AssertionError during decode if payload takes around more than 2 and less than 64 grids. Successful decodes could have null bytes at the end.

[Cerlancism]

Hello once again.

I was experimenting around with this library, I found that the decoding part has quite a bit of issues.

Take the following 3 test payloads.

test1.txt

test

test2.txt

test test

test3.txt

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

test test

All these ran using the example image with 0.45 alpha.

test1.txt and test3.txt successfully encoded and decoded. However the decoded test1.txt and test3.txthas 3 and 5 null bytes respectively at the end of file.
test2.txt passed encoding but suddenly fails at decoding as it throws AssertionError. I tested around the length of message and found it always would fail if it takes up around more than 2 grids but less than 64.

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/runner/BPCS-Test/bpcs_mobeets/bpcs/bpcs.py", line 55, in <module>
    decode(opts.infile, opts.outfile, opts.alpha)
  File "/home/runner/BPCS-Test/bpcs_mobeets/bpcs/bpcs_steg_decode.py", line 34, in decode
    write_conjugated_message_grids(outfile, grids, alpha)
  File "/home/runner/BPCS-Test/bpcs_mobeets/bpcs/array_message.py", line 190, in write_conjugated_message_grids
    messages, conj_map_grids, nbits_per_map = separate_conj_map_from_message(grids, alpha)
  File "/home/runner/BPCS-Test/bpcs_mobeets/bpcs/array_message.py", line 174, in separate_conj_map_from_message
    x = get_n_message_grids(nbits_per_map, ngrids)
  File "/home/runner/BPCS-Test/bpcs_mobeets/bpcs/array_message.py", line 154, in get_n_message_grids
    assert x > 0
AssertionError

I have setup a REPL.it workspace to show my test results:
https://repl.it/@Cerlancism/BPCS-Test#main.sh

I have reduced the size of image to 150x150 to speed up in REPL.it. Similar results shows for the provided example image.

Update

One of my friend catkane-doodles has forked this repo and he seemed to have fixed this. However, he did a complete revamp. Not sure if we could PR in.