Way too big. Finally you add on SIZE_WIDTH because in addition to the hidden data you read you also read in the size of that data. Zero is a valid length. This garden contains more than it seems. We really should check a lot more things to ensure we have a compatible image, but this is just a POC. The program maps each character to a stream of n bits. The first IDAT was at offset 57. Do it several thousand more times and we can hide quite a lot of data. We will not be using an alpha channel. Now we've got one byte of message data hidden in our image. The CRC can be used to check for corruption of the data. We find the next IDAT at offset 10008. In the example I will show you, we will hide data in the least significant bits of a PNG image. For specifics the libpng documentation is here. Animated Portable Network Graphics (APNG) is a file format which extends the Portable Network Graphics (PNG) specification to permit animated images that work similarly to animated GIF files, while supporting 24-bit images and 8-bit transparency not available for GIFs. I solved this with a short python script and the unzipping utility unar: We obtain the flag.png nested in 1000 tar file which has the flag. Finally we or that with buffer up to this point. Each pointer in that array points to one row of image data. A quick hexdump with xxd shows that there are two different patterns: e28083 and 20: Treating e28083 as 0 and 20 as 1 gives us the flag in binary: flag: picoCTF{not_all_spaces_are_created_equal_dd5c2e2f77f89f3051c82bfee7d996ef}. You can view the libpng documentation here. Data added after this block will not change anything besides the size of the file. A PNG file comprises a signature, making it possible to indicate that it is a PNG file, followed by a series of elements called chunks. There should be a flag somewhere. Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all color types. See what you can make of it. Then, in the properties of the libpng project, under C++ general, you have to add the zlib source folder to additional include directories. This field can be of zero length. An example from Al Qaeda is here. This chunk header consists of two 32-bit fields, the first of which is the length (in bytes) of the data in the chunk (not including the header or the trailer), and the second is a 4-byte code that identifies the type of the chunk. Success! There are three varieties of PNG image; we will examine truecolor images. The chunk type: a 4-character (4-bytes) code comprised by ASCII alphanumeric characters (A-Z, a-z, 65 to 90 and 97 to 122) allowing the nature of the chunk to be established 3. Chunk Data - The data bytes appropriate to the chunk type, if any. This is performed by the following function: There isn't much to mention about the first part other then that the variable buffer will contain the individual bytes being encoded from the hidden message file into the PNG image in question. To get it working you open up the project in VS in a new solution. This is why I initialized x outside the loop. For this reason we can't edit the IDAT chunks of a raw PNG or you'll get some really funky results (I tried just for giggles and it's more than a little noticeable when you try to encode something). It can be extracted with zsteg: This is a really weird text file TXT? A brief overview of PNG datastream structure: You can also find the file in /problems/c0rrupt_0_1fcad1344c25a122a00721e4af86de13. Hopefully it saves someone some trouble. There should be a flag somewhere. Digital steganography is defined as hiding messages within digital media. Read/write access to PNG images in pure Ruby. Since this does not specify a chunk, we must begin at the start and check each chunk, with the knowledge of the format of chunks and each field’s length: 4bytes(length)-4bytes(chunk type)-lengthbytes(data)-4bytes(crc). He holds a bachelors degree in Computer Science and Engineering from the Ohio State University. * The C++ standard pow function uses doubles and I needed an integer version. A quick file type check with file reveals that we have a PNG file instead of a TXT file: Simply changing the filename to flag.png yields the flag. PNG compression method 0 (the only compression method presently defined for PNG) specifies deflate/inflate compression with a sliding window of at most 32768 bytes. The three chunk types we will concern ourselves with are the IHDR, IDAT, and IEND chunks. Since we cannot identify CRCs, to find the end of the chunk, we must look for the next chunktype field. PNG格式的图片具有如下特点: 1. To get it to compile it is dependent on zlib, which I also included. Sorry, I realize that was confusing. PNG images are comprised of chunks. Its also found in /problems/investigative-reversing-2_5_b294e24c9063edbf722b9554e7750d19 on the shell server. This may help you understand the zTXt chunk. Specifically, apply the filter udp.stream eq 6 and then right-click the follow udp option: I stopped using YellowPages and moved onto WhitePages… but the page they gave me is all blank! In this case I wanted to have 32 bits for the size so SIZE_WIDTH is 32. Ancillary chunks. flag: picoCTF{now_you_know_about_extensions}. * Contains the data for a PNG file object A common method of hiding flags in these types of challenges is to place messages after the IEND chunk. If fread comes back with 0 it means we've reached the end of the file and we have to break out of the nested loop. Once they're both in there compile zlib. The length counts only the data field, not itself, the chunk type, or the CRC. We can used xxd to extract the encoded hex and decode it with a short python script: Revisit the last transmission. See what you can make of it. We have recovered a binary and 1 file: image01. The IDAT chunk contains the actual image data which is the output stream of the compression algorithm. According to the specification, a PNG file should end at the IEND chunk, however ExifTool will preserve any data found after this when writing unless it is specifically deleted with -Trailer:All=. The four-byte chunk type field contains the decimal values 73 72 68 82. We have to shift it left the appropriate number of spaces. flag: picoCTF{the_answer_lies_hidden_in_plain_sight}. Fun Fact: This is a common technique used by clandestine organizations and terrorist groups such as Al Qaeda alike to covertly share information. This is a challenge where the flag is hidden in the least significant bit of each pixel value. 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