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

 wrote:

>> 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.
>
> Hmmm... how would the user know anything about the secret unless he  
> could somehow get to the resulting combined password?
>
> For example, if my password is "chocolate" and the application  
> secret is "fudge", I can't recover any part of the combination  
> "chocolate-fudge" unless ...

Assuming that you how the compound password is generated, you now  
know that it starts with "chocolate" - any attack now has a smaller  
space to search.  The more sophisticated compounding schemes work  
such that even knowing part of the secret, and the compounding  
method, the search space is the same size as if you don't know any  
part of the secret.

> So, in your opinion, this isn't a crazy idea?  It should work?  But  
> it could be made more secure if Andrus is particularly paranoid.

Yah, that's all I meant to indicate.  As others have observed, a  
determined user can sniff the compound password out if they really  
wish.  I suspect the only really secure approach is some sort of  
challenge-response algorithm, or a one-time pad in the application -  
in either case, whatever the black-hat user sniffs off the wire or  
with a debugger changes every session.

- John D. Burger
   MITRE