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• An introduction to reverse engineering

  22 janvier 2011

  (This blog is still in hibernation, but I needed somewhere to post this)

  Reverse engineering is one of those wonderful topics, covering everything from simple "guess how this program works" problem solving, to poking at silicon with scanning electron microscopes. I’m always hugely fascinated by articles that walk through the steps involved in these types of activities, so I thought I’d contribute one back to the world.

  In this case, I’m going to be looking at the export bundle format created by the Tandberg Content Server, a device for recording video conferences. The bundle is intended for moving recordings between Tandberg devices, but it’s also the easiest way to get all of the related assets for a recorded conference. Unfortunately, there’s no parser available to take the bundle files (.tcb) and output the component pieces. Well, that just won’t do.

  For this type of reverse engineering, I basically want to learn enough about the TCB format to be able to parse out the individual files within it. The only tools I’ll need in this process are a hex editor, a notepad, and a way to convert between hex and decimal (the OS X calculator will do fine if you’re not the type to do it in your head).

  Step 1 : Basic Research
  After Googling around to see if this was a solved issue, I decided to dive into the format. I brought a sample bundle into my trusty hex editor (in this case Hex Fiend).

  

  A few things are immediately obvious. First, we see the first four bytes are the letters TCSB. Another quick visit to Google confirms this header type isn’t found elsewhere, and there’s essentially no discussion of it. Going a few bytes further, we see "contents.xml." And a few bytes after that, we see what looks like plaintext XML. This is a pretty good clue that the TCB file consists of a . Let’s scan a bit further and see if we can confirm that.
  
  In this segment, we see the end of the XML, and something that could be another filename - "dbtransfer" - followed by what looks like gibberish. That doesn’t help too much. Let’s keep looking.
  
  Great - a .jpg ! Looking a bit further, we see the letters "JFIF," which is recognizable as part of a JPEG header. If you weren’t already familiar with that, a quick google for "jpg hex header" would clear up any confusion. So, we’ve got the basics of the file format down, but we’ll need a little bit more information if we’re going to write a parser.

  Step 2 : Finding the pattern
  We can make an educated guess that a file like this has to provide a few hints to a decoder. We would either expect a table of contents, describing where in the bundle each individual file was located, some sort of stop bit marking the boundary between files, byte offsets describing the locations of files, or a listing of file lengths.

  There isn’t any sign of a table of contents. Let’s start looking for a stop bit, as that would make writing our parser really easy. Want I’m going to do is pull out all of the data between two prospective files, and I want two sets to compare.
  I’ve placed asterisks to flag the bytes corresponding to the filenames, since those are known.

  1E D1 70 4C 25 06 36 4D 42 E9 65 6A 9F 5D 88 38 0A 00 *64 62 74 72 61 6E 73 66 65 72* 42 06 ED 48 0B 50 0A C4 14 D6 63 42 F2 BF E3 9D 20 29 00 00 00 00 00 00 DE E5 FD

  01 0C 00 *63 6F 6E 74 65 6E 74 73 2E 78 6D 6C* 9E 0E FE D3 C9 3A 3A 85 F4 E4 22 FE D0 21 DC D7 53 03 00 00 00 00 00 00

  The first line corresponds to the "dbtransfer" entry, the second to the "contents.xml" entry. Let’s trim the first entry to match the second.

  38 0A 00 *64 62 74 72 61 6E 73 66 65 72* 42 06 ED 48 0B 50 0A C4 14 D6 63 42 F2 BF E3 9D 20 29 00 00 00 00 00 00

  01 0C 00 *63 6F 6E 74 65 6E 74 73 2E 78 6D 6C* 9E 0E FE D3 C9 3A 3A 85 F4 E4 22 FE D0 21 DC D7 53 03 00 00 00 00 00 00

  It looks like we’ve got three bytes before the filename, followed by 18 bytes, followed by six bytes of zero. Unfortunately, there’s no obvious pattern of bits which would correspond to a "break" between segments. However, looking at those first three bytes, we see a 0x0A, and a 0x0C, two small values in the same place. 10 and 12. Interesting - the 12 entry corresponds with "contents.xml" and the 10 entry corresponds with "dbtransfer". Could that byte describe the length of the filename ? Let’s look at our much longer JPG entry to be sure.

