Polyalphabetic Ciphers History Alberti



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Polyalphabetic Ciphers History

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

Alberti’s cipher disk

  • F
  • E
  • D
  • C
  • B
  • A
  • T
  • R
  • A
  • P
  • E
  • M
  • Z
  • J

Johanne Trithemius

  • German Monk
  • Known for magic
  • Wrote Polygraphia 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.

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

Giovan Battista 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

Giovanni Battista Porta

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

Giovanni Battista Porta

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

Blaise de Vigenere

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

Vigenere

  • Polyalphabetic cipher
    • Tableaux like Trithemius
    • But scrambled alphabet on borders
      • Makes it like Alberti’s disk
    • Discussed several key methods
      • Words, phrase, poetry
      • Date of dispatch
      • Ordered, like Trithemius
    • New contribution!
    • Autokey

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

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.


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