Cryptology - Unofficial St. Mary's College of California Web Site
Cryptology - Unofficial St. Mary's College of California Web Site
Cryptology - Unofficial St. Mary's College of California Web Site
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164 CHAPTER 8. POLYALPHABETIC CIPHERS<br />
(a) On November 8, 1913 The Adjutant General’s Office in the War<br />
Department, Washington sent the following to the Chief <strong>of</strong> <strong>St</strong>aff,<br />
U.S. Army.<br />
HBYID EEKRA VVOIU RROGI FUIIJ ONADJ XKRBO SKPYU VFZGF JRZBV<br />
ZYKFS WYOMV MCIVP CYIUO IOPVV RSFSW PWKLQ SQVFG VKQTF VGKZI<br />
ZHSMR FIQQO XRYRZ TMSXD RBOWM DOEBK GIPHI KYWN.<br />
AAPH.<br />
What did it say<br />
(b) The HQ <strong>of</strong> Coast Defenses <strong>of</strong> Chesapeake Bay, Fort Monroe VA,<br />
struggled with this new cipher, as they reported in a letter <strong>of</strong> Nov 9,<br />
1913. To attempt a decipherment <strong>of</strong> a Larrabee message they first<br />
crossed “out the inadmissible code letters”, and then used (probably)<br />
the Red code book to treat the remaining letters as code groups.<br />
This gave “Appropriations for the 4th quarter British Minister J. F.<br />
Reynolds Landis Henry J. Nichols Facts do not justify having party<br />
arrested” as the message. Trying a slightly different technique gave<br />
“Congress did not appropriate merited unknown unknown permanent<br />
arrangement.”<br />
They eventually realized their mistake:<br />
“It was evident that the meaning was not that which was intended.<br />
[And, after further consideration] It is thought that this<br />
message might be translated by the Larrabee Cipher Code Card<br />
which was sent to the Depot Quartermaster, Newport News, Va.,<br />
on March 10, 1913, but which has apparently been misplacedby<br />
[sic] that <strong>of</strong>ficer as he is unable to furnish it.”<br />
Then on November 12 the same HQ sent<br />
ESAID FRTRQ DTTFV CRHOI DOSCN FNENF ITBRZ GMSHI RQSMH FHYNJ<br />
ZQMEI LFSJJ AOIZA DJWEF WJVCI SRLIG BHLYM SXVLA VWFFV ERKKS<br />
DURVJ GQEDF ULRGO PMSVR OSZGU QLVLQ JHQCI WHFIM EIIU<br />
Have they figured it out What does the message say<br />
(c) How is the Larrabee cipher related to the Vignere<br />
24. (a) Two very long message have been enciphered with a Vigenère cipher<br />
using the same five letter keyword. How do the ciphertexts’ frequency<br />
charts compare (“Very long” here indicates that the plaintext’s<br />
frequencies are close to that <strong>of</strong> typical English.)<br />
(b) Continuing, how the two ciphertext’s Φ values compare<br />
(c) Two very long message have been enciphered with Vigenère ciphers<br />
using five letter keywords. If the two ciphertexts’ frequency charts<br />
are very similar, can we conclude that the keywords were the same<br />
If yes, why If no, why not, and what may we conclude<br />
25. Explain in your own words why an endless, senseless key is needed for<br />
perfect secrecy.