  70 4A 00 *77 77 77 5C 73 6C 69 64 65 73 5C 64 37 30 64 35 34 63 66 2D 32 39 35 62 2D 34 31 34 63 2D 61 38 64 66 2D 32 66 37 32 64 66 33 30 31 31 35 65 5C 74 68 75 6D 62 6E 61 69 6C 73 5C 74 68 75 6D 62 6E 61 69 6C 30 30 2E 6A 70 67*

  0x4A - 74, corresponding to a 74 character filename. Looks like we’re in business.

  At this point, it’s worth an aside to talk about endianness. I happen to know that the Tandberg Content Server runs Windows on Intel, so I went into this with the assumption that the format was little-endian. However, if you’re not sure, it’s always worth looking at words backwards and forwards, just in case.

  So we know how to find our filename. Now how do we find our file data ? Let’s go back to our JPEG. We know that JPEGs start with 0xFFD8FFE0, and a quick trip to Google also tells us that they end with 0xFFD9. We can use that to pull a sample jpeg out of our TCB, save it to disk, and confirm that we’re on the right track.
  

  This is one of those great steps in reverse engineering - concrete proof that you’re on the right track. Everything seems to go quicker from this point on.

  So, we know we’ve got a JPEG file in a continuous 2177 byte segment. We know that the format used byte lengths to describe filenames - maybe it also uses byte lengths to describe file lengths. Let’s look for 2177, or 0x8108, near our JPEG.

  

  Well, that’s a good sign. But, it could be coincidental, so at this point we’d want to check a few other files to be sure. In fact, looking further in some file, we find some larger .mp4 files which don’t quite match our guess. It turns out that file length is a 32bit value, not a 16bit value - with our two jpegs, the larger bytes just happened to be zeros.

  Step 3 : Writing a parser

  "Bbbbbut...", I hear you say ! "You have all these chunks of data you don’t understand !"

  True enough, but all I care about is getting the files out, with the proper names. I don’t care about creation dates, file permissions, or any of the other crud that this file format likely contains.

  

  Let’s look at the first two files in this bundle. A little bit of byte counting shows us the pattern that we can follow. We’ll treat the first file as a special case. After that, we seek 16 bytes from the end of file data to find the filename length (2 bytes), then we’re at the filename, then we seek 16 bytes to find the file length (4 bytes) and seek another 4 bytes to find the start of the file data. Rinse, repeat.

  I wrote a quick parser in PHP, since the eventual use for this information is part of a larger PHP-based application, but any language with basic raw file handling would work just as well.

  tcsParser.txt
  This was about the simplest possible type of reverse engineering - we had known data in an unknown format, without any compression or encryption. It only gets harder from here...

• MoviePy Create Video that keeps audio on Twitter

  26 janvier 2019, par SDS

  I’m exporting lots of videos and was planning to use them on Twitter.

  When I read the documentation the first time I (foolishly) thought I only had to worry about the dimensions and the rest would take care of itself.

  I have many files that look and play great on my PC but as soon as I load them into Twitter there is no audio.
  Twitter’s Media Best Practices are located here :

  Recommended Video Codec: H264 High Profile
  
  Recommended Frame Rates: 30 FPS, 60 FPS
  
  Recommended Video Resolution: 1280x720 (landscape), 720x1280 (portrait), 720x720 (square)
  
  Recommended Minimum Video Bitrate: 5,000 kbps
  
  Recommended Minimum Audio Bitrate: 128 kbps
  
  Recommended Audio Codec: AAC LC
  
  Recommended Aspect Ratio: 16:9 (landscape or portrait), 1:1 (square)
  
  Advanced:
  
  
  
  Frame rate must be 60 FPS or less
  
  Dimensions must be between 32x32 and 1280x1024
  
  File size must not exceed 512 mb
  
  Duration must be between 0.5 seconds and 140 seconds
  
  Aspect ratio must be between 1:3 and 3:1
  
  Must have 1:1 pixel aspect ratio
  
  Only YUV 4:2:0 pixel format is supported
  
  Audio must be AAC with Low Complexity profile. High-Efficiency AAC is not supported
  
  Audio must be mono or stereo, not 5.1 or greater
  
  Must not have open GOP
  
  Must use progressive scan

  Here’s the specs on a specific video :

  Video ----
  
  Length: 00:00:12
  
  Frame width 1280
  
  Frame height 720
  
  Data rate 238kbps
  
  Total bitrate 368kbps
  
  Frame rate 24.00 frames/second
  
  Audio --
  
  Bit rate 129 kbps
  
  Channels 2 (stereo)
  
