Polyalphabetic Ciphers History

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Polyalphabetic CiphersHistory
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Alberti

• Rennaisance Architect
• First fountains of Trevi, Rome
• Churches
• 1467 essay on cryptology
• Frequency analysis
• Cipher disk for polyalphabetic cipher
• Scrambled alphabet
• Changed key letter only after 3 or 4 words

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Albertis cipher disk
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Johanne Trithemius

• German Monk
• Known for magic
• Wrote Polygraphia in 1518, published posthumously
  in 1518
• Introduced tableaux of shifted alphabets
• But just went from one alphabet to the next
• First letter with first alphabet, second with
  second, etc.

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Tritemius Tableaux
a b c d e f g h i j k l m n o p q r s t u v w x y
z ------------------------------------------------
--- A B C D E F G H I J K L M N O P Q R S T U V W
X Y Z B C D E F G H I J K L M N O P Q R S T U V W
X Y Z A C D E F G H I J K L M N O P Q R S T U V W
X Y Z A B D E F G H I J K L M N O P Q R S T U V W
X Y Z A B C E F G H I J K L M N O P Q R S T U V W
X Y Z A B C D F G H I J K L M N O P Q R S T U V W
X Y Z A B C D E G H I J K L M N O P Q R S T U V W
X Y Z A B C D E F H I J K L M N O P Q R S T U V W
X Y Z A B C D E F G I J K L M N O P Q R S T U V W
X Y Z A B C D E F G H J K L M N O P Q R S T U V W
X Y Z A B C D E F G H I K L M N O P Q R S T U V W
X Y Z A B C D E F G H I J L M N O P Q R S T U V W
X Y Z A B C D E F G H I J K M N O P Q R S T U V W
X Y Z A B C D E F G H I J K L N O P Q R S T U V W
X Y Z A B C D E F G H I J K L M O P Q R S T U V W
X Y Z A B C D E F G H I J K L M N P Q R S T U V W
X Y Z A B C D E F G H I J K L M N O Q R S T U V W
X Y Z A B C D E F G H I J K L M N O P R S T U V W
X Y Z A B C D E F G H I J K L M N O P Q S T U V W
X Y Z A B C D E F G H I J K L M N O P Q R T U V W
X Y Z A B C D E F G H I J K L M N O P Q R S U V W
X Y Z A B C D E F G H I J K L M N O P Q R S T V W
X Y Z A B C D E F G H I J K L M N O P Q R S T U W
X Y Z A B C D E F G H I J K L M N O P Q R S T U
V X Y Z A B C D E F G H I J K L M N O P Q R S T U
V W Y Z A B C D E F G H I J K L M N O P Q R S T U
V W X Z A B C D E F G H I J K L M N O P Q R S T U
V W X Y
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Giovan Batista Belaso

• Otherwise unknown
• Small booklet in 1553
• Used Trithemius tableaux of shifted alphabets
• Added the idea of a key word or phrase
• The longer the better

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Giovanni Batista Porta

• Wrote several books
• Scientific subject
• Magic
• Book on cryptology De Furtivis Literarum
• 1563
• Included classification of methods
• Cryptanalysis
• How to solve monoalpha with no word breaks
• Likely word method
• Digram cipher (using non-letter symbols)

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Giovanni Batista Porta

• Polyalphabetic cipher
• Combined elements of
• Alberti scrambled alphabet
• Trithemius - letter-by-letter change of alphabet
• Belaos key word/phrase
• Scrambled alphabet could be from Alberti style
  disk or from table

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Blaise de Vigenere

• From age of 24 served the Duke of Nevers (France)
  in Court and as Diplomat
• 2 years in Rome as diplomat introduced him to
  cryptology
• Quit the court in 1570 (age 47) and wrote on many
  subjects
• Wrote Traicte de Chiffres in 1585
• Collected much previous work (with credit)

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Vigenere

• Polyalphabetic cipher
• Tableaux like Trithemius
• But scrambled alphabet on borders
• Makes it like Albertis disk
• Discussed several key methods
• Words, phrase, poetry
• Date of dispatch
• Ordered, like Trithemius
• New contribution!
• Autokey
• Text itself is the key!
• Key letter to start, then each letter in message
  is key for the next.

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Practice of Cryptography

• Polyalphabetic not used until 19th century
• Era of Black Chambers
• Government agencies for cryptanalysis
• French, Italian, German, Austrian
• Encryption typically used ciphers and
  nomenclators (combined code and cipher)
• Introduction of telegraph (1840s) led to use of
  better methods, polyalphabetic

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Vigenere

• Ironically has name attached to easy version of
  polyalphabetic, as done by Porta, not his better
  autokey system.
• We will look at this method and how to break it.