COMP 133 - Assignment 3 |
IGDLK MJSGC FMGEP PLYRC IGDLA TYBMR KDYVY XJGMR TDSVK ZCCWG ZRRIP
UERXY EEYHE UTOWS ERYWC QRRIP UERXJ QREWQ FPSZC ALDSD ULSWF FFOAM
DIGIY DCSRR AZSRB GNDLC ZYDMM ZQGSS ZBCXM OYBID APRMK IFYWF MJVLY
HCLSP ZCDLC NYDXJ QYXHD APRMQ IGNSU MLNLG EMBTF MLDSB AYVPU TGMLK
MWKGF UCFIY ZBMLC DGCLY VSCXY ZBVEQ FGXKN QYMIY YMXKM GPCIJ HCCEL
PUSXF MJVRY FGYRQ
Problem 2. The following text was encrypted using a column transposition cipher. What are all of the possible values for the number of columns used assuming the plaintext fills the set of columns exactly? What is the plaintext?
ENGNT ESEDH IEEBA ESORO ENWHN CASAE HOAEP DDKRC HSPOH SDECI POYWI
SHNEM TCTFY NNAOI TAPTS
Problem 3. In order to make things more difficult for Eve, Alice and Bob decide to perform a double encryption as follows: First, they encrypt the plaintext using a Caesar shift and then they encrypt the resulting ciphertext using a monoalphabetic substitution cipher. Is this system more secure, less secure or about the same as a single encryption using a monoalphabetic substitution cipher? Is it more secure, less secure or about the same as a single encryption using a Caesar shift? Explain your answers.
Problem 4. Vigenere is associated with the autokey method of generating long keys for enciphering using the Vigenere table. In the standard Vigenere method the key is a repeated phrase placed above the text, e.g.,
Key: KING KING KING KING
Plaintext: HERE ISTH EMES SAGE
Ciphertext: RMEK SAGN OURY CITK
There are two versions of the auto key method. In the first, after using the key to start the enciphering, one uses the plaintext as the running key, e.g.,
Key: KING HERE ISTH EMES
Plaintext: HERE ISTH EMES SAGE
Ciphertext: RMEK PWKL MEXZ WMKW
In the second method, after using the key to start, one uses the ciphertext as the running key, e.g.,
Key: KING RMEK ZEXR DQBJ
Plaintext: HERE ISTH EMES SAGE
Ciphtertext: RMEK ZEXR DQBJ VQHN
One of these two methods is much less secure than the other. Which one and why? (Assume an attacker knows which system is being used.)
Problem 5. The following pairs of ciphertexts were computed by permuting the letters of two words using the same permutation, e.g. DAB GOD decodes to BAD DOG since in both cases the letters are reversed. Find the original pairs in each case.
Bonus #1. Find the longest word you can think of that appears in a standard dictionary of American English which has all of its letters in alphabetical order. An example is LOST since L appears before O which appears before S which appears before T in alphabetical order. Why is it a bad idea to use such a word as a keyword in a column transpostion cipher?
Bonus #2. Create an (hopefully amusing or apt) anagram using your name or a variation on your name or a nickname. For example, my full name is David Daniel Martin Krizanc and if you let me cheat a little and use D. Dany Krizanc I can get CRAZY AND KIND. If you can come up with something better for me I would be interested as well.
| Report problems to dkrizanc at wesleyan dot edu
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