[Cryptography] retro crypto

Ben Laurie ben at links.org
Sun Jan 12 00:44:57 EST 2020


On Wed, 8 Jan 2020 at 21:59, John Denker via cryptography <
cryptography at metzdowd.com> 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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