Cryptographic system with concealed work factor

Abstract

Methods and apparatus for providing cryptographic keys for cryptographically processing information are provided. Means are provided for receiving a first cryptographic signal. Means responsive to said first cryptographic signal are provided for generating the cryptographic key. The cryptographic key has an associated first work factor for a person without knowledge of said first cryptographic signal. The cryptographic key also has an associated second work factor which is smaller than the first work factor for a person with knowledge of the first cryptographic signal. Thus, the work factor of the key can be reduced to a level which is small enough to satisfy governmental export or import requirements without reducing the protection level or strength of the system. A single cryptographic key generator (engine) can be easily adapted for use in different countries, where different work factors are required.

What is claimed is:  
     
         1 . Apparatus for providing a cryptographic key for cryptographically processing information, comprising: 
 means for receiving a first cryptographic signal; and    means responsive to said first cryptographic signal for generating said cryptographic key, wherein: 
 said cryptographic key has an associated first work factor for a person without knowledge of said first cryptographic signal; and  
 said cryptographic key has an associated second work factor which is smaller than said first work factor for a person with knowledge of said first cryptographic signal.  
   
     
     
         2 . Apparatus in accordance with  claim 1 , further comprising: 
 a key generator for generating a first key according to at key generator scheme.    
     
     
         3 . Apparatus in accordance with  claim 2 , wherein said means responsive to said first cryptographic signal comprises: 
 a key space reducer operatively associated with said key generator for reducing a key space of said first key in accordance with a key space reducing scheme.    
     
     
         4 . Apparatus in accordance with  claim 3 , further comprising: 
 a key distributor operatively associated with said key space reducer for distributing said reduced key space over a larger key space in accordance with a key space distribution scheme to provide said cryptographic key.    
     
     
         5 . Apparatus in accordance with  claim 4 , wherein: 
 said larger key space is indicative of said first work factor; and    said reduced key space is indicative of said second work factor.    
     
     
         6 . Apparatus in accordance with  claim 3 , wherein said second work factor is the same as a work factor of said reduced key space.  
     
     
         7 . Apparatus in accordance with  claim 3 , wherein said key space reducer comprises: 
 means for performing a digital logic function on said first key to provide said reduced key space.    
     
     
         8 . Apparatus in accordance with  claim 4 , wherein said key space distributor comprises: 
 means for performing a digital logic function on said reduced key space to provide said cryptographic key.    
     
     
         9 . Apparatus of  claim 3 , further comprising: 
 means for providing a key space size input variable to said key space reducer; wherein 
 said key space reducer controls the size of said reduced key space in accordance with said input variable.  
   
     
     
         10 . Apparatus in accordance with  claim 9 , wherein said key space reducer comprises at least one of: 
 (a) an AND gate for ANDing of said input variable with said first key to provide said reduced key space; or    (b) an OR gate for ORing of said input variable with said first key to provide said reduced key space.    
     
     
         11 . A method for providing a cryptographic key for cryptographically processing information, comprising the steps of: 
 receiving a first cryptographic signal; and,    generating said cryptographic key in response to said first cryptographic signal, wherein: 
 said cryptographic key has an associated first work factor for a person without knowledge of said first cryptographic signal; and  
 said cryptographic key has an associated second work factor which is smaller than said first work factor for a person with knowledge of said first cryptographic signal.  
   
     
     
         12 . A method in accordance with  claim 11  further comprising: 
 generating said first key according to a key generator scheme.  
 
     
     
         13 . A method in accordance with  claim 12 , further comprising: 
 reducing a key space of said first key in accordance with a key space reduction scheme.    
     
     
         14 . A method in accordance with  claim 13 , further comprising: 
 distributing said reduced key space over a larger key space in accordance with a key space distribution scheme to provide said cryptographic key.    
     
     
         15 . The method in accordance with  claim 14 , wherein: 
 said larger key space is indicative of said first work factor; and    said reduced key space is indicative of said second work factor.    
     
     
         16 . The method in accordance with  claim 13 , wherein said second work factor is the same as a work factor of said reduced key space.  
     
     
         17 . The method in accordance with  claim 12 , wherein said key generator scheme generates said first key randomly.  
     
     
         18 . The method in accordance with  claim 13 , wherein said key space reducing scheme comprises the step of performing a digital logic function on said first key to provide said reduced key space.  
     
     
         19 . The method in accordance with  claim 14 , wherein said key space distribution scheme comprises the step of: 
 providing a second key; and    distributing said reduced key over said larger key space in accordance with said second key to provide said cryptographic key.    
     
     
         20 . The method in accordance with  claim 14 , wherein said key space distribution scheme comprises the step of performing a digital logic function on said reduced key space to provide said cryptographic key.  
     
     
         21 . The method in accordance with  claim 11 , comprising the further step of: 
 dropping bits from said cryptographic key to provide a reduced bit length cryptographic key.    
     
     
         22 . The method in accordance with  claim 13 , comprising the further steps of: 
 providing a key space size input variable; and    controlling a size of said reduced key space in accordance with said input variable.    
     
     
         23 . The method in accordance with  claim 22 , wherein said key space reducing scheme comprises at least one of: 
 (a) ANDing of said input variable with said first key to provide said reduced key space; or    (b) ORing of said input variable with said first key to provide said reduced key space.    
     
     
         24 . The method in accordance with  claim 11 , wherein said cryptographic key is a B-bit key, and said first work factor is no greater than S=2 B .  
     
     
         25 . The method in accordance with  claim 13 , wherein: 
 said cryptographic key is a B-bit key, and said first work factor is no greater than S=2 B ;    said reduced key space corresponds to a key space of a (B−F)-bit key; and    said second work factor is no greater than W=2 B-F .