How to save custom data as a valid jpeg image in C#

Question

I need to store XML data on a server that only accepts jpeg images. I thought of writing my XML data inside a valid jpeg file. After all, other than the jpeg header, the content of the image file is arbitrary data right?

Is it possible to produce a valid jpeg file, but have its "body" filled with custom bytes?

Of course, I also need to be able to decode the custom jpeg file and restore the data.

I'm not familiar with the jpeg file format, so I'd appreciate an explicit example.

score 3 · Answer 1 · edited May 23 '17 at 12:18

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Perhaps just appending the data to a small jpeg will work?

Create a small jpeg.
Append your (obfuscated/encrypted) XML to the file.
Upload to server.

FWIW, you can easily see this works using a Hex editor. Just create a small jpeg and append your xml to the end. Then open it using any image editor.

This is a perfectly valid thing to do to a jpeg file:

Will random data appended to a JPG make it unusable?

edited May 23 '17 at 12:18

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answered Feb 02 '11 at 15:50

SuggestionBox

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That does work indeed. I was worried that the extra bytes would get shopped off by the server, but they weren't. It's not exactly how I wished to do it since I am not sure the resulting image has a 100% jpeg-conformity state. – Frederic Feb 02 '11 at 16:32
This is a good alternative. I prefer not set this as the answer since it's more like a workaround than an answer to the question. – Frederic Feb 03 '11 at 11:51

score 2 · Answer 2 · edited May 23 '17 at 10:27

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Uhmmm it is a strange architecture... but anyway I think this post would be useful: How to Add 'Comments' to a JPEG File Using C# so the proposal is add the data as a metadata of a jpeg blank image.

edited May 23 '17 at 10:27

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answered Feb 02 '11 at 15:46

Felice Pollano

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I think he meant to encode XML data with the binary image data directly (not for OCR processing) – Philibert Perusse Feb 02 '11 at 15:47
I do not want to add metadata as it would become accessible to the server and might confuse the user if the server displays it. – Frederic Feb 02 '11 at 15:50

score 0 · Answer 3 · answered Feb 02 '11 at 16:04

If you want to add your data as the actually jpeg data, you first create a BitmapSource with BitmapSource.Create and put your data in the buffer parameter. Than use the JpegBitmapEncoder to save it as a jpeg file (an example is here).

However, as far as I know, the .Net jpeg encoder is not lossless (even if you set it's quality to 100%) so you will need a third party library that can encode JPEG lossless.

score 0 · Answer 4 · answered Feb 02 '11 at 16:05

I don't know of a JPEG specific way but there is a PNG/GIF method to encode arbitrary data and pixels. Check out this post. Some sites allow you to upload PNGs and GIFs renamed to JPEG so you could try that. http://blog.nihilogic.dk/2008/05/compression-using-canvas-and-png.html

He's saving javascript but you could use and text, really.

How to save custom data as a valid jpeg image in C#

4 Answers4