Only one IHDR chunk and one IEND chunk are allowed in a PNG datastream. * This is just a standard implementation using modular exponentiation. It shifts the bit left the appropriate number of spots in the byte. There should be a flag somewhere. PLTE chunk:支持IDAT chunk:图像信息必须使用5种过滤方式中的方式0 (None, Sub, Up, Average, Paeth)IEND chunk:当IEND数据块被找到时,这个PNG图像才认为是合法的PNG图像。可选数据块:MIDP可以支持下列辅助数据块,然而,这却不是必须的。 bKGD cHRM gAMA hIST iCCP iTXt pHYs By reversing the program, we can recover this mapping, therefore, obtain the flag: We have recovered a binary and 1 file: image01. Finally, here is the PNG_file class in its entirety so you can look at it without having to download: There is one function I didn't explain in the code portion. Here we'll take a look at hiding information in images. Finally you add on SIZE_WIDTH because in addition to the hidden data you read you also read in the size of that data. In the middle part: All this is doing is extracting the LSB of the byte of image data, which is our encoded bit. flag: picoCTF{now_you_know_about_extensions}. Replace the length field with 00 00 FF A5. * Feel free to reuse at your leisure. Once they're both in there compile zlib. */, Dirty function for calculating the size of a file, Last Visit: 31-Dec-99 19:00     Last Update: 1-Jan-21 11:21, Download libpng_for_windows_source - 1.2 MB, Hi Sir can you please upload the encode.h file please. You may just have to stare at it for a moment. The size variable contains the size of the file to be encoded. This is why I initialized x outside the loop. Where it gets different is inside the inner for loop. Keep it up once again. Its also found in /problems/investigative-reversing-1_0_329e7a12e90f3f127c8ab2489b08bcf1 on the shell server. We have to shift it left the appropriate number of spaces. Remember that in the first row of image data we encoded the size of our hidden message in bytes. 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. You can also find the file in /problems/glory-of-the-garden_5_eeb712a9a3bc1998ffcd626af9d63f98 on the shell server. 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. Can you unzip this file and get the flag? The next thing to do is encode data into the image we read into memory. The next part is: You may have to stare at it for a moment, but what this is doing is iterating over each of the 32 bits of the size, checking if they are a 1, if they are or-ing that PNG byte with 1 to encode that one into the least significant bit and if the bit isn't one and-ing the PNG byte with 0xFE, which has the effect of setting the least significant bit to 0. Chunk struct 数据块结构. Can you retrieve the flag? IHDR(Image Header) chunk:描述影像的维度、色彩深度、色彩格式、压缩类型等. 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. A valid PNG datastream shall begin with a PNG signature, immediately followed by an IHDR chunk, then one or more IDAT chunks, and shall end with an IEND chunk. We have recovered a binary and a few images: image, image2, image3. Now the meet of the encode function is a bit more complex so I'll do my best to break it down line for line: The outer loop (primary variable is y)  controls the row of image data we're encoding into. In the example I will show you, we will hide data in the least significant bits of a PNG image. */, #define PNG_SIG_LENGTH 8 //The signature length for PNG, #define SIZE_WIDTH 32 //The number of bits used for storing the length of a file, Integer power function 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. That being said, we need libpng to decompress and then unfilter the image for us. A PNG file with CgBI extension has a structure where PNG signature is followed by CgBI chunk and then by IHDR chunk. That gives us the temporary byte: Now we or that with whatever buffer is up to this point. Each chunk consists of three or four fields. 索引彩色模式:PNG-8格式与GIF图像类似,同样采用8位调色板将RGB彩色图像转换为索引彩色图像。图像中保 … We have recovered a binary and an image. We found this file. If we kept iterating through the process we'd end up with the fully decoded byte: The header is called png_file.h and is at the beginning of the code section. Since we read the encoded bits from most significant bit to least significant bit our buffer would look like this before the or: 11001100 <- buffer at the end of the operation. It is most likely IDAT as they must be consecutive. Now we have to align that properly in our buffer. That's where the: comes into play. This is an optional chunk. Way too big. The decode function is essentially just the inverse of the encode function. Success! read_ptr->width is multiplied by y because that's the total number of rows we've read. Read/write access to PNG images in pure Ruby. PNG(Portable Network Graphics)イメージのデータ構造がどうなっているのか調べてみた。 