[FoRK] Crypto protocol for only good news?

Matt Jensen < mattj at newsblip.com > on > Tue Aug 15 13:59:15 PDT 2006

I'm looking for a technique for revealing info, but only if the info  
has one particular value.

For example: Suppose a couple is pregnant with their second child. The  
first kid is a girl, and though the couple will love the second kid  
regardless of gender, they'd be extra excited if it turned out to be a  
boy.  (Plus, they could start outfitting the nursery.)

So, we now want to know if there's some procedure whereby they can  
have the sonographer tell them if it's a boy, but not tell them  
anything if it's a girl.  Obviously, not telling them anything tells  
them it's a girl.  Anyway around this?

Ideally, you'd like a 100% chance of learning that it's a boy.   
However, in order trust an answer like "I can't tell you the result  
because our protocol forbids it", it seems you have to accept a lower  
chance of getting the good news.

I suppose you could have the sonographer secretly flip a coin, and if  
it's heads, she tells the parents the gender.  But if it's a girl you  
actually have the sonographer lie to you, and tell you she got tails,  
so she can't tell you the gender.  But even that tells you something:  
there are two "tails" outcomes for  a girl and only one "tails"  
outcome for a boy.

I feel like there's a better answer to this question, but I'm missing  
it.  Ideas?

-Matt Jensen

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