Assignment #4, Due 27 Sep

Show your work on all problems. Feel free to use a calculator.

1.

Suppose that you are Eve, listening in to Alice and Bob having a conversation using the Massey-Omura ``no-key'' system and a multiplicative substitution cypher, as illustrated in class.

Alice: LJCQUXYUAKNUXQ
(recall that this is the plaintext encrypted with Alice's key)

Bob: XTSUCVKCOIBCVU
(this is the previous ciphertext encrypted with Bob's key)

Alice: NPWIEBAEYOLEBI
(this is the previous ciphertext decrypted with Alice's decryption key)

Mount a known-plaintext attack to get at least one of the keys and find the original message. Show all your steps.

2.

Exercise 16.1(a) on page 96 of A Friendly Introduction .

3.

Exercise 17.1 on page 101 of A Friendly Introduction . (Big hint: $\phi(1147) = 1080$.) Use successive squaring. You may need to adjust the equation you get from the Euclidean Algorithm so that the numbers are positive.

4.

Exercise 18.1 on page 106 of A Friendly Introduction . (Big hint: 7081 = (73)(97).) See note on problem above.

5.

Now that your organization has implemented your cryptographic system, you discover that someone outside appears to have been reading your messages. Your security chief assures you that it could not have been a cyphertext-only attack. Write a paragraph explaining in detail one possible way this could have happened.

About this document ...

Assignment #4, Due 27 Sep

This document was generated using the LaTeX2HTML translator Version 97.1 (release) (July 13th, 1997)

Copyright © 1993, 1994, 1995, 1996, 1997, Nikos Drakos, Computer Based Learning Unit, University of Leeds.

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The translation was initiated by Joshua Holden on 9/21/2000

Footnotes

...attack.

Nor was it the attack used in problem 1.