  Audio sample rate 44.100 kHz
  
  File --
  
  Size 551 KB
  
  Item type MP4 File

  The code is fairly straightforward. I grab an image and make a short intro frame with it, then add two short clips and put it all together as you can see here :

  image_clip = ImageClip(my_image)   image_clip.set_duration(seconds).write_videofile('F:/sm_Temp_Files/trash_%s.mp4' % fname,fps=24)
  
  image_video_clip = mpy.VideoFileClip('F:/sm_Temp_Files/trash_%s.mp4' % fname)
  
  video_path = (my_video_path)
  
  video_clip = mpy.VideoFileClip(video_path, target_resolution = (h,w),audio=True)
  
  outro_clip = mpy.VideoFileClip('Logo_Intro_w_Stinger_Large.mp4',target_resolution = (h,w),audio=True)
  
  clips = [image_video_clip,video_clip,image_video_clip,video_clip,image_video_clip,outro_clip]
  
  slided_clips = [CompositeVideoClip([clip.fx( transfx.crossfadein, transition_seconds)]) for clip in clips]
  
  c = concatenate_videoclips(slided_clips)                
  
  c.write_videofile('F:/sm_Short_Video/sm_%s.mp4' % fname,fps=24)

  I’ll admit, I’m out of my league to know what to do next. When Googling it looks like others have had similar issues but show fixes in FFMpeg, like this example.

  Yes, I know MoviePy is written on top of FFMpeg but for a novice like me it would be really nice if there was tweaks I could do inside of MoviePy to get the audio to work on Twitter.

  Thank you in advance !

• Is it possible to merge 3 videos with ffmpeg by -f concat and crossfade without video content

  10 août 2019, par user3792705

  I want to merge 3 .mov files quickly without losing any resolution. I want to be able to distinguish the 3 pieces of videos after merge.

  "ffmpeg -f concat" does not lose resolution and quick without crossfade.

  But, I can’t distinguish 3 videos.

  As far as I know ffmpeg filter can be used add crossfade, but it have to use video start/end content to do the merger, which might involve transcoding. It won’t be fast compared with ’concat’, which won’t do transcoding, but simply copying.

  Here is the content (ffmpeg -i video.mov) of one of 3 videos :

  ffmpeg version 4.1.3 Copyright (c) 2000-2019 the FFmpeg developers
  
    built with Apple LLVM version 10.0.1 (clang-1001.0.46.3)
  
    configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.3 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/openjdk-12.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/openjdk-12.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
  
    libavutil      56. 22.100 / 56. 22.100
  
    libavcodec     58. 35.100 / 58. 35.100
  
    libavformat    58. 20.100 / 58. 20.100
  
    libavdevice    58.  5.100 / 58.  5.100
  
    libavfilter     7. 40.101 /  7. 40.101
  
    libavresample   4.  0.  0 /  4.  0.  0
  
    libswscale      5.  3.100 /  5.  3.100
  
    libswresample   3.  3.100 /  3.  3.100
  
    libpostproc    55.  3.100 / 55.  3.100
  
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '..../(edited)/VMEK8375.MOV':
  
    Metadata:
  
      major_brand     : qt  
  
      minor_version   : 0
  
      compatible_brands: qt  
  
      creation_time   : 2019-06-30T01:28:04.000000Z
  
      com.apple.quicktime.model: iPhone
  
      com.apple.quicktime.software: ZHIYUN
  
      com.apple.quicktime.creationdate: 2019-06-30T09:28:04Z
  
    Duration: 00:00:07.61, start: 0.000000, bitrate: 4386 kb/s
  
      Stream #0:0(und): Video: hevc (Main) (hvc1 / 0x31637668), yuv420p(tv, smpte170m/bt709/bt709), 1280x720, 4329 kb/s, 30.01 fps, 30 tbr, 600 tbn, 600 tbc (default)
  
      Metadata:
  
        creation_time   : 2019-06-30T01:28:04.000000Z
  
        handler_name    : Core Media Video
  
        encoder         : HEVC
  
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 94 kb/s (default)
  
      Metadata:
  
        creation_time   : 2019-06-30T01:28:04.000000Z
  
        handler_name    : Core Media Audio

  If I don’t care about crossfade with video content, just some ’nice’ black screen in between (It would be nice I add some text, like date + time on the black screen) is good enough for me. Is it possible to do ’concat’ and simple crossfade without video ’content’ ?