def get_bit(bytearr, offset):
""" Returns the bit at offset into a bytearray """
byte_idx = offset / 8
bit_idx = offset - (byte_idx * 8)
byte = bytearr[byte_idx]
bits = BitArray(byte)
return bits.get_bit(bit_idx)
def decode(trojan):
""" Decode a payload from a trojan AudioFile that has been encoded
using the Modify LSB algorithm. Returns a Payload.
"""
payload_data = []
payload_size_bits = []
payload_size = 0
paybit_stack = []
payload_spacing = 0
paybit_space = 0
for tidx, tbyte in enumerate(trojan.data):
# First 32 bytes of the trojan will have the file size of the
# payload encoded in their lsb. [0, 31]
if 0 <= tidx <= 31:
paysize_bit = get_lsb(tbyte)
payload_size_bits.append(paysize_bit)
# sys.stdout.write("%s" % paysize_bit)
# There is no payload data at trojan byte index 32. Let's compute
# the integer value of the payload_size here for use later.
elif tidx == 32:
payload_size = BitArray.from_bits(payload_size_bits).to_int()
payload_spacing = int(math.floor((len(trojan.data) - 32) / (payload_size * 8)))
print "[modify_lsb] Payload spacing is %s" % payload_spacing
print "[modify_lsb] decode - payload_size = %s" % payload_size
# print "[modify_lsb] decode - payload_size_bits = %s" % \
# ''.join([str(b) for b in payload_size_bits])
# print "[modify_lsb] bits read"
# Trojan bytes [33, n] will have the payload encoded in their lsbs,
# where n is payload_size * 8, where payload_size is the number of
# bytes in payload.
elif 33 <= tidx and len(payload_data) < payload_size:
if paybit_space == payload_spacing:
payload_bit = get_lsb(tbyte)
paybit_stack.append(payload_bit)
paybit_space = 0
# We've read enough bits to make a byte. DO IT.
if len(paybit_stack) == 8:
paybyte = BitArray.from_bits(paybit_stack).to_int()
payload_data.append(paybyte)
paybit_stack = []
paybit_space += 1
# There's no more payload data to be read from the trojan, let's break
else:
break
print "[modify_lsb] decode - len(payload_data) = %s (result)" % len(payload_data)
return Payload(bytearray(payload_data))
