[Cryptography] A two key file/program

[Ángel]

On 2019-06-08 at 19:38 -0700, Allen Schaaf wrote:
> Hi Vitor and the rest of you, 
> 
> Thanks for the various responses. It is quite helpful in clarifying my
> thinking.
> 
> Not being a programmer, some of the answers are beyond me, alas.
> 
> One thing that I now realize that I did not include is the need for
> more than just two people to access the file. Just for discussion,
> lets assume that there are five people, A, B, C, D, E, with an access
> key. What is needed is one of the ten combinations to cover the the
> presence/absence issues. So A/B, A/C, A/D, A/E, B/C, B/D, B/E, C/D,
> C/E, D/E, all ten possibilities would cover presence possibilities.
> 
> Thanks,
> 
> Allen

Shamir's Secret Sharing can do that.

As stated however, by sharing the passwords you are weaking your
process. Suppose that when auditing the account for 'John', the pair A/B
found some crookedness. This could be that John made that. Or that C/D
did that with John account. Or D/E. Or even A/E, and A is now knowingly
blaming another employee for its miscreants.

And even if John was the guilty on, he will claim he is not and sue
back, and you cannot prove otherwise.