Cryptography - basic-mod1 - writeup
description
We found this weird message being passed around on the servers,
we think we have a working decrpytion scheme.
Download the message here. Take each number mod 37 and
map it to the following character set: 0-25 is the alphabet (uppercase),
26-35 are the decimal digits, and 36 is an underscore.
Wrap your decrypted message in the picoCTF flag
format (i.e. picoCTF{decrypted_message})
writeup
Lets write a simple python program which reads the message
into a string first.
Then I will read each number into an array and take each
number in the array mod 37
After that I will try to map each array field to the following scheme:
0-25 is the alphabet (uppercase),
26-35 are the decimal digits, and 36 is an underscore.
At the end I will warp the decoded message in the picoCTF
flag format like so: (i.e. picoCTF{decrypted_message})
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
| #!/usr/bin/env python3
import string
# read the file into a string
with open('message.txt', 'r') as file:
enc_msg = file.read()
# split the string by ' ' and insert the values into an array
enc_arr = enc_msg.split( )
# print out the array for test purposes
enc_arr_len = len(enc_arr)
# define empty mod_arr
mod_arr = []
# calculate modulo 37 of every number
for i in range(0, enc_arr_len):
mod_arr.append(round(int(enc_arr[i]) % 37))
# get length of resulting array
mod_arr_len = len(mod_arr)
# define array for final string
final_string = []
for i in range(0, mod_arr_len):
# try to map numbers from 0 to 25 to uppercase characters of the alphabet
if 0 <= int(mod_arr[i]) <= 25:
if(int(mod_arr[i]) == 0):
final_string.append('A')
elif(int(mod_arr[i]) == 1):
final_string.append('B')
elif(int(mod_arr[i]) == 2):
final_string.append('C')
elif(int(mod_arr[i]) == 3):
final_string.append('D')
elif(int(mod_arr[i]) == 4):
final_string.append('E')
elif(int(mod_arr[i]) == 5):
final_string.append('F')
elif(int(mod_arr[i]) == 6):
final_string.append('G')
elif(int(mod_arr[i]) == 7):
final_string.append('H')
elif(int(mod_arr[i]) == 8):
final_string.append('I')
elif(int(mod_arr[i]) == 9):
final_string.append('J')
elif(int(mod_arr[i]) == 10):
final_string.append('K')
elif(int(mod_arr[i]) == 11):
final_string.append('L')
elif(int(mod_arr[i]) == 12):
final_string.append('M')
elif(int(mod_arr[i]) == 13):
final_string.append('N')
elif(int(mod_arr[i]) == 14):
final_string.append('O')
elif(int(mod_arr[i]) == 15):
final_string.append('P')
elif(int(mod_arr[i]) == 16):
final_string.append('Q')
elif(int(mod_arr[i]) == 17):
final_string.append('R')
elif(int(mod_arr[i]) == 18):
final_string.append('S')
elif(int(mod_arr[i]) == 19):
final_string.append('T')
elif(int(mod_arr[i]) == 20):
final_string.append('U')
elif(int(mod_arr[i]) == 21):
final_string.append('V')
elif(int(mod_arr[i]) == 22):
final_string.append('W')
elif(int(mod_arr[i]) == 23):
final_string.append('X')
elif(int(mod_arr[i]) == 24):
final_string.append('Y')
elif(int(mod_arr[i]) == 25):
final_string.append('Z')
# try to map numbers from 26 to 35 to decimal digits
elif 26 <= int(mod_arr[i]) <= 35:
if(int(mod_arr[i]) == 26):
final_string.append('0')
elif(int(mod_arr[i]) == 27):
final_string.append('1')
elif(int(mod_arr[i]) == 28):
final_string.append('2')
elif(int(mod_arr[i]) == 29):
final_string.append('3')
elif(int(mod_arr[i]) == 30):
final_string.append('4')
elif(int(mod_arr[i]) == 31):
final_string.append('5')
elif(int(mod_arr[i]) == 32):
final_string.append('6')
elif(int(mod_arr[i]) == 33):
final_string.append('7')
elif(int(mod_arr[i]) == 34):
final_string.append('8')
elif(int(mod_arr[i]) == 35):
final_string.append('9')
# try to map 36 to an underscore _
elif int(mod_arr[i]) == 36:
final_string.append("_")
final_flag = ''.join(final_string)
print("picoCTF{" + final_flag + "}")
|
Flag:
1
| picoCTF{R0UND_N_R0UND_B0D5F596}
|
