Microsoft employee Raymond Chen says that an encrypted copy of Microsoft Bob is included on the Windows XP CD to take up space.

But Windows XP doesn't take up the entire CD; there is a lot of free space remaining.

Is the story true?

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    According to the article, the copy of Bob was not to "take up the entire CD", but to create "ballast data" for the Windows Installer to verify. – Dour High Arch Dec 16 '12 at 4:51
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    Given the source, I'd say this is probably true. – Arkady Jan 24 '13 at 14:02
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    ...yet unverifiable. A disk image, encrypted, is (should be) undistinguishable from random noise. Therefore, unless the (unknown) passphrase somehow surfaces (improbable), there is no useful way to verify this. – Piskvor left the building Mar 13 '15 at 16:07
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    «A disk image, encrypted, is (should be) undistinguishable from random noise.» not necessarily, given that commonly used encryption algos of the era were later discovered to be susceptible to cryptanalysis. – vartec Mar 16 '15 at 21:41
  • Looking at the article, it should likely be in a file.... (Or at least in something that would be kept in a CD image - you can make an image of the CD and find areas not referenced by the filesystem....) – Gert van den Berg Apr 17 '18 at 8:17

The story that Chen claimed it is true. But he was most likely joking. Above all it would be impossible to check because Chen — in this supposed joke story — claims the encryption key was created by just mashing the keyboard.


Also the "slowdown" would be by adding 30 MBytes, which is less than 5% data compared to a full CD.

So just assume Chen made an April Fools joke... in June 2008.

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    If it is true, it would be easy to verify, as an encryption key created by mashing the keyboard is not very secure and could be brute-forced with heuristics of the keys hit when mashing the keyboard. – allo Apr 19 '18 at 11:53
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    @allo Have fun trying. – MichaelK Apr 19 '18 at 12:06
  • @DavePhD Wouldn't the floppies contain the "entire Bob"? – Tobia Tesan Apr 19 '18 at 13:38
  • @DavePhD Eh, I think it's pretty clear that in that context "floppy disk images" means "disk image", i.e. "dump of the floppy disk contents". Couldn't even think of the other interpretation until you pointed it out. – Tobia Tesan Apr 19 '18 at 18:00
  • @TobiaTesan ok, you're probably right, I'll delete all my comments – DavePhD Apr 19 '18 at 18:40

The answer is that only half of MS Bob is on the Windows XP disk.

Raymond Chen gets it wrong in his blog. Dave Plummer, the employee that implemented it, spoke about this on a recent YouTube video. However, even Dave gets it wrong.

OEMBIOS.BIN is the dummy data. There are two versions of it that correspond to the retail and OEM version of Windows XP. Each is only 13 MB, so each are only half of MS Bob. To verify that you have the Bob blob, the code decrypts OEMBIOS.DAT with a hardcoded key and bespoke Feistel cipher routine. Unencrypted, the file is just an INI that indicates which variant of Windows XP that you have, but it also contains a list of 100 hash values. The hash values are for each 1/100th of the 13 MB OEMBIOS.BIN. On boot, winlogon.exe calls into licdll.dll, which picks a random hash from the .DAT and checks the corresponding 131 KB (1/100th of 13 MB) worth of the .BIN.

One motivation for this, as mentioned by Dave Plummer, was to maximize the difference between the retail and OEM disc versions. Without this, the differences would amount to less than a kilobyte. One could theoretically collect and compress all of the required files to produce any OEM variant of the install CD, and have it be just a few KB. Back when piracy was very Usenet centric, pumping up the file sizes would at least cause such a file to span multiple messages (due to Usenet's 15 MB limit.)

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    This is interesting information, but not very clearly sourced. It would be good to link to the YouTube video, and make clear which parts of your answer are sourced from it. Since you claim that the YouTube video is also incorrect in some of the details, you will also need to provide a third source for whichever parts of your answer disagree with both Chen and Plummer. – IMSoP Feb 11 at 11:21
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    Please provide some references to support your claims. – Oddthinking Feb 11 at 18:43

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