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7.3. BELASO, THE UNKNOWN AND PORTA, THE GREAT 113
(4) Decipher FFFHVFR. 5
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The influence of this system was great, in part because of Trithemius’ notoriety
and in part because his was the first published book on cryptology.
Letter-by-letter enciphering quickly became common and the basis for many
cipher systems.
In some ways this system is better than Alberti’s because a new alphabet
is used with each letter. Even very recognizable words like Mississippi will
become well-enciphered. (Alberti’s method with key F gives FRNXXNXXNUUN,
putting the keyletter first, while Trithemius’ method gives MJUVMXYPXYS.)
It also has clear weaknesses. For example, if the plaintext contains alphabetically
anti-consecutive letters, as in pon of pontoon or fed of federal then
the cipher text will contain a consecutive copies of the same letter. 6 While not
terribly common, this allows easy entries to decrypting the cipher. (This last
is Porta’s idea.) Nonetheless, Trithemius contributed the idea of changing the
key with each letter.
7.3 Belaso, the Unknown and Porta, the Great
Giovan Batista Belaso, unlike the others appearing in this chapter, is someone
we know little about. He was from Brescia, Italy and lived during the 1500’s.
What we do know is that in 1553 he published La cifra del. Sig. Giovan Balista
Belaso in which he described a ciphersystem with an easily remembered and
easily changed key. He called the key a countersign, although we will just
call it the keyword. His cipher became known as the Vigenère cipher and is
the most important and most used cipher in history. (Vigenère will appear in
Chapter 8.)
Belaso’s idea is an extension of Trithemius’s: to encipher pick a keyword
and use its letters cyclically as the key in a Caesar cipher to encipher the text.
Example: Encipher eek, eek, I saw a mouse near that computer using
the keyword TYPE. 7
plaintext e e k e e k i s a w a m o u s e n e a r t h a t c o m p u t e r
key T Y P E T Y P E T Y P E T Y P E T Y P E T Y P E T Y P E T Y P E
ciphertext X C Z I X I X W T U P Q H S H I G C P V M F P X V M B T N R T V
Answer: XCZIX IXWTU PQHSH IGCPV MFPXV MBTNR TV.
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5 (2) RFEWESMSM, (3) diagonal, (4) federal.
6 pontoon is enciphered as PPPWSTT.
7 Belaso was careful to use longer keywords, or better, phrases, like OPTARE MELIORA and
BIRTUTI OMNIA PARENT.