I have been struggling with this assignment for a while now and I feel completely lost.
I have this image:
There is a hidden message in it, hidden using LSD coding. I understand the concept of changing the least significant bits, (the one furthest to the right) but I can't quite figure out how to extract a message. I have looked at the image with this tool and located some weird looking pixels at the very bottom. The problem is I don't know how to locate them and extract a message from it. I think bmp is bottom up so they should be at the top of the hex code. Anyways I would be happy if anyone could help me or point me in the right direction.
LSD steganography uses the least siginificant digit (ie, the least significant bit). Go through the image file, ignoring the headers, just looking at the bytes forming the actual image. From each byte extract the least significant bit. Every eight bytes you will have 8 bits, which make a single byte of the hidden massage. Assemble the bits into bytes, and assemble the bytes into the hidden message.
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I've wondered how QR codes are working, so i did a research and tried to paint my own in an table in word.
On Wikipedia I found this picture
I understand the configuration, but how you actually store a letter doesnt make sense to me.
With the example letter w.
On even rows black is 0 and on odd rows 1.
So the example should give this binary number 01110011 which would be 115 but w is number 32.
So how do I get the right number
I dont know much about this topic but I found you a video where dude explains it. And from what I understood, there are those cells that are read in order of numbers depending on arrow (there are 4 options here and you posted those yourself). So you simply follow those numbers and write 1s and 0s on paper which results in 8bit number. That video has much more detail.
It is also worth pointing out that it is MSB, meaning if we follow your example (you just considering numbers, not colors since you mislabeled it), it has arrow pointing up, meaning you write right/down to up/left which leads to number : 01110011 which has most significant bit at the left which means its 115
could anyone tell me that
is it right way if i calculate the crc by the method given below:
i did this because dataword length is large.
is it right?
The bottom set of divisions are what you want for a CRC, and arrive at the correct remainders. Your quotients each need one more bit, but they are not used anyway.
The top division was not completed, but it is not relevant, since you need to append the four zeros first as you did in the bottom left, or the four-bit CRC as you did in the bottom right.
Ultimately, You are doing the same thing what a division does. Refer https://www.wikihow.com/Divide-Binary-Numbers binary division for more. However, the data word to be sent to the receiver should not be altered.
I didn't know before about artistic or artwork QR codes, while checking some of these codes, they are completely different from the regular standard QR code, but how is it possible to create this kind of QR code without loosing it's value (the scan result is the same) ?
These QR Codes are the most ones that amazed me:
http://www.hongkiat.com/blog/qr-code-artworks/
The only thing in common is the 3 corners, and they're different in style.
So my question is, what are the elements that we should preserve while creating such QR Codes ?
The most important things are:
Dark-on-light
Very nearly square modules
Modest light border
Substantially preserve the three-finder patterns
... and the first line of modules around them, which carries format info
... and the bottom-right alignment pattern, is helpful
The rest, the interior, can be substantially obscured and still be readable, certainly with high error correction. But messing with the elements above will tend to make it unreadable much more rapidly
I'm trying to create (or, if I've somehow missed it in my research, find) an algorithm to encode/decode a bmp image into/from a QR code format. I've been using a guide (Thonky) to try to understand the basics of QR codes and I'm still not sure how to go about this problem, specifically:
Should I encode the data as binary or would numeric be more reasonable (assuming each pixel will have a max. value of 255)?
I've searched for information on the structured append capabilities of QR codes but haven't found much detail beyond the fact that it's supported by QR codes -- how could I implement/utilize this functionality?
And, of course, if there are any tips/suggestions to better store an image as binary data, I'm very open to suggestions!
Thanks for your time,
Sean
I'm not sure you'll be able to achieve that, as the amount of information a QR Code can hold is quite limited.
First of all, you'll probably want to store your image as raw bytes, as the other formats (numeric and alphanumeric) are designed to hold text/numbers and would provide less space to store your image. Let's assume you choose the biggest possible QR Code (version 40), with the smallest level of error correction, which can hold up to 2953 bytes of binary information (see here).
