Homework SourseStudents must break this cipher text by hand not by a decode
Students must break this cipher text by hand, not by a decode software other than the Excel worksheet provided. The main purpose of this homework is NOT to learn the cryptanalysis technique, but to learn the importance of encryption.So, if you cannot find the correct answer, do not waste too much time on it, and submit a report what you have done as an incomplete work.
*** HW1 ciphertext *** j r g d g i d x g q a n n g z g t g t t s i t g j r a n m n o e d d i o m n w j r a j v k s e x j m d x k m n w j r g m t t g d t g o g n j a j m z g o v g k i n l a q g t j a m n x m s m j j r g k o j t g n w j r g n j r g v a t t m g t a w a m n o j j r g w i z g t n s g n j i b a b g u ***** Hint for Homework #1 ******
To break a cipher text, you always begin with a simple method - character substitution (shift to right a certain number just like Caesar did, or to the left). If it does not work, you can try more organized substitution. Rearrange your substitution part in the encryption table - begin with \'a\' for \'A\' (plaintext) and skip to character (2, 3, 4, 5, 6, ...) to the right or to the left and put the next character... until you fill out all the substitution chart. Here, when you try the number of skip, remember that alphabet has 26 characters, which is 2 X 13. So, do not use even numbers as a skip number (because A will be repeated eventually), also do not use 13 (same reason). Always begin with 3, 5, 7, 9, 11, 13 (left or right direction). This method assumes that there is no key used, and the first cipher letter is always \'a\'. Also remember this, the most frequently used letter in English is ETAOIN. However, if the number of characters is small, this statistic means very little. Of course, if you have 30-40 characters in the cipher, 0 frequency of big three (E T A) is also unusual. From the Wikipedia, the most frequently used first letter is T and A. The most frequently used double-letter is LL EE SS OO. This work will be very time consuming, so an Excel worksheet for this problem is provided. You can play with it, and hopefully find the answer quickly. This cipher text does not use any complex encryption method. So, all students can find the answer.
| cipher | # used | Plain | |
| a | 10 | ||
| b | 2 | ||
| c | 0 | ||
| d | 6 | ||
| e | 2 | ||
| f | 0 | ||
| g | 23 | ||
| h | 0 | ||
| i | 6 | ||
| j | 17 | ||
| k | 4 | ||
| l | 1 | ||
| m | 11 | ||
| n | 14 | ||
| o | 6 | ||
| p | 0 | ||
| q | 2 | ||
| r | 8 | ||
| s | 4 | ||
| t | 13 | ||
| u | 1 | ||
| v | 3 | ||
| w | 5 | ||
| x | 4 | ||
| y | 0 | ||
| z | 3 |
Solution
Each letter or group of letters is replaced by another group or group of letters, order of letter is preserved.
In Caesar cipher shift the letters in alphabet order modulo n.
For the given table please find the solution as below:
cipher
# used
Plain
a
10
k
b
2
D
c
0
C
d
6
J
e
2
G
f
0
F
g
23
D
h
0
H
i
6
O
j
17
A
k
4
O
l
1
M
m
11
X
n
14
B
o
6
U
p
0
P
q
2
S
r
8
Z
s
4
W
t
13
G
u
1
V
v
3
Y
w
5
B
x
4
B
y
0
Y
z
3
c
Thank you.
| cipher | # used | Plain | |
| a | 10 | k | |
| b | 2 | D | |
| c | 0 | C | |
| d | 6 | J | |
| e | 2 | G | |
| f | 0 | F | |
| g | 23 | D | |
| h | 0 | H | |
| i | 6 | O | |
| j | 17 | A | |
| k | 4 | O | |
| l | 1 | M | |
| m | 11 | X | |
| n | 14 | B | |
| o | 6 | U | |
| p | 0 | P | |
| q | 2 | S | |
| r | 8 | Z | |
| s | 4 | W | |
| t | 13 | G | |
| u | 1 | V | |
| v | 3 | Y | |
| w | 5 | B | |
| x | 4 | B | |
| y | 0 | Y | |
| z | 3 | c |
