SECURITY and CRYPTOGRAPHY 15-827                  16 OCT 01
             Lecture #9                                  M.B.
                                                      4615 Wean


 READING: 
"The Hunting of the Snark," by Lewis Carroll.

"Secure Human Identification Protocols," by Hopper and (apparently) Blum.



 PhonOID: I'm having trouble along several dimensions:

1. I want the protocol to be FUN.
What's FUN?  FUN is being able to whip out something quickly that
completely stumps everyone.

2. I want it to work against CHOSEN CHALLENGE attack.
There are at least 2 possible ways to try and achieve this:

  2.1 RESTRICT the class of allowable challenges thru the use of an error
correcting code that eliminates most challenges. or

  2.2 Compute a CHECKSUM, say the sum of the challenge digits (mod 10). Use
it, for example, to find a starting point. A change in one digit would then
have a serious effect on the response.

 For example, HEALTH = 5, while WEALTH = 4. Thus the protocol on Challenge
HEALTH might start at T, while on the Challenge WEALTH it would start on L.
 This, however, may be difficult for the arithmetically challenged.


Sample protocol based on above ideas.
Challenge = a word or sentence having 5 or 6 characters.

    CHALLENGE= JUMP      DUMP      BUMP      HUMP           
    HIDDEN   =    5         8         6         2           
    RESPONSE = <2,2,4,6> <7,0,2,4> <1,0,4,6> <2,1,4,4>      
                                                            
    CHALLENGE= JUMPS     DUMPS     BUMPS     HUMPS          
    HIDDEN   =    0         3         1         7           
    RESPONSE = <6,1,2,7> <9,0,4,8> <2,0,4,2> <2,1,4,8>      
                                                            
    CHALLENGE= STUMP     LUMP      DREAM     REAM           
    HIDDEN   =    6         5         9         9           
    RESPONSE = <2,2,4,8> <2,2,4,4> <7,5,9,2> <8,8,0,2>      
                                                            
    CHALLENGE= STUMPS    LUMPS     SCREAM    CREAM          
    HIDDEN   =    1         0         0         5           
    RESPONSE = <2,2,4,2> <6,1,2,6> <7,8,9,4> <6,4,7,2>