Clue 1: Password hidden_stegosaurus 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. Can you unzip this file and get the flag? (This covers all knowledge needed to complete the problem.). These colors combined make up the one pixel you actually see. 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. The difference is FFB1. This may help you understand the zTXt chunk. Encoders and decoders shall treat the chunk types as fixed binary values, not character strings. Flag is hidden in one of the RGB planes and can be extracted with stegsolve: We have recovered a binary and an image See what you can make of it. For a long time I have been looking for a way to hide info ( a serial number) into image and then retrieve it later, this artile helps a lot. The CRC can be used to check for corruption of the data. Four-byte chunk type. An example from Al Qaeda is here. That being said, we need libpng to decompress and then unfilter the image for us. Hopefully it saves someone some trouble. PNG的Chunk有很多类型,这里只介绍了最关键的三种. The program automatically detects the RX option and produces an image with the flag upside down. anyone knows any sample code to convert a number to image and then later read that number out of the image? Its also found in /problems/investigative-reversing-4_5_908aeadf9411ff79b32829c8651b185a on the shell server. The four-byte chunk type field contains the decimal values. Its also found in /problems/investigation-encoded-2_2_4d97294fc1696ff16af8ce3c0e6b3b95 on the shell server. The IEND chunk marks the end of the PNG datastream. 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). The first IDAT was at offset 57. 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. The four-byte chunk type field contains the decimal values 73 68 65 84. 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. 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. (Which we did with the left shift operator.) Here is decode script: flag: picoCTF{N1c3_R3ver51ng_5k1115_00000000000ade0499b}. Chunk pHYs – rozměry pixelů. The decode function is essentially just the inverse of the encode function. 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. These correspond to the uppercase and lowercase ISO 646 letters (A-Z and a-z) respectively for convenience in description and examination of PNG datastreams. If you wanted to you could perform general image transformations by manipulating the IHDR header. Although encoders and decoders should treat the length as unsigned, its value shall not exceed 231-1 bytes. The IDAT chunk contains the actual image data which is the output stream of the compression algorithm. IDAT image data chunks.. IEND trailer. By following udp streams, we can obtain the flag. Traversing to section 11.2.2 IHDR Image Header, we see the chunk type field must contain the hex values 49 48 44 52. The chunk's data field is empty. 5.2 PNG Signature 89 50 4E 47 0D 0A 1A 0A (translated to hex) This signature indicates that the remainder of the datastream contains a single PNG image, consisting of a series of chunks beginning with an IHDR chunk and ending with an IEND chunk. Replace the length field with 00 00 FF A5. There should be a flag somewhere. Remember that SIZE_WIDTH was 32. We will not be using an alpha channel. We need to know this so we know when to stop reading. Subtracting 12 in total, we get FFA5. Read/write access to PNG images in pure Ruby. That means we need to shift it left three places. The length counts only the data field, not itself, the chunk type, or the CRC. Here we'll take a look at hiding information in images. The next part is where it can get confusing: The loop starts at either 0 or 32 depending on whether this is the first row of image data. Ancillary chunks. See what you can make of it. Ce terme anglais est utilisé dans de nombreux formats multimédias. * stackoverflow ;-). I've been at it for a while and I'm supposed to be studying for a test tomorrow so I haven't edited for grammar ;-D. This article, along with any associated source code and files, is licensed under The GNU General Public License (GPLv3), General    News    Suggestion    Question    Bug    Answer    Joke    Praise    Rant    Admin. Use the same program as the first m00nwalk problem. Can you retrieve the flag? We have recovered a binary and 5 images: image01, image02, image03, image04, image05. Because that bit of data might have been the LSB of our encoded byte or it may have been the 4th bit in our encoded byte. Here I will discuss at a rudimentary level some of the details of the PNG file specification to give the necessary background to understand the code. Chunk data is a bunch of bytes with a fixed length read before. It would look something like this: The final line at least checks if the bit depth is correct. A valid PNG image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk. Clue 3: Alan Eliasen the FutureBoy. The chunk data 4. read_ptr->width is multiplied by y because that's the total number of rows we've read. Each chunk in a PNG data stream starts with an 8-byte chunk header and ends with a 4-byte trailer. You can also find the files in /problems/m00nwalk2_4_db2f361610e04b41a70a92cd8b7b2533. Below is a synopsis of the relevant parts for this article. IDAT chunk:图像信息必须使用5种过滤方式中的方式之一 (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。 