Re: retro crypto

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On Wed, 8 Jan 2020 at 21:59, John Denker via cryptography <
[email protected]> wrote:

> Using 1970s technology, you can build a cipher machine on
> rotor-like principles.  It has the virtue of "not running
> any kind of malware because it can't".
>
> For example:  Use LFSRs (linear feedback shift registers)
> to drive the address lines on a bunch of EPROMs.  XOR the
> EEPROM outputs.
>
> Discussion:  If you use a LFSR directly, as a stream cipher,
> it is straightforward to ascertain the current state, whereupon
> you can predict the future state for all time.  But if you
> muddy it up using an EPROM (as a form of S-box) then that's
> not so easy.
>

Are you assuming the contents of the EPROM is secret? If so, why not use it
as a OTP? If not, then surely this construction is trivially insecure?

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote">=
<div dir=3D"ltr" class=3D"gmail_attr">On Wed, 8 Jan 2020 at 21:59, John Den=
ker via cryptography &lt;<a href=3D"mailto:[email protected]">crypt=
[email protected]</a>&gt; wrote:<br></div><blockquote class=3D"gmail_quo=
te" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204=
);padding-left:1ex">Using 1970s technology, you can build a cipher machine =
on<br>
rotor-like principles.=C2=A0 It has the virtue of &quot;not running<br>
any kind of malware because it can&#39;t&quot;.<br>
<br>
For example:=C2=A0 Use LFSRs (linear feedback shift registers)<br>
to drive the address lines on a bunch of EPROMs.=C2=A0 XOR the<br>
EEPROM outputs.<br>
<br>
Discussion:=C2=A0 If you use a LFSR directly, as a stream cipher,<br>
it is straightforward to ascertain the current state, whereupon<br>
you can predict the future state for all time.=C2=A0 But if you<br>
muddy it up using an EPROM (as a form of S-box) then that&#39;s<br>
not so easy.<br></blockquote><div><br></div><div>Are you assuming the conte=
nts of the EPROM is secret? If so, why not use it as a OTP? If not, then su=
rely this construction is trivially insecure?=C2=A0</div></div></div>

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