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INI File  |  1986-09-04  |  2KB  |  26 lines
  1. [Read? Yes/No/Reply/Mod/Quit/Help] [yes]                                       
  2.                                                                                
  3. Just got my Sept 86 Byte, enjoyed your article on the data encryptor           
  4.  In the text box entitled 'Cryptography Background' on page 98 there           
  5.  is a slightly misleading statement:  the key size of the key formed           
  6.  by expanding two keys of length 999 and 1001 is 999,999.  This is             
  7.  of course true,  but this new key does not consist of 999,999                 
  8.  INDEPENDENT alphabets.  The effective key length is only 2000                 
  9.  characters (although complicated by the permutation).  The cipher             
  10.  is vulnerable to attack in much the same way as the N-tape Vignere            
  11.  cipher (Konheim,p.165)                                                        
  12.                                                                                
  13.  Although I have not fully analysed the pseudo-random number                   
  14.  generator that you use in the encryptor, I think there is a risk              
  15.  of partial information being recoverable from sufficient ciphertext           
  16.  because bit N can be guessed correctly over 50% of the time given             
  17.  bits N-k...N-1.  Note that RSA, etc, do not suffer from this                  
  18.  weakness.                                                                     
  19.                                                                                
  20.                                --- Mike Naunton.                               
  21.                                                                                
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