[Cryptography] Secret sharing for family members

[Dave Horsfall]

On Fri, 16 Oct 2020, Jerry Leichter wrote:

> Neither writable CD's nor SSD media are good for long-term storage. 
> Both will deteriorate and become unreadable in a fairly small number of 
> years.  Given the underlying nature of the task here - making data 
> available to heirs - the limits on the lifetime of the media is 
> significant.

I've heard that writable DVDs will deteriorate over time (by design), and 
that it's best to store them in the freezer.  You also need to keep that 
purple plastic wrapper on, to protect them from UV sunlight when out of 
the freezer and not being used.

> Others have suggested, for other reasons, going with paper.  Paper, if 
> reasonably carefully stored, should remain readable for many decades. 
> Go with acid-free paper and store it in a safety deposit box and it 
> should be good for many centuries.

I think that "acid-free" paper is also known as green-washing i.e. be seen 
to be doing something good, whilst not actually doing so.

The ancient Egyptians (with their hieroglyphics) got it right, centuries 
later; apparently they weren't using crypto, but were just showing off.

> I wouldn't go fancy with QR codes - a technology that might fade.  Just 
> use a couple of randomly chosen worss - easy for anyone to type, easy to 
> get enough entropy for a portion of a key.

Yep, just another technology that will fade.  The QR codes that I check on 
my iPhone from time to time in the shops are usually unreadable.

> I don't know of any stock program to do this kind of secret splitting 
> and recombining, but the algorithms are simple enough.  You could 
> include a listing of such a program on the sheet of paper just in case 
> the program itself isn't readily available years from now.  I'd suggest 
> FORTRAN as it's likely to survive us all. :-)

Argghh!  You mentioned the "F" word :-)  But yes, it will be along for
ages, as were the dinosaurs.

-- Dave