Polyalphabetic Ciphers History

1. Polyalphabetic CiphersHistory

2. 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

3. Alberti’s cipher disk A Z B C T J R D E P E M F A

4. 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.

5. 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

6. 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

7. 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)

8. 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

9. 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)

10. 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 • Text itself is the key! • Key letter or word to start, then each letter in message is key for the next.

11. 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

12. 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.