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2.2.3 Cryptanalysis of Monoalphabetic Ciphers
• Cryptographer’s Dilemma
– an encryption algorithm has to be regular in order for it to be
algorithmic and in order for cryptographers to be able to remember
it
– the regularity gives clues to the cryptanalyst
– there is no solution to this dilemma
• Cryptography : principle of timeliness
Principle of timeliness(Adequate Protection) :
A security measure must be strong enough to keep out the
attacker for the life of the data. Data with a short time
value can be protected with simple measures
2.3 Polyalphabetic Substitution Ciphers
• Weakness of monoalphabetic ciphers
– their frequency distribution reflects the distribution of the
underlying alphabet
• Polyalphabetic substitution cipher
– one way to flatten the distribution
– to combine distributions that are high with ones that are low
• T is sometimes enciphered as‘a’and sometimes as ‘b’
• frequency of ‘a’is high and that of ‘b’is low
A cipher is more cryptographically secure would display a
rather flat distribution, which gives no information to a
cryptanalyst