First option, as you suggest, you store the image as a bitmap. This format allows no compression at all and requires (in the case of an RGB image without alpha channel) 3 bytes per pixel. If we take into account the file header size (14 to 54 bytes), and ignore the padding (each row of image data must be padded to a length being a multiple of 4), that allows you to store roughly 2900/3 = 966 pixels. If we consider a square image, this represents a 31x31 bitmap, which is small even for a thumbnail image (for example, my avatar at the end of this post is 32x32 pixels).
Second option, you use JPEG to encode your image. This format has the advantage of using a compression algorithm that can reduce the file size. This time there is no exact formula to get the size of an image fitting in 2.9kB, but I tried using a few square images and downsizing them until they fit in this size, keeping a good (93) quality factor: this gives an average of about 60x60 pixel images. (On such small images, it's normal not to see an incredible compression factor between jpeg and bmp, as the file header in a jpeg file is far larger than in a bmp file: about 500 bytes). This is better than bitmap, but remains quite small.
Finally, even if you succeed in encoding your image in this QR Code, you will encounter an other problem: a QR Code this big is very, very hard to scan successfully. As a matter of fact, this QR Code will have a size of 177x177 modules (a "module" being a small white or black square). Assuming you scan it using a smartphone providing so-called "HD" frames (1280x720 pixels), each module will have a maximum size on the frame of about 4 pixels. If you take into account the camera noise, the aliasing and the blur due to the fact that the user is never perfectly idle when scanning, the quality of the input frames will make it very hard for any QR Code decoding algorithm to successfully get the QR Code (don't forget we set its error correction level on low at the beginning of this!).
Even though it's not very good news, I hope this helps you!
There is indeed a way to encode information on several (up to 16) QR Codes, using a special header in your QR Codes called "Structured append". The best source of information you can use is the norm about QR Codes (ISO 18004:2006); it's possible (but not necessarily easy) to find it for free on the web.
The relevant part (section 9) of this norm says:
"Up to 16 QR Code symbols may be appended in a structured format. If a symbol is part of a Structured Append message, it is indicated by a header block in the first three symbol character positions.
The Structured Append Mode Indicator 0011 is placed in the four most significant bit positions in the first symbol character.
This is immediately followed by two Structured Append codewords, spread over the four least significant bits of the first symbol character, the second symbol character and the four most significant bits of the third symbol character. The first codeword is the symbol sequence indicator. The second codeword is the parity data and is identical in all symbols in the message, enabling it to be verified that all symbols read form part of the same Structured Append message. This header is immediately followed by the data codewords for the symbol commencing with the first Mode Indicator."
Nevertheless, i'm not sure most QR Code scanners can handle this, as it's a quite advanced feature.
You can define a fixed image size, reduce jpg header parts and using just vital information about it, so you can save up to 480bytes of a ~500bytes normal header.
I was using this method to store people photos for a small-club ID cards, images about 64x64 pixels is enough.
I have an incomplete QRCode (about 30%). Is it possible to decode just the fragment of it? I would really like a code snippet - the language doesn't matter.
If you mean, can you decode the entire contents of a QR code even if part of the code is obscured or changed, then yes you can -- sometimes.
QR codes can be encoded with varying levels of redundancy, which are known as levels L, M, Q and H, and correspond to about 7%, 15%, 25% and 30% redundancy. This means you can lose up to that much of the barcode and still decode it. The more you lose, the harder it is to decode, but remains possible within those limits.
Note that certain regions of the QR code can't be lost. The finder patterns (squares at corners) must be findable; they can tolerate some distortion but there's no error correction to help that. Also, the regions around the finder patterns encode format and version. They have a different redundancy (2x encoding using BCH, not Reed-Solomon), but, if you lose too much of those tiny areas you'll not be able to decode, regardless of the main error correction.