TheCodeBreakers
Britannica.) Kasiski died on May 22, 1881, almost certainly without
realizing that he had wrought a revolution in cryptology.
That revolution began when Kasiski shrewdly noted a phenomenon:
the conjunction of a repeated portion of the key with a repetition in the
plaintext produces a repetition in the ciphertext:
key RTJNRTJNRUNRUNRTJNRUNRUNRTJNRtJNRUN
plaintext tobeornot tobethatisthequestion ciphertext KIO v i EE io K Io
v NUB N v J N u VKHVMQZ I A
Each time that the key RUNR engages the repeated plaintext to be, the
repeated ciphertext tetragraph KIOV results. Like causes produce like
effects. Similarly, when the repeated key-fragment UN operates upon the
repeated th's, the ciphertext registers repeated NU'S.
Clearly, the keyword must repeat one or more times for a given part of
it to encipher two identical bits of plaintext several letters distant from
one another. The number of letters between the two resultant ciphertext
annaabi.ee 