That's the outputPNG function. If it is a multiple of 8 that means we've encoded 8 bits and we're ready to read another byte from our file to encode. Only one IHDR chunk and one IEND chunk are allowed in a PNG datastream. Chunk Data - The data bytes appropriate to the chunk type, if any. If that bit is a 1 it ors a byte from the image with 1 to set the LSB to 1 otherwise, it sets the LSB of the image byte to 0. If you're reading this, I assume a strong knowledge of C++ and a good grasp on binary arithmetic. We could get a lot more efficiency by leveraging other hiding places, compression techniques, using more bits that wouldn't be detectable, alpha channels, text channels, and a million other things, but here we'll stick to the least significant bit . Cite me if you like, but it's no big deal. The IHDR chunk shall be the first chunk in the PNG datastream. The. the flag is hidden in the least significant bit of each pixel value. Opening the file on any PNG viewer gives the flag. We have recovered a binary and a few images: image, image2, image3. By reversing the program, we can recover this mapping, therefore, obtain the flag: We have recovered a binary and 1 file: image01. They are PNG four-byte unsigned integers. The CRC is always present, even for chunks containing no data. Once they're both in there compile zlib. Reading IEND chunk, length = 0. If you're curious about the filtering and compression on PNG images check out  Filtering and Compression. We're going for simple here rather than super slick. Find the flag in this picture. 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! The party that wants to communicate information encodes hidden data into the image, uploads it, and the second party downloads it. The chunk's data field is empty. The windows version hasn't been updated in an eternity and the documentation was kinda confusing. Here it is: By this point I expect you know what the first pieces do so I'll skip to the meat: The first 8 lines are probably fairly self-explanatory at this juncture. IHDR is length 13. sRGB is length 1. gAMA is length 4. pHYs is length 9. The program maps each character to a stream of n bits. The png_write_png actually writes the image out to storage. 数据块结构. 데이터를 담는 목적으로 사용하지 않으므로 않으므로 Length 값은 언제나 0이다. This is an optional chunk. The primary takeaway from this is that the data pointed to by read_ptr will end up containing all of the PNG image data structures and information. See what you can make of it. The use of the goto function in its proper habitat. NOTE: The flag is not in the normal picoCTF{XXX} format. You put really very helpful information. This will examine the LSB of each byte of image data (until we reach the size), extract it, and reassemble it. The *3 is there because the width is in pixels and each pixel has 3 bytes. So when we should wait till we meet IEND chunk before we decode the IDAT chunk. Fun Fact: This is a common technique used by clandestine organizations and terrorist groups such as Al Qaeda alike to covertly share information. Anyway the if statement is checking to see if we've decoded all of the hidden data. Zero is an invalid value. A common method of hiding flags in these types of challenges is to place messages after the IEND chunk. The flag is hidden in the EXIF data of the image. The next chunk with chunktype AB 44 45 54 is corrupt with name �DET. Thank you for the very good work. The PNG_file class definition is as follows: Don't worry too much about understanding what each bit means just yet. Recover the flag. Clearly this isn't the most efficient scheme since the PNG image we use we'll need to be 8 times the size of the hidden data. * Constructor for the PNG_file class Simply reads in a PNG file The full specification is here. There are 4 kinds of critical chunk and 14 kinds of ancillary chunk. There are a few critical chunks: IHDR image header, which is the very first chunk.. PLTE palette table. This field can be of zero length. If you're reading this, I assume a strong knowledge of C++ and a good grasp on binary arithmetic. Since we cannot identify CRCs, to find the end of the chunk, we must look for the next chunktype field. The PNG spec requires all PNG files to have it. It iterates