Weaknesses of a Cipher
List a weakness of the Caesar Shift Cipher.
(Sample) There are only 25 possible keys.
Explain the technique of brute force.
You try every possibility until you find the correct one.
Decrypt the following Caesar shift:
DOVSPCLZPUHWPULHWWSLBUKLYAOLZLH
Caesar-H (shift of 7)
who lives in a pineapple under the sea
Explain how you would use keyword ORANGE during ADFGVX encryption.
Rearrange columns to AEGNOR
Reduce 4579 mod 12
7
List a weakness of a monoalphabetic substitution cipher.
(Sample) You can easily use frequency analysis to decrypt if the message is long enough.
Explain the technique of frequency analysis.
You find the frequencies of each ciphertext letter and compare them to the frequencies of letters in the English language.
Decrypt the following Caesar Shift:
WDQJXYSCUQDIHUBQJYDWJEJXUZQMI
Caesar-Q (shift of 16)
gnathic means relating to the jaws
Given the Crib: np→PQ for a Playfair cipher, what is the most likely arrangement of N, P, and Q in the keygrid?
They are in the same row, but not the first (in the keyword).
Bonus: probably in row 4
Find determinant of
15 7
7 5
26
List a weakness of the Vigenere Cipher.
(Sample) If the message is long enough, you can use repeated strings to determine the length of the keyword and eventually figure out the actual letters using frequencies.
Explain the technique of cribbing.
You make a guess of a certain word/phrase and its location in the ciphertext message to try to figure out the key.
Decrypt the following Vigenere cipher with the keyword LIGHT
EPKYXTATVLFJYABECZLYZZNHKOEUYD
there is no substitute for hard work
-Thomas Edison
Given the Crib: bo→DN in a Playfair cipher, what is likely arrangement of B, O, D, and N in the grid?
a rectangle
Bonus: B and D most likely in row 2; O and N most likely in keyword or row 4
Find the matrix product AxB if
A = 25 8 and B = 12 39
3 19 7 1
356 983
169 136
List a weakness of the Affine Cipher.
(Sample) There are only 312 keys because many multipliers do not have inverses mod 26.
OR
(Sample) You only need a two letter crib to find the key.
Explain the Babbage-Kasiski method.
You first find repeated strings of letters and calculate their distances. You then find the gcds of the distances to determine a possible keyword length. Once you have the length, you use the frequencies of each letter position to determine the actual letter in the keyword.
Decrypt the following Affine cipher that was encrypted using C = 9*P + 11 (mod 26):
PLXVTHJIRVGEQIHJMAHMLT
make yourself proud today
Which of these would be the worst keyword for the ADFGVX cipher: MIST, MOST, MUST, or RUST?
MOST because it is already alphabetized.
Use the Extended Euclidean Algorithm to find the inverse of 113 mod 199.
118