BMP Wrap is a free tool which can temporarily convert any file into a valid BMP image, and restore the original file later.
This is a familiar idea, but BMP Wrap does have one or two features that make it stand out. In particular, as a single 20KB executable, it’s probably the smallest steganography application we’ve ever seen.
Unsurprisingly for a program of this size, there’s no GUI. Just drag and drop a file onto BMP Wrap and the program will package it into a BMP of the same (Secret.docx gets you Secret.bmp, while the original file is untouched).
This isn’t quite as convincing as a full steganography app, because it’s not using a source image as cover. BMP Wrap-created BMPs are valid images and will open in graphics programs, but the image you’ll get depends on the binary data in the original file, probably just a mosaic of noise.
Still, BMP Wrap isn’t just embedding the raw source file in a BMP outline. Even if someone bothers to open the image, and sees it doesn’t contain a picture, and is suspicious enough to open the BMP in a hex editor, they won’t see anything of the source. That’s going to be secure enough for most situations.
Assuming your files aren’t intercepted by the CIA, once they reach the destination they can be restored with another copy of BMP Wrap. Drag and drop a bitmap onto the program and it’ll restore the original in seconds.
One feature you don’t get with BMP Wrap is batch processing, but you can fake this by using the program from a batch file, like:
FOR %%A IN (*.ZIP) DO BMPWRAP %%A
There’s plenty of scope for automation, if you need it.
BMP Wrap runs on all modern versions of Windows, and most of the old ones, too.