README.md

Basic-mod2

Write-up author : jon-brandy

DESCRIPTION:

A new modular challenge! Download the message here. Take each number mod 41 and find the modular inverse for the result. Then map to the following character set: 1-26 are the alphabet, 27-36 are the decimal digits, and 37 is an underscore. Wrap your decrypted message in the picoCTF flag format (i.e. picoCTF{decrypted_message})

HINTS:

1. Do you know what the modular inverse is?
2. The inverse modulo z of x is the number, y that when multiplied by x is 1 modulo z.
3. It's recommended to use a tool to find the modular inverses

STEPS:

1. First, download the message.txt file.

104, 290, 356, 313, 262, 337, 354, 229, 146, 297, 118, 373, 221, 359, 338, 321, 288, 79, 214, 277, 131, 190, 377

3. The pattern quite similiar to the previous one, the basic-mod1 challenge. But for this one, we use the modular inverse algorithm.
4. I tried to solve it using python code like this:

import os
import string
import math

os.system("cls")
stringChar = string.ascii_lowercase # string of all lowercase letters
stringChar += "0123456789_" # string of all lowercase letters and numbers
rawFlag = [104, 290, 356, 313, 262, 337, 354, 229, # raw flag
           146, 297, 118, 373, 221, 359, 338, 321,
           288, 79, 214, 277, 131, 190, 377]
convertedFlag = "" # converted flag variable

# convert raw flag to string of all lowercase letters and numbers
for items in rawFlag:
    index = pow(items, -1, 41) # convert to index of stringChar (mod 41)
    convertedFlag += stringChar[index - 1] # convert to character and add to string

print(convertedFlag) # print converted flag

5. Finally, after executed the file, we got the flag!

---

FLAG:

picoCTF{1nv3r53ly_h4rd_8a05d939}