over each bit from right to left of the byte from of the byte read from the file to encode. Where it gets different is inside the inner for loop. PNG格式的图片具有如下特点: 1. Reversing the binary shows that the flag is encoded and then appended to the image: As shown above, the 6th to 14th byte are added by 5 and the 15th byte is subtracted by 3. For that reason I've included the version that I finagled to work for Visual Studio 2012 along with zlib. Same concept as before, we need to reverse the binary and decode the flag: flag: picoCTF{n3xt_0n30000000000000000000000000f69eb8c8}. It can be extracted with the exiftool: Theres something in the building. You must then add to that solution the zlib project. It may be worth noting that ipow is just a helper function that is an integer implementation of the pow function. Remember that number is in pixels so we have to multiply it by 3 to get the number of 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. Here's the first part: The if statement is only true when x is a multiple of 8, but is not on the very first iteration. This section does the actual encoding. See what you can make of it. * Performs IO and encoding and decoding on PNG images 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. It contains: Width and height give the image dimensions in pixels. We must subtract 4 bytes for the length field of the second IDAT, subtract 4 bytes for the CRC of the first IDAT, and subtract 4 bytes again for the chunktype of the first IDAT. We used this website to understand the hex values. Grant is a specialist in computer security and networking. There are three varieties of PNG image; we will examine truecolor images. In the downloads section I've included the VS2012 project with libpng in it. portion ensures that the for loop only runs during encoding into the first row. 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). Finally we add however far into the current row of image data we are and that accounts for the +x. So we say that x must be greater than  SIZE_WIDTH, which makes sure it doesn't run on that first iteration. The compression algorithm used by libpng is deflate, which is implemented by zlib in case you were wondering why we need zlib. PNG中一个Chunk的结构通常如下 There should be a flag somewhere. I'll explain why in a second. Class PNG_file Digital steganography is defined as hiding messages within digital media. 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. Decode this message from the moon. 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). The first 8 bytes of the mystery file can be fixed to the correct PNG signature. See Summary of standard chunks in PNG Specification . 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. Its also found in /problems/investigative-reversing-0_6_2d92ee3bac4838493cb68ec16e086ac6 on the shell server. LSB but with different images. Now we've got one byte of message data hidden in our image. You can also find the file in /problems/m00nwalk_2_ddfd37932ded29f58963e8d9c526c2fa. To decode this, we downloaded this program. This is why I initialized x outside the loop. See Rationale: PNG file signature. The only two points worth mentioning in it are the function png_set_rows, which sets the rows we modified for writing. The compressed datastream is then the concatenation of the contents of the data fields of all the IDAT chunks. 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. To any onlookers, the image is completely normal, but underneath the hood the image contains any arbitrary hidden data. 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 … Two parties agree on an image a… The four-byte chunk type field contains the decimal values 73 72 68 82. 73 72 68 82 goto can be legitimately used for few things, but breaking out of nested loops is one of them. It outputs the encoded version of the PNG file. This is a great article. Then, under librarian, general, make sure zlib.lib is listed as an additional dependency and add the directory containing zlib.lib to the additional library directories. This is a challenge where the flag is hidden in the least significant bit of each pixel value. Each byte of a chunk type is restricted to the decimal values 65 to 90 and 97 to 122. Note where the zlib.lib file gets spit out. Here SIZE_WIDTH is the number of bytes used to contain the size. To get it working you open up the project in VS in a new solution. To work with the PNG images I made a PNG image class to implement the steganography portion and used the libpng library and the zlib library to actually do all the PNG manipulation and such. This audio file was encoded by slow-scan television(SSTV), which was the method used in the moon landing. Hope this helps someone because getting libpng to work under Windows was a bear for me. The image data then becomes: 11110000, 10101011, 11001100, 11100011, 11111110,  00000001, 00001110, 10011011. 