Resolved: on-disc data for Sherston Software protection

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scarybeasts
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Resolved: on-disc data for Sherston Software protection

Post by scarybeasts » Fri Mar 06, 2020 6:32 am

Hi,

I finally got my hands on a couple of Sherston Software discs (thanks Eelco!) and as promised, I'm reporting back what is actually on the disc surface.
Most (but not all) Sherston Software titles are protected by a CRC error on track 39, sector 9. The data in that sector is not deterministic and the Sherston loader checks that two subsequent reads of the sector return different bytes.

Non-deterministic byte reads can be caused by weak bits or fuzzy bits: https://forum.kryoflux.com/viewtopic.php?t=972

What do the Sherston disc use? It's weak bits. Or specifically, patches of weak bits. Here's a picture showing the amplified raw waveforms coming off the read head:
IMG_8045.JPG
The protected sector contains a few bytes of code, then an 0xFF then alternating patches of "empty" disc surface and 0xFF bytes (~6-8 bytes each?). Subsequent reads will return different results:
IMG_8046.JPG
In terms of protected disc format archiving, this is a useful finding. Patches of "empty" disc surface can be represented by simply using 0 byte values in FM byte streams. Since this isn't fuzzy bits, the complexity of variable flux timings isn't required.


Cheers
Chris

[bonus: one of the discs I received is "Animated Alphabet", which draws pictures and makes sounds for an object or two for each letter of the alphabet. W is for Watch and the sound effect for that sounded immediately awesome -- it's toggling the cassette motor relay for the tick tick! :D ]

eelco108
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Re: Resolved: on-disc data for Sherston Software protection

Post by eelco108 » Fri Mar 06, 2020 6:53 am

Nice! And that's a pretty clever protection mechanism ...

So how did Sherston actually write these weak bits? Did they have their own rigged floppy drive, or can this be done by software (clever 8271 programming or whatever) ? Or are these 'weak' bits just unwritten and therefore random? Just trying to understand.

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billcarr2005
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Re: Resolved: on-disc data for Sherston Software protection

Post by billcarr2005 » Fri Mar 06, 2020 10:01 am

Excellent detective work! =D>


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vanekp
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Re: Resolved: on-disc data for Sherston Software protection

Post by vanekp » Fri Mar 06, 2020 4:40 pm

very interesting, is there any kryoflux image made or available of this? would be interesting to look at.
Peter.

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scarybeasts
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Re: Resolved: on-disc data for Sherston Software protection

Post by scarybeasts » Sun Jun 28, 2020 1:26 am

eelco108 wrote:
Fri Mar 06, 2020 6:53 am
Nice! And that's a pretty clever protection mechanism ...

So how did Sherston actually write these weak bits? Did they have their own rigged floppy drive, or can this be done by software (clever 8271 programming or whatever) ? Or are these 'weak' bits just unwritten and therefore random? Just trying to understand.
I can now answer this. I chatted to one of the Sherston Software authors, Simon Hosler, who also did the weak bits protection!
Short story is they made a little hardware widget. Long story is here (along with some tricks to make weak bits without hardware tricks): https://scarybeastsecurity.blogspot.com ... ction.html


Cheers
Chris

eelco108
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Re: Resolved: on-disc data for Sherston Software protection

Post by eelco108 » Sun Jun 28, 2020 11:31 am

Thanks for the write-up! Very ingenious indeed, especially as it worked for both the 8271 and 1770 disk controllers.

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vanekp
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Re: Resolved: on-disc data for Sherston Software protection

Post by vanekp » Sun Jun 28, 2020 7:29 pm

Thanks for that, very interesting indeed.
Peter.

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