Byte wise modification in a specific range of a jpg image (treated as a binary object)-CodePudding

I am testing to write data at the end of a .jpg file. As i know all the .jpg files begin by FFD8 and end by FFD9. Using that fact i succeeded in writting "Hello World!" at the end of my .jpg file with a function that looks like this:

def writte_at_the_end_of_jpg_fct (img_path:str, msg:str):
      with open(img_path, 'ab') as fimg:
      fimg.write(b"Hello World!")

And Indeed i get:

But now how can i do to remove only the data that i ve added at the end of the file (that means "Hllo World!"

I ve tried this:

def erase_data_in_img_fct(img_jpg_file_path: str) -> None:
    with open(img_jpg_file_path, 'rb') as fimg:
        content = fimg.read()
        offset = content.index(bytes.fromhex('FFD9'))
        end_i = fimg.seek(offset   2)

    with open(img_jpg_file_path, 'ab') as fimg:
        fimg = fimg[0:end_i]

But it didn't work, i get this error:

TypeError: '_io.BufferedWriter' object is not subscriptable

I have searched a lot of time on the web an answer to my problem and didn't found it.

Thank you

CodePudding user response：

You should use bytes.rindex instead because the ffd9 bytes may occur multiple times in the file:

$ diff -u old.py new.py 
--- old.py  2022-06-08 08:07:33.381031019  0100
    new.py  2022-06-08 08:07:45.581315987  0100
@@ -1,8  1,8 @@
 def erase_data_in_img_fct(img_jpg_file_path: str) -> None:
     with open(img_jpg_file_path, 'rb') as fimg:
         content = fimg.read()
-        offset = content.index(bytes.fromhex('FFD9'))
         offset = content.rindex(bytes.fromhex('FFD9'))
         end_i = fimg.seek(offset   2)
 
-    with open(img_jpg_file_path, 'ab') as fimg:
-        fimg = fimg[0:end_i]
     with open(img_jpg_file_path, 'wb') as fimg:
         fimg.write(content[:end_i])