
"""
Fix the signature and checksums on a fuzzed PNG image.
"""
out = [b"\x89PNG\r\n\x1A\n"]
data = bytes(data[8:])
chunk = 0
while len(data) >= 8:
chunklen = data[:4]
out.append(chunklen)
chunklen = struct.unpack("!I", chunklen)[0]
if chunk == 0:
chunkname = b"IHDR" # make sure the first tag is correct
else:
chunkname = data[4:8]
#chunkname = bytes(_coerce_ascii(c) for c in data[4:8])
out.append(chunkname)
data = data[8:]
if len(data) < chunklen:
break
else:
chunkdata = data[:chunklen]
chunkcrc = zlib.crc32(chunkname) & 0xFFFFFFFF
chunkcrc = zlib.crc32(chunkdata, chunkcrc) & 0xFFFFFFFF
out.append(chunkdata)
out.append(struct.pack("!I", chunkcrc))
data = data[chunklen+4:] # skip the old crc
chunk += 1
out.append(data)
return b"".join(out)
_KNOWN_EXTS["png"] = fix_png
def auto_fixer(data, ext):
"""
Attempt to automatically fix fuzzed data based on the file extension.
If *ext* represents a known extension, it will be passed to the appropriate
function to fix it's signature and/or checksums. If *ext* is not know, data
is returned unmodified.
"""
try:
fixer = _KNOWN_EXTS[ext.lower().lstrip('.')]
except KeyError:
return data
return fixer(data)