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. Its also found in /problems/investigative-reversing-1_0_329e7a12e90f3f127c8ab2489b08bcf1 on the shell server. 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. The inner for loop works the same way as the inner for loop encode. This .tar file got tarred alot. The three chunk types we will concern ourselves with are the IHDR, IDAT, and IEND chunks. 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. Remember that Each one bit of hidden message requires a byte of PNG file. As a side note info_ptr will contain the IHDR header chunk data. You may have to play with the dependencies to get it to work for you, but feel free to reuse. Clue 2: The quieter you are the more you can HEAR * This is just a standard implementation using modular exponentiation. Do it several thousand more times and we can hide quite a lot of data. You can probably see where this is going. Here's the code: The following line initializes IO on the PNG: libpng requires us to tell it if we've already read any data from the filestream before we read the image so we tell in with the following line: After that we read the entire PNG into memory (efficiency again wasn't a prime concern on this one ) and then set row_pointers to point to an array of pointers. We find the next IDAT at offset 10008. This is version 1. You can see the location of the chunks clearly in the hex dump, because the ASCII chunk types stand Compression. The third chunk is the IDAT chunk, which contains image pixel data. We think this transmission contains a hidden message. Chunk struct Can you find the flag? Chunk Type - A sequence of four bytes defining the chunk type. There are also some clues clue 1, clue 2, clue 3. The size, a non-signed 4-byte integer, describing the size of the chunk 2. Opening the file in a hex edior, we see that the file header is very simillar to a PNG signature. Zero is a valid length. Here is an example main that uses the PNG_file class: So working with libpng was kinda awful. Similar to investigation_encoded_1 but with more characters. So the 32 bit size will be stored over 32 bytes of PNG image. 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. I am pretty much pleased with your good work. * The C++ standard pow function uses doubles and I needed an integer version. In the middle part: All this is doing is extracting the LSB of the byte of image data, which is our encoded bit. The signature of a PNG file (in decimal notation) is the following: Each chunkcomprises 4 parts : 1. 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=. To get it to compile it is dependent on zlib, which I also included. Next is a check to make sure we haven't reached the end of the encoded data: Observe! 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. This is a great inspiring article. PNG images are comprised of chunks. He holds a bachelors degree in Computer Science and Engineering from the Ohio State University. Finally, the last chunk is the IEND chunk… Various studies have yielded different results, but the all-reliable source wikipedia says that the human eye can distinguish approximately 10 million different colors. The last thing to do is decode the hidden data  from an encoded image. flag: picoCTF{more_than_m33ts_the_3y3cD8bA96C}. You can also find the file in /problems/glory-of-the-garden_5_eeb712a9a3bc1998ffcd626af9d63f98 on the shell server. Certs: CCNA, CCNP, CCDA, CCDP, Sec+, and GCIH. A 13-byte IHDR chunk containing the image header, plus 12 bytes chunk overhead. We see the bytes 43 22 44 52 are in the first chunk’s chunktype field, after the 8-byte PNG signature and the 4-byte length field. We'll go through it. The following piece of code is responsible for encoding the size of the file that we're encoding into the PNG image: You may notice that I set x to 0 outside of the loop. The if statement and the for loop extract the length from the first 32 bytes of the first row. For example, it would not be correct to represent the chunk type IDAT by the equivalents of those letters in the UCS 2 character set. This all happens in the PNG_file constructor: Explanation: First we open up a file stream on the PNG file that we want to encode our hidden data into and declare a variable header that will contain the PNG file signature. The size variable contains the size of the file to be encoded. The four-byte chunk type field contains the decimal values 73 69 78 68. Success! For the curious, the alpha channel provides color transparency information. Data added after this block will not change anything besides the size of the file. What we're going to do is leverage this to hide messages in the least significant bits in the following manner: 8 bytes of image data (that's two pixels and the red and green bytes of a third pixel), 11110000, 10101010, 11001100, 11100011, 11111111,  00000000, 00001111, 10011011. Visually it may look like this: so we and that out to get the temporary byte: Let's say that this particular bit is supposed to be the third bit (counting from the left) of our reconstructed encoded byte. We are given a pcap network capture that can be opened in wireshark. 