14 One-time pad

 

This chapter covers

  • One-Time Pad ciphers
  • The Vernam cipher, which approximates a one-time pad
  • Diffie-Hellman key exchange
  • How to construct the large primes needed for Diffie-Hellman and Public Key cryptography

The best-known stream cipher is the One-Time Pad. Many writers restrict this term to mean only a cipher where the plaintext and the key stream are exclusive-ORed byte-by-byte. This is historically inaccurate. The first one-time pad cipher was published in 1882 by Frank Miller, a Sacramento CA banker, for the purpose of saving money by shortening telegraph messages. Miller’s telegraph code used 5-digit code groups to represent words and phrases that were common in commercial telegrams. To obtain secrecy, Miller proposed a cipher which consisted of adding a 3-digit number to each 5-digit group. His code values were small enough so that the sum could never exceed 99999. That is, the codes were all less than 99000. So the one-time pad was originally a decimal system, not a binary system.

14.1  The Vernam Cipher

 
 

14.2  Key Supply

 
 
 
 

14.2.1    Circulating Key

 
 
 
 

14.2.2    Combined Key

 
 

14.2.3    Selection Key

 
 
 

14.3  Indicators

 
 

Diffie-Hellman Key Exchange

 

14.4.1    Constructing Large Primes, Old

 

14.4.2    Constructing Large Primes, New

 
 
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