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130 CHAPTER 7.
VIGENÈRE CIPHERS
20. A common reinvention of the Vigenère Cipher reduces it to a progressive
key cipher. From [Pall] we have “The Pass-Word Cipher.” To use it, select
a keyword, say PRUDENTIA, and then encipher using the table produced,
using a new line for each new word.
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
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
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
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
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 a b c d e
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
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
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
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
The example given is JXU KGDVAWJK HPODIT JSV BFSY CT PCWNOUFMA
BDYYUH VT EH LZWQ RDGG VIZSPX YT HVS YHYGS. Decipher it.
21. Graf Gronsfeld suggested that the Vigenère keyword be replaced by a short
series of numbers. For example, come here enciphered with keynumber
1403 is DSMH IIRH. Now known as the Gronsfeld Cipher, in effect it is
a Vigenère Cipher as a composite of Shift Ciphers, rather than the usual
composite of Caesar Ciphers.
(a) Encipher Gronsfeld with keynumber 296.
(b) Decipher JVIMZ HTXLL LR. The keynumber was 34870.
(c) Decipher EMSLH MCOF. The keynumber was 2625.
(d) Considering the example given in the question, what makes this a
good cipher
(e) For a sizable ciphertext, we can use the techniques for decrypting a
Vigenère Cipher to decrypt a Gronsfeld Cipher. But because of the
restricted choice of keys (really, only words made up from the first
ten letters of the alphabet), the Gronsfeld is considerably less secure
than the Vigenère.
The following ciphertext has been enciphered using a Gronsfeld Cipher
in which the keynumber uses only (some of) 0’s, 1’s, 2’s and
3’s. By trial-and-error, can you decrypt the message (Hint: each
ciphertext letter could come from at most four plaintext letters: D
from a, b, c, or d. Neatly list them, and see if you cannot build a
message.)
LHEBE SQGLL OUSWB KRZBA TRIGA YFP.