Fwd: How to allow users to log on only from my application

<korryd(at)enterprisedb(dot)com> wrote:

> Say that your application offers a way for each user to set/change
> his own password.
>
> When I (using your application) change my password, you could
> combine my new password with a secret value and then send the
> result to the PG server (so now the PG server thinks that my
> password is my_password+your_secret).

This is a special case of (2,2) secret sharing:

http://en.wikipedia.org/wiki/Secret_sharing

Here the secret is the actual password, a+b, shared into two parts, a
and b. The above scheme suffers from the problem that the user now
knows quite a lot about the secret. If this is an issue, there are
more sophisticated combining schemes that give the user no advantage
over someone who knows neither half of the secret.

- John D. Burger
MITRE