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162 CHAPTER 8. POLYALPHABETIC CIPHERS
Which text is in German, and which is in French (Compute Φ of
each and, remembering that monoalphabetic encipherments do not
change the Φ value of the text, use part (a).)
21. Here is a variant on the auto-key method. Rather than use the message
as the key, which perhaps seems rather dangerous, instead use the secret
message
Encipher mysterious with priming key m and use your answer to help you
discuss the security of this method.
22. Joseph Willard Brown wrote
“Ciphers are undiscoverable in proportion as their changes are frequent,
and as the messages in each change are brief. When alphabetic
ciphers are used, the aim should be never to allow any letter to appear
twice alike. The number of letters under each key is to be as
small as possible [Brown page 99].”
Explain the meaning of this quote.
23. On March 8, 1913, the Secretary of State, William Jennings Bryan, sent
the following message to all American Diplomatic and Consular Officers.
“Gentleman:
For the purpose of affording a means of direct secret communication
between officers of the Army and Navy and the Diplomatic and
Consular Service, the respective Departments are providing the necessary
officers with the Larrabee Cipher Code, a copy of which is
transmitted herewith.
The following arrangements as to key words will be observed:
(1) Key words may be arranged between officers using code either
directly or through their Departments in Washington, when more
direct secret communication is not possible.
(2) In the absence of other agreement, the key word is to be the name
of the month in which the message is sent.
The Navy Department is supplying the Larrabee Cipher to the commanding
officers of all ships in the service.”
Enclosed with this letter was a chart, Figure 8.2, and instructions:
“Write down the words, message, or cipher to be converted, and write
over them, letter for letter, the key word agreed upon, repeating
the key word as often as necessary. For each letter to be converted
enter the table with the letter of the key word found above it, as a
marginal letter. If converting message into cipher [read] in the upper
line abreast the marginal letter, the letter of the message, the letter
of the cipher is [then] directly below it. If converting cipher into
message [read] the lower line abreast the marginal letter [, then] the
letter of the cipher the letter of the message is directly above it.