基本的なデータ構造 PNGイメージは最初の8バイトを除いてチャンクと呼ばれる構造… The four-byte chunk type field contains the decimal values 73 72 68 82. What I mean by this is that we don't want to come in on the first iteration (where we've just finished extracting the size) and have this conditional result to true because at that point in execution nothing would be in our buffer. On the other side you get size multiplied by BYTE_SIZE. The png_write_png actually writes the image out to storage. These colors combined make up the one pixel you actually see. Here is the decode script: flag: picoCTF{4n0th3r_L5b_pr0bl3m_0000000000000aa9faea3}. You can view the libpng documentation here. Traversing to section 11.2.2 IHDR Image Header, we see the chunk type field must contain the hex values 49 48 44 52. Clue 2: The quieter you are the more you can HEAR In this case I wanted to have 32 bits for the size so SIZE_WIDTH is 32. The chunk's data field is empty. The other obvious problem is this chunk’s length: AA AA FF A5. Reading IEND chunk, length = 0. It would look something like this: The final line at least checks if the bit depth is correct. Chunk Data のサイズ 常に 0: 0x0004 (4) Chunk Type: 16進数で常に 49 45 4E 44 (ASCIIコードでは "IEND" である) 0x0008 (4) CRC (Cyclic Redundancy Check) Chunk Type と Chunk Data を もとに計算 … In the downloads section I've included the VS2012 project with libpng in it. So the first thing we need to do is uncompress and unfilter our PNG image. The final section of the loop is just a rudimentary check to see if we don't have any more rows of image to put data into, which means our hidden message is too big. A valid PNG image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk. 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=. This tells us the calculated CRC value from the data field, and the current CRC(expected). The least significant bit of each of these 32 bytes is combined into one 32 bit unsigned int that represents the size of the encoded file. Here SIZE_WIDTH is the number of bytes used to contain the size. Grant is a specialist in computer security and networking. The lines below read in the PNG signature and then check to make sure that the signature is valid using the libpng function png_sig_cmp: Following that we set up some necessary libpng data structures. Chunk type can be anything 1. Here is decode script: flag: picoCTF{N1c3_R3ver51ng_5k1115_00000000000ade0499b}. chunk IDAT at offset 0x150008, length 45027 chunk IDAT at offset 0x15aff7, length 138 chunk IEND at offset 0x15b08d, length 0 No errors detected in sctf.png (28 chunks, 36.8% compression). A 0-byte IEND chunk marking the end of the file, plus 12 bytes chunk overhead. You can see the location of the chunks clearly in the hex dump, because the ASCII chunk types stand You may have to play with the dependencies to get it to work for you, but feel free to reuse. The if statement and the for loop extract the length from the first 32 bytes of the first row. PNG file signature The first eight bytes of a PNG file always contain the following (decimal) values: 137 80 78 71 13 10 26 10 This signature indicates that the remainder of the file contains a single PNG image, consisting of a series of chunks beginning with an IHDR chunk and ending with an IEND chunk. The IEND chunk must appear LAST. 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). PNG中一个Chunk的结构通常如下 Do it several thousand more times and we can hide quite a lot of data. The use of the goto function in its proper habitat. To get it to compile it is dependent on zlib, which I also included. He holds a bachelors degree in Computer Science and Engineering from the Ohio State University. See what you can make of it. The length counts only the data field, not itself, the chunk type, or the CRC. The last thing to do is decode the hidden data  from an encoded image. We are given a pcap network capture that can be opened in wireshark. flag: picoCTF{more_than_m33ts_the_3y3cD8bA96C}. http://www.libpng.org/pub/png/spec/1.2/PNG-Compression.html It contains: Width and height give the image dimensions in pixels. the flag is hidden in the least significant bit of each pixel value. Chunk Data - The data bytes appropriate to the chunk type, if any. The program maps each character to a stream of n bits. This is a great article. In order to read in one byte of hidden data we have to read 8 bytes of image data so we multiply by BYTE_SIZE (which is 8). 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