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import numpy as np
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import mjpeg
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from common import *
__all__ = ['StegBase', 'sample_key']
sample_key = [46812L, 20559L, 31360L, 16681L, 27536L, 39553L, 5427L, 63029L, 56572L, 36476L, 25695L, 61908L, 63014L,
5908L, 59816L, 56765L]
class StegBase(object):
"""
This is the base class for some of the JPEG-algorithms that behave
similarly such as JSteg, OutGuess and F3.
"""
def __init__(self, key=sample_key):
"""
Constructor of the JPEGSteg class.
"""
self.t0 = None
self.cov_jpeg = None
self.cov_data = None
self.hid_data = None
self.key = key
def _get_cov_data(self, img_path):
"""
Returns DCT coefficients of the cover image.
"""
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self.cov_jpeg = mjpeg.Jpeg(img_path)
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self.key = self.cov_jpeg.getkey()
self.cov_data = self.cov_jpeg.getCoefBlocks()
return self.cov_data
def _get_hid_data(self, src_hidden):
"""
Returnsthe secret data as byte sequence.
"""
raw = [0, 0, 0, 0] + np.fromfile(src_hidden, np.uint8).tolist()
raw_size = len(raw)
for i in xrange(4):
raw[i] = raw_size % 256
raw_size /= 256
self.hid_data = np.array(raw, dtype=np.uint8)
if np.size(self.hid_data) * 8 > np.size(self.cov_data):
raise Exception("Cover image is too small to embed data.Cannot fit %d bits in %d DCT coefficients" % (
np.size(self.hid_data) * 8, np.size(self.cov_data)))
return self.hid_data
def _post_embed_actions(self, src_cover, src_hidden, tgt_stego):
"""
This function isn't named very accurately. It actually calls the
_raw_embed function in inherited classes.
"""
try:
cov_data = self._get_cov_data(src_cover)
hid_data = self._get_hid_data(src_hidden)
# print hid_data.dtype,type(hid_data),hid_data.tolist()
cov_data, bits_cnt = self._raw_embed(cov_data, hid_data)
if bits_cnt != np.size(hid_data) * 8:
raise Exception("Expected embedded size is %db but actually %db." % (
np.size(hid_data) * 8, bits_cnt))
self.cov_jpeg.setCoefBlocks(cov_data)
self.cov_jpeg.Jwrite(tgt_stego)
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# size_cov = os.path.getsize(tgt_stego)
size_cov = np.size(cov_data) / 8
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size_embedded = np.size(hid_data)
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self._display_stats("embedded", size_cov, size_embedded,
time.time() - self.t0)
except TypeError as e:
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raise e
except Exception as expt:
print "Exception when embedding!"
raise
def _post_extract_actions(self, src_steg, tgt_hidden):
"""
This function isn't named very accurately. It actually calls the
_raw_extract function in inherited classes.
"""
try:
steg_data = self._get_cov_data(src_steg)
# emb_size = os.path.getsize(src_steg)
emb_size = np.size(steg_data) / 8
# recovering file size
header_size = 4 * 8
size_data, bits_cnt = self._raw_extract(steg_data, header_size)
if bits_cnt < header_size:
raise Exception("Expected embedded size is %db but actually %db." % (
header_size, bits_cnt))
size_data = bits2bytes(size_data)
size_hd = 0
for i in xrange(4):
size_hd += size_data[i] * 256 ** i
raw_size = size_hd * 8
if raw_size > np.size(steg_data):
raise Exception("Supposed secret data too large for stego image.")
hid_data, bits_cnt = self._raw_extract(steg_data, raw_size)
if bits_cnt != raw_size:
raise Exception("Expected embedded size is %db but actually %db." % (
raw_size, bits_cnt))
hid_data = bits2bytes(hid_data)
# print hid_data.dtype,type(hid_data),hid_data.tolist()
hid_data[4:].tofile(tgt_hidden)
self._display_stats("extracted", emb_size,
np.size(hid_data),
time.time() - self.t0)
except Exception as expt:
print "Exception when extracting!"
raise
def _looks_like_jpeg(self, path):
try:
with open(path, 'r') as f:
return f.read(2) == '\xff\xd8'
except IOError:
return False
def _display_stats(self, verb, cov_size, emb_size, duration):
print(
"%dB %s in %.2fs (%.2f kBps), embedding ratio: %.4f" %
(emb_size, verb, duration, (emb_size / duration) / 1000.,
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float(emb_size) / cov_size))
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# set & get
def set_key(self, key):
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assert key != None
self.key = key
def get_key(self):
return self.key
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# dummy functions to please pylint
def _raw_embed(self, cov_data, hid_data):
pass
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def _raw_extract(self, steg_data, num_bits):
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pass
def _dummy_embed_hook(self, cov_data, hid_data):
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pass
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def _dummy_extract_hook(self, steg_data, num_bits):
pass
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