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7.8. SUMMARY 123
Repetition Start Positions Distance Factors
YVGYS 3, 283 280 2 × 2 × 2 × 5 × 7
STY 7, 281 271 2 × 137
GHP 28, 226 198 2 × 3 × 3 × 11
EZM 48, 114 66 2 × 3 × 11
EZM 48, 198 150 2 × 3 × 5 × 5
ZUDLJK 52, 148 96 2 × 2 × 2 × 2 × 2 × 3
LJK 55, 151 96 2 × 2 × 2 × 2 × 2 × 3
LEEBMMTG 99, 213 114 2 × 3 × 19
CSSVMRS 107, 203 96 2 × 2 × 2 × 2 × 2 × 3
SEZM 113, 197 84 2 × 2 × 3 × 7
ZMX 115, 163 48 2 × 2 × 2 × 2 × 3
RWA 138, 234 96 2 × 2 × 2 × 2 × 2 × 3
GEE 141, 249 108 2 × 2 × 3 × 3 × 3
Figure 7.3: A Kasiski Table
7.8 Summary
The Vigenère cipher is the most successful cipher in history. It is really nothing
but a Caesar cipher with the key being changed in a pattern provided by a
keyword or keyphrase, but produces a polyalphabetic cipher. One continually
enciphers by using the next letter of the keyphrase to encipher the next letter
of the message, repeating the keyphrase when necessary. When the key consists
of many letters the frequency counts produces are quite flat, successfully hiding
the pattern of the repetition of the key.
Misnamed though it was, when used correctly the Vigenère cipher was all but
unbreakable for 300 years and remained in use even after techniques for routinely
decrypting its messages were known. A sign of the value of an idea is the number
of times it is rediscovered, and the numerous times the Vigenère was reinvented,
sometimes in its standard form, sometimes in the form that the Beaufort and
the Variant Vigenère take, show the idea behind this polyalphabetic cipher to
be a very good one indeed.
It wasn’t until Kasiski’s test that a general test for decrypting a carefully
enciphered Vigenère cipher was known. To perform this test, find the distances
between repetitions in the ciphertext. The largest number that divides most of
these distances is likely the length of the keyword (or a multiple of it). From
here, divide the ciphertext into groups of letters that were enciphered by the
same keyletter and decrypt these individual groups as simple Caesar ciphers.
As we will see, the next 70 years of cipher history would be dominated by
guarded by ciphers. The ciphers must be capable of frequent changes. The rules by which
these changes are made must be simple. Ciphers are undiscoverable in proportion as their
changes are frequent and as the messages in each change are brief.” From Albert J. Myer’s
Manual of Signals.