[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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