Secret Key Cryptography

14 One-time pad

This chapter covers

• One-time pad ciphers
• The Vernam cipher, which approximates a one-time pad
• Diffie-Hellman key exchange
• Constructing 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 ...