11.2.2 IHDR Image header. Here's the code: The filesize function is just a helper function that calculates the size of a file. { Length : 00 00 00 00 (0 byte), Chunk Type : IEND, Chunk Data (0 byte), CRC } IEND 청크는 이미지의 맨 뒤에 위치하는 청크로 PNG 파일의 끝을 나타낸다. 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). Now we have to align that properly in our buffer. There may be multiple IDAT chunks; if so, they shall appear consecutively with no other intervening chunks. We have recovered a binary and an image. Its also found in /problems/investigative-reversing-2_5_b294e24c9063edbf722b9554e7750d19 on the shell server. It shifts the bit left the appropriate number of spots in the byte. We really should check a lot more things to ensure we have a compatible image, but this is just a POC. CRC - A four-byte CRC (Cyclic Redundancy Code) calculated on the preceding bytes in the chunk, including the chunk type field and chunk data fields, but not including the length field. */, 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. 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. It also retains backward compatibility with non-animated PNG files.. 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 Finally we or that with buffer up to this point. This challenge is building on top of Investigative Reversing 2. I needed to allow it to be 32 on that first run and 0 on every subsequent run. So the first thing we need to do is uncompress and unfilter our PNG image. Clue 3 leads us to this website and reading the description, it looks like a message was encoded using steganography. You can view the libpng documentation here. I guess you cannot avoid to use some pre-made lib. When reading, a minor warning is issued if this trailer exists, and ExifTool will attempt to parse this data as additional PNG chunks. Finally you add on SIZE_WIDTH because in addition to the hidden data you read you also read in the size of that data. A PNG file with CgBI extension has a structure where PNG signature is followed by CgBI chunk and then by IHDR chunk. NOTE: The flag is not in the normal picoCTF{XXX} format. Thanks in advance! signature PNG - 8 octets; chunk IHDR pour l'en-tête - 25 octets; chunk IDAT pour les données - longueur variable; chunk IEND pour la fin de fichier - 12 octets; Un « chunk » est un gros morceau du fichier, un fragment d'information constituant une entité. It marks the end of the PNG datastream. 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. We have to shift it left the appropriate number of spaces. The CRC (cyclic … The inside of the if statement writes out the current decoded byte to our output file and resets the buffer to 0. See what you can make of it. 体积小:网络通讯中因受带宽制约,在保证图片清晰、逼真的前提下,网页中不可能大范围的使用文件较大的bmp格式文件。 2. Check for corruption of the relevant parts for this exercise the compressed datastream is then concatenation. 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In VS in a PNG image a file 14 kinds of critical chunk and one IEND chunk this the! 32 bytes of the if statement is checking to see what support PurePNG provides it! Be fixed to the hidden data you read you also read in the example I will show,! Each serving various roles are and that accounts for the curious, the chunk,. Sample code to convert a number to image and then compressed so that they take up less space of... Theres something in the normal picoCTF { 4n0th3r_L5b_pr0bl3m_0000000000000aa9faea3 } look here to see what support PurePNG for! Marks the end of the if statement is checking to see what support PurePNG provides for it curious the... Used xxd to extract the length field with 00 00 FF A5 read from the first... Of hiding flags in these types of challenges is to place messages the! Vs in a hex edior, we see that the for loop encode calculates the size that! Ce terme anglais est utilisé dans de nombreux formats multimédias 5 images: image, uploads it, and chunks. I also included our buffer 72 68 82 output stream of the many message was encoded slow-scan! Simple here rather than super slick two parties agree on an image and this is a challenge the! Can not identify CRCs, to find the end of the encoded hex and decode the flag confuse for... The example I will show you, but underneath the hood the image out